Chapter 3 Work book

1 | P a g e

1- What is meant by ? a- Faraday’s Law of electromagnetic induction . ……………………………………………………………………………………………………………………………………………………………………………………………… b- Lenz rule ……………………………………………………………………………………………………………………………………………………………………………………………… c- Mutual induction coefficient between two coils is 0.2 Henry . ……………………………………………………………………………………………………………………………………………………………………………………………… d- Self induction coefficient of a coil is 0.4 Henry . ……………………………………………………………………………………………………………………………………………………………………………………………… e- Henry ……………………………………………………………………………………………………………………………………………………………………………………………… f- Effective value of A.C. intensity is 2 Ampere. ……………………………………………………………………………………………………………………………………………………………………………………………… g- Efficiency of transformer is 80 % ……………………………………………………………………………………………………………………………………………………………………………………………… 2- Give reason for: a- The induced electromotive force in a coil increases if its core is made of soft iron . …………………………………………………………………………...………………………………………………………………………………………………………………… b- Give reason for ,when the magnet was pushed toward the aluminum ring ,it moved to the right

………………………………………………………………………………………………………………………………………………… c- Standard ohmic resistance must be double wounded. ……………………………………………………………………………………………………………………………………………………………………………………………… d- In a straight wire the current grows quickly and it grows slowly in a coil when the electric circuit is switched on. ……………………………………………………………………………………………………………………………………………………………………………………………… e-Induced e.m.f. may not be produced in the coil in case of switching on or switching off the circuit.

Chapter 3 Work book

2 | P a g e

f-An induced e.m.f. produces between terminals of a wire cutting lines of magnetic flux lines. ……………………………………………………………………………………………………………………………………………………………………………………………… g- The core of transformer made of soft iron sheets isolated from each other. ……………………………………………………………………………………………… h- No induced e.m.f. is produced in a straight wire moved in magnetic field. ……………………………………………………………………………………………… i- The coil of electric motor rotates in one direction. ……………………………………………………………………………………………… j- Efficiency of the transformers usually is less than 100%. …………………………………………………………………………………………… k- The average induced e.m.f. in dynamo coil during one complete cycle = zero. ……………………………………………………………………………………………… l- The coil of the electric motor continues in rotating when it is perpendicular to the lines of magnetic field, in spite that the torque in this position = zero. ………………………………………………………………………………………… m- The electric transformer does not work if its primary coil is connected with a direct current source. ………………………………………………………………………………………………. n- Step up transformers are used at power stations while step down used at houses. ……………………………………………………………………………………………… o- The core of transformer made of soft iron sheets isolated from each other. ……………………………………………………………………………………………… 3- The opposite figure represents a simple dynamo, a student wants to change it into a d.c. motor so he disconnects the voltmeter and connects in its place a battery and a switch . When he close the switch the coil does not rotate as it should in a d.c. motor: 1. What is the reason for this ? 2. How would you help the student to make the coil rotate? Show by drawing. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 4- What will expect when it happens with interpretation when ? a- Closing the key S in the drown circuit.

…………………………………………………………………… …………………………………………………………………… ……………………………………………………

b. The electric circuit is switched off, which illustrated in the front of you. c- Passing a high alternating current through a coil, which surround a piece of metal.

Chapter 3 Work book

3 | P a g e

……………………………………………………………………………………………… d. The replacement of the two metallic rings of the alternating current generator by metallic cylinder splitted into two halves insulated. ……………………………………………………………………………………………………………………………………………………………………………………………… e- Remove the iron core from the primary and the secondary coil from the transformer

………………………………………………………………………………………………………………………………………………………………

5- First: in the shown figure a- What the effect of the iron core ? b- What happen when the key is switched off ? c- Mention one application based on this figure ? 6- In the opposite circuit; what happens for the illumination of the two lamps joining with the two coils A and B, once as the magnet moves towards coil A, and another when moving towards the coil B.

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 7- If we have a secondary coil connected with M.C.G and primary coil can be moved in or out Secondary coil where connected in series to D.C. source , variable resistance and key , Explain what happens when a) Key opened or closed when primary coil inside secondary coil b) The variable resistance decreases c) The variable resistance increases d) Insert primary coil in secondary coil e) With primary coil out of secondary coil ……………………………………………………………………………………………………………………………………………………………………………………………… 8-

Chapter 3 Work book

4 | P a g e

9- Using the shown figure , A metal ring fall freely down to direction towards a solenoid: 1- Determine the direction of induced current in the ring when we look to the upper face . 2- What is the rule used to determine the direction of induced current ? 3- State one method to change direction of induced current in the ring when fall freely down again.

10 - Choose the correct answer : A- Two identical magnet allow to fall down through two rings as shown in figure then ( A fall 1st– B fall 1st – reach together 1st). b- The unit of the induced electromotive force in a coil is --------.[volt. ampere -1 -joule coulomb-1 – volt second. ampere-1 - Henry] . c- when S is closed What happens to the distance X 1- decreases 2- increases 3- Decreases then increases 4-fixed d- When decreasing the resistance of rheostat the light intensity of the lamp will 1- decrease 2-increase 3-unchanged 4-turned off e- In the shown figure ,which change will cause the pointer of the galvanometer deflect 1- Moving the wire in direction 1 2- Moving the wire in direction 2 3- Moving the magnet in the direction 2 4- Moving the magnet in the direction 3 f- When a magnet fall down as shown in fig. through the resistance the potential of point (a)is ( < , > , = )the potential of point (b)

Chapter 3 Work book

5 | P a g e

g- When maximum induced e.m.f. obtained from the dynamo, the plane of its coil relative to the magnetic field is: ( – parallel – inclined by an angle 45°). h- The continuation of motion of the motor coil is due to: (Mutual induction – inertia – self-induction – electromagnetic induction). i- When induced e.m.f. obtained from the dynamo vanishes, the plane of its coil relative to the magnetic field is: ( – parallel – inclined by an angle 45°). j- When the instantaneous induced e.m.f. obtained from the dynamo equals its effective value , the plane of its coil relative to the magnetic field is: ( – parallel – inclined by an angle 45°). k- Eddy currents are useful in designing: (Electric transformer – electric generator – Ruhmkorff's coil – induction furnace). l. If the time required for A C to reach half its maximum value (t) sec. the time needed to change from zero to max. is (4t – 2t –t-3t). m- The ratio between the number of coils to the number of sections of the commutator Cylinder in the DC generator equals: (1:1 , 2:1 ,1:2) respectively. n- In the simple generator, the direction of the current is reversed when be induced e.m.f. is ( maximum - of effective value - zero) . o- The ratio between the number of turns of the coils of a transformer that transfers 220 V into 17.6 V is 10 : 1 . The efficiency of the transformer is ------------------- . (12.5 - 80 - 95%) p- transformer is used to step up voltage from 120V to 3000V . if the current passes through its coil is 2A and the current passes through its secondary coil is 0.06A . So the efficiency of the transformer is ( 85% , 80% , 75%) q- If the value of E.m.f max. of A.C. generator equal 100 volt, then average E.m.f. during half cycle is ( 100- 63 -70.7-75). While during quarter cycle is ( 50,36.5,70.7,63) volt while during complete cycle is ( zero ,100,63,70.7) r-

Chapter 3 Work book

6 | P a g e

11- What is the function for 1-Fleming right hand rule: …………………………………………………………………………………...………… 2- Fleming’s Left Hand Rule : …………………………………………………………………………………...………… 3- Lenz’s rule: …………………………………………………………………………………..………… 4-Induction Furnaces: …………………………………………………………………………………..………… 5- Commutators in the dynamo: ……………………………………………………………………………………………… 6- Commutators in DC motor: ……………………………………………………………………………………………… 7- Step up Transformer: …………………………………………………………………………………..………… 8-Step down Transformer: …………………………………………………………………………………..………… 9- Electric Motor: …………………………………………………………………………………..………… 10- AC generator: ……………………………………………………………………………………………… 11- Graphite brushes in AC dynamo: ………………………………………….…………………………………………………… 12- Graphite brushes in DC motor: ……………………………………………………………………………………………… 13- The reverse current induced in the motor's coil during its rotation. …………………………………………..………………………………………………… 12- If the magnet moves it induces emf in the two coils ,find the direction of induced current in the two resistors AB and CD when 1- the magnet move toward the right 2- the magnet moves toward the left 13 - Write the scientific idea which each of the following based : 1- Fluorescent lamp: ……………………………………………………………………………………………… 2- Ruhmkorff's coil: ……………………………………………………………………………………………… 3- Induction furnaces: ………………………………………………………………………………………………

Chapter 3 Work book

7 | P a g e

4- AC dynamo: ……………………………………………………………………………………………… 5- Electric motor: ……………………………………………………………………………………………… 6-Transformer: ……………………………………………………………………………………………… 14- Write the physical quantity and equivalent unit for each of the following :

a- Volt.Sec………………………………………………………………… b- Ohm. Sec………………………………………………………………… c- Weber /sec………………………………………………………………… d- Tesla.m2………………………………………………………………… e- Ohm . Joule / Volt…………………………… f- Henry.Amp/Sec………………………………………………………… g- Ohm.Coulomb/m2………………………………………………………… h- Kg/Amp.sec2……………………………………………………………… i- V.Sec/ Amp.………………………………………………………………… j- J.Sec / C …………………………………………………………………….

k-Henry .Amp/Sec…………………………….………………………. m-J/V.………………………………………………………………………. n-Henry.Amp/m2……………………………………………………... o- N.m/Amp…………………………………………………………… p-Tesla.m2/Sec………………………………………………………………… q- Ohm.Coulomb…………………………………………………………. r-Volt.Sec/m2………………………………………………………… 15-Al-Azhar, 1989: What is the idea upon the mode of working of the A.C. generator is built? Deduce the relation which can be used to determine the instantaneous e.m.f. in this device. …………………………………………………………………………………………………………………………………………………………………………………………… 16- Al-Azhar,1990: Describe the construction of a device whose function is based on the conversation of the mechanical energy into electric energy by electromagnetic induction. Then explain its function. ……………………………………………………………………………………………………………………………………………………………………………………………… 17- Al-Azhar,1992: Compare between the primary coil and the secondary coils of a step up transformer with respect to: a- The no of turns. b- The current intensity. c- The e.m.f. …………………………………………………………………………………………………………………………………………………………………………………………… 18- Egypt, 1993: What are the modifications introduced to the AC generator to convert it into a unidirectional current generator. ………………………………………………………………………………………………

Chapter 3 Work book

8 | P a g e

19- Egypt, 1995: What are eddy currents? How can we use them in a useful apparatus? And how can we minimize them in another one? Mention also the name of each apparatus. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 20- Explain how to increase the following: a- The efficiency of the electric transformer. ……………………………………………………………………………………………… b- The efficiency of the electric motor. ……………………………………………………………………………………………… 21- Draw a labeled diagram of step down transformer and state reasons for the loss of energy in an electric transformer and then show how to reduce the effect of each of them. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 22- Draw a labeled diagram for the dynamo of the alternating current , then deduce the law of the maximum induced electromotive force. …………………………………………………………………………………………………………........................................................................................................................................................................................................................................................................................................................................................................................... 23- Mention the factors which each of the following depends: a- The self-inductance of a coil . ……………………………………………………………………………………………… b- Mutual inductance between two coils . ……………………………………………………………………………………………………….………………………………………………………… c- The induced e.m.f. produced in a straight wire moved in magnetic field. ……………………………………………………………………………………………… d- The induced e.m.f produced in a coil rotating in magnetic field . ……………………………………………………………………………………………… 24- Draw the figure in your answer book. On such figure sketch the variation of the (emf) induced in the coil with the time. ………………………………………………………………………………………………………………………………………………………………………………………………

B

0 emf

0

t

t

Chapter 3 Work book

9 | P a g e

25- What is the indication of negative sign and numericalvalue of the following:

e.m.f = - 20 ΔtΔϕ

(e.m.f is the induced e M. f produces in a coil). ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 26- Give reasons for :"The efficiency of transformer usually is less than 100%" ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..….… 27- Deduce the mathematical relation to calculate: a- The induced e.m.f. in the coil of the dynamo and follow the change during a complete cycle from the beginning motion. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… b- The induced e.m.f in a straight conductor moving in a magnetic field. ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 28- Egypt, May 2014: Figure (1) shows the resultant current through the external circuit of an electric generator . Figure (2) shows the resultant current through the external circuit of the same electric generator after making a certain modification 1- What is the difference between the two

currents? 2- What is the modification made on the electric generator? 3- Why can’t the ammeter be used to measure the resultant current in both

figures? ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 29- An isolated coil wounded around soft iron rod , what happens to in the following cases: 1- When D.C flow through the coil. 2- When A.C flow through the coil 3- When the coil is double wounded .

Chapter 3 Work book

10 | P a g e

1- Increasing the frequency of rotation of the coil's dynamo to double ……………………………………………………………………………………………………………………………………………………………………………………………… 2- Step up transformer used frequency in a solenoid coil around piece of metal ……………………………………………………………………………………………………………………………………………………………………………………………… 3- Replace the two metal rings by splited metal ring or commutator. ……………………………………………………………………………………………………………………………………………………………………………………………… 4- Using several coils of small angles in dynamo. …………………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 31-First: State the physical quantity which measured by the following units: Ω.Sec 2- Volt.Sec

.……………………………………………………………………………………………………………………………………………………………………………………………… ……………………………………………………………………………………………………………………………………………………………………………………………… 32- The opposite sketch represents the induced emf genreated in dynamo coil rotate by angular velocity (ω ) in 20 milli second.starting from zero postion ,Find: 1- the frequency of electric current 2- the induced emf produced after 2.5 milli sec 3-the avarege induced emf produced through 5 milli sec …………………………………………………. …………………………………………………. ………………………………………………….

Chapter 3 Work book

11 | P a g e

33- Mention the rule used to determine the following ..? 1- The direction of rotation of Motor 2- The induced current direction in the dynamo ………………………………………………………………………………………………………………………………………………………………………… 34-Compare between : Step-up transformer step-down

transformers Voltage Current Number of turns Application

35- Find the mathematical relation and the name of the slope of each of the following

Chapter 3 Work book

12 | P a g e

Problems 1- A coil of 100 turns is placed in a magnetic flux which changes from 0.02 wb to 0.08 wb in a time 0.003 sec. Find the induced e.m.f. (Ans. 2000 V ) ……………………………………………………………………………………………………………………………………………………………………………………………… 2- A coil of 300 turns and of area 60 cm2 is placed where its plane normal to magnetic field of 0.004 T. Find the induced e.m.f. in the coil if it is inverted in a time of 0.05 sec. (Ans. 0.288 V ) ……………………………………………………………………………………………………………………………………………………………………………………………… 3- A coil of 100 turns of resistance 3 Ohm and area 80 cm2. Find the rate of change of mag. flux density in order to induce a current of 0.5 mA in the coil? (Ans. 0.001875 Tesla /sec. ) ……………………………………………………………………………………………………………………………………………………………………………………………… 4- A coil 40 turns and area 30 cm2 is placed with its plane Perpendicular to a magnetic field of 0.05 Tesla. Find the average induced e.m.f. when the magnetic field is changed. a- to 0.06 T in a time of 0.1 sec. b- by 0.03 T in a time of 0.1 sec. (Ans. 0.012 V & 0.036V ) ……………………………………………………………………………………………………………………………………………………………………………………………… 5- The vertical component of the earth's magnetic field in a certain region is 0.03 milli Tesla. Calculate the Potential difference between the Wheels of a car, which are apart 1.5m and the car's speed, is 20 m/s. ( 0.9 V ) ……………………………………………………………………………………………………………………………………………………………………………………………… 6- The effective value of voltage produced by an AC. Dynamo is 100V. Find the induced e.m.f when the coil rotates by an angle. a- 30° b- 45° c- 60° d- 90° e- 120° from its vertical original position. (Ans. 141.4 V, 70.7, 100 , 122.46 ,141.4v 122.46) …………………………………………………………………………..………………………………………………………………………………………………………………………………………………………………………………………………………………… 7- A coil of 200 turns and dimensions 20 x 5 cm rotates in a magnetic field of density 0.36 Tesla with angular frequency 900 Revolution/minute, find the average induced e.m.f. during 1/4 cycle from the vertical position when the coil plane perpendicular to the field? ( Ans. 43.2 V ) ……………………………………………………………………………………………………………………………………………………………………………………………… 8- A circular coil of 20 turns and diameter 10 cm is placed in a magnetic field (0.2 T) where the field is parallel to the axis of the coil, Calculate: a- The magnetic flux through the coil? b- The time should the field drop (Vanish) to induce an average e.m.f. of 0.5 V in the coil. ( Ans. 1.57x10-3 Weber & 3.14 x10-4 Sec )

Chapter 3 Work book

13 | P a g e

9- Al-Azhar, 1995: A solenoid of length 1.1 m consists of 700 turns and cross sectional area 10 cm2 carries a current of intensity 2A, Find: a- The magnetic flux density at a point within the coil and on its axis. b- The induced e.m.f. when the current intensity drops to zero in 0.01 s.

c-The self – inductance of the coil. (1.6 x 10-3 T, 0.112 Volt& 5.6 x10-4 H) ……………………………………………………………………………………………………………………………………………………………………… 10- If the maximum induced e.m.f. produced in the coil of AC. Dynamo is equal to 110V. Calculate the instantaneous e.m.f. when the coil rotates. a- 1/12 b- 1/4 c- 1/2 rotation from zero position.(55 V , 0 , 110V) …………………………………………………………………………………………………………………………………………………………………………………………… ……….……………………………………………………………………. 11- A coil of dynamo consists of 350 turns , the cross - sectional area of each is 200 cm2 ,the coil rotates with frequency 50 rotation/sec in a uniform magnetic field of flux density 0.5 tesla . calculate : 1. The maximum induced e.m.f generated .(π=22/7). 2. The instantaneous value of the induced e.m.f generated after a time of 1/600 sec from the position at which the coil plane is normal to the magnetic field lines . (Ans. 1100 V , 550 V ) …………………………………………………………………………………………………………………………………………………………………… 12- Two facing coils, when the current intensity in one of them is changed from 4 A to zero in 0.01 sec. An induced e.m.f. equal 40 volts is generated in the other coil. Calculate the mutual induction between the two coils. ( Ans. 0.1 H ) …………………………………………………………………………………… …………………..……………………………………………………………….. 13- If the effective intensity of current in a circuit (Ieff) equal 2.828 ampere. Find the value of each of the following: i- The maximum value for current (Imax). ii- The instantaneous value of the induced current when the angle (θ) confined between the direction of velocity and the direction of the magnetic flux density = 30° ( Ans . 4 A & 2 A)

………………………………………………………………………………………………………………………………………………………………………….……………………

Chapter 3 Work book

14 | P a g e

14-2nd : If you have an electric generator of 100 turns, and cross sectional area of the coil is 0.025 m2. If rotates with frequency 700 cycles per minute in a magnetic field of flux density 0.3 Tesla. Calculate the induced e.m.f when 1. The plane of the coil is in a perpendicular position to the direction of the lines of the magnetic flux. 2. The plane of the coil is in a parallel position to the direction of the lines of the magnetic flux. 3. The angle between the direction of the coil plane and the lines of the flux is 30° Then calculate the effective value (emf )eff. π=22/7 ( Ans . zero , 55 V & 47.63 , Volt 38.89 V ) ……………………………………………………………………………………………………………………………………………………………………………………………………….……………………………………………………………………………………………………………………………………………….. 15- A coil with 25 turns of wire is wrapped around a hollow tube with a cross section area 1.8 cm2. Each turn has the same area as the tube. A uniform magnetic field is applied at right angle to the plane of the coil. If the field increases uniformly from zero to 0.55 Tesla in 0.75 second. l. Find the magnitude of the induced e.m.f in the coil. 2.If the resistance of the coil is 3 ohm ,find the magnitude of the induced current in the coil. ( Ans . 0.0033 Volt & 0.0011 Amp) ……………………………………………………………………………………………………………………………………………………………………… 16- A dynamo coil of length 40 cm and width 30 cm has 300 turns, it produces a current of frequency 50 /11 Hz if the effective value of e.m.f across its two terminal is 200√𝟐𝟐volt. Calculate:The maximum value of the induced e.m.f. 1- The magnetic flux density. 2- The maximum value of e m.f when the coil rotates around an axis parallel to its length with velocity 3 m/sec ( Ans. 400 V , 0.389 Tesla & 280.08 V) ……………………………………………………………………………………………………………………………………………………………………………………………… 17- A current of intensity 4 Amp passes through an inductive coil of 800 turns to produce a magnetic flux density of 2x10-4Weber. If the current vanishes in 0.08 sec. calculate: 1- induced emf in the coil. 2. The self inductance of the coil. 3. What is the rule used in determining the direction of the induced current in the coil. ( Ans. 2 Volt , 0 .04 Henry & Lenz’s Rule ) ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 18- An electric current of intensity 5 Amperes flowing through a coil of 500 turns produces a magnetic flux 10-4 Weber. If the current vanishes in 0.5 second, calculate: 1- The induced e.m.f. generated in the coil. 2- The self induction coefficient of the coil. ( Ans. 0.1 Volt & 0.01 H )

Chapter 3 Work book

15 | P a g e

19-The coil of an alternating current dynamo of dimensions 5 cm, 10 cm. It consists of 420 turns, it is put in a uniform magnetic flux of flux density 0.4 Tesla such that the plane of the coil was perpendicular to the field. If the coil rotates at a rate of 1000 rotation/minute, Calculate the induced e.m.f. in each of the two following positions: a) After ¼ of rotation from the first position. b) After rotation of angle of 30º from the initial position. c) Then calculate the average value of the induced e.m.f. through ¼ of rotation from the first position. (Ans. 88 Volt , 44 Volt & 56 Volt ) ……………………………………………………………………………………………………………………………………………………………………………………………… 20- A coil of A.C dynamo of dimensions (5 x 20 cm2 ) and of 800 turns place in uniform mag. field of 0.4 tesla , the axis of rotation in the plane of the coil is perpendicular to the field . if rotates with uniform velocity of 1500 rotation per half minute Calculate the e.m.f after 30o and after 1/4 cycle from the initial position . Calculate also the average induced e.m.f during 1/4 cycle from the initial position and the effective value of ac voltage. ( Ans. 502.5 V , 1005 V , 640 V & 710 V ) …………………………………………………………………………………………………………………………………………………………………………………………… 21- A coil of 10 ohm resistance , consists of 200 turns of area 10 cm2 , the coil inserted in mag. field of 0.3 tesla perpendicular to the turns plane . Calculate the average charge passing through the coil during interval of 1/4 cycle of the coil. ( Ans. 6 x 10-3 C ) ……………………………………………………………………………………………………………………………………………………………………………………………… 22- Electric generator has a coil of 500 turns, the cross sectional area of each turn equal 7/11 m2. It is placed in a uniform magnetic field of magnetic flux density 5X10-4 Tesla. If the coil rotates with frequency 3000 cycle/minute. Calculate: a- The maximum induced emf in the coil. b- The effective value of the current. c- No. of times to reach maximum value of induced e.m.f. in one second d- No. of times to reach zero induced e.m.f. in one second. (Ans. 50 V , 35.35 V , 100 times & 101 times ) ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 23- In the shown transformers 1- Mention the type o the transformers? why? 2- Why the core made of thin sheet? 3-What happen if the circuit of secondary coil is opened

………………………………………………………… ………………………………………………………… …………………………………………………………

Chapter 3 Work book

16 | P a g e

24- A dynamo coil of 200 turns , cross sectional area 2x 2.5 cm . revolve in a fixed magnetic field of flux density 14 tesla , with a rate 1500 cycle /min , calculate : 1- frequency . 2- periodic time . 3- angular velocity. 4- no of times reach I to zero each second. 5- no of times reach I to maximum each second. 6- e.m.f. (max). 7- eff(e.m.f) 8- e.m.f. when coil to flux 9- e.m.f. when the coil to flux lines. 10- e.m.f. when the to coil makes angle 30 with flux 11- e.m.f. when the coil makes angle 30 with flux. 12- e.m.f. when the coil rotates 30 from horizontal position 13- e.mf after 1/150 sec from starting position , 14- Time needed till e.m.f. becomes +110 volt 1sttime 15- Energy consumed in R=30 ohm in complete cycle (Ans. 25 Hz ( 1/25 sec 50 π. 51 times, 50 times , 220 Volt , 155.54 volt , 220 Volt, 110 volt , 95.26 volt, 110 volt , 1/150 sec 95.26 volt ) ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 25- Egypt 1990: An electric power station generates 100 Kilowatt at P.D. 200V. It is desired to transfer such power through a wire whose resistance is 4 ohm. calculate the efficiency of the transfer process, given that the ratio between the no of turns of the transformer used at the power station = 5: 1 . ………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 26- Al-Azhar, 1993: The input voltage of a transformer is 220V and its output voltage is 17.6V. Find the efficiency of this transformer given that the ratio between it turns is 10:1. ( Ans. 80 % )

Chapter 3 Work book

17 | P a g e

27- Egypt, 1996: A step down transformer of efficiency 80% and the no. of turns of its primary coil is 4000 turns, connected to the end of AC. Power lies to step down the voltage from 3000V to 120V. if the electric power produced from the transformer is 15 Kilowatt, Calculate: a- The no. of turns of the sec. coil. b- The current intensity in each of the primary and secondary coils. ( Ans. 200 Turns , 6.25 A & 125 A ) ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 28- A step down transformer is used to operate an electric lamp of power 24 watt, which works on a potential difference of 30 volts. If an electric source of 240 volts is used and the number of turns of the primary coil is 480 turns calculate: 1- The electric current intensity that passes through each of the primary and secondary coils. 2-The number of turns of the secondary coil.( Ans. 0.1 A ,0.8 A&60 Turns ) ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 29- A step-down transformer of efficiency 90% and the number of turns of its primary coil are 4500 turns, connected to the end of A.C. power lines to step down the voltage from 2000 volts to 120 volts. If the electric power connected to the transformer is 20 Kilowatt, calculate: a- The number of turns of the secondary coil. b- The current intensity in each of primary and secondary coils. ( Ans. 300 turns , 10 A & 150 A) ……………………………………………………………………………………………………………………………………………………………………………………………… 30- A step down transformer works on source of e.m.f. 240 volts. If the, number of turns of the primary coil is 5000 turns and the number of turns of the secondary coil is 250 turns and the efficiency of the transformer was 75%. 1. Calculate the e.m.f. produced in the secondary coil. 2. Mention three methods by which the efficiency of any transformer can be improved. ( Ans . 9 V ) ……………………………………………………………………………………………………………………………………………………………………………………………… 31 - An electric transformer operates on potential difference 220 volt has two secondary coils , one of them operates a small fan of ( 6 volt, 0.4 A), the other operates electric recorder of (12 volts , 0.35 A) . if the number of turns of the primary coils is 1100 turns . Find : 1) the number of turns of each secondary coil. 2) the current intensity that flows through the primary coil that operating the electric fan and the recorder at the same time . ( Ans. 30 , 60 , 0.03 A) ………………………………………………………………………………………………………………………………………………………………………………………………

Chapter 3 Work book

18 | P a g e

32-The figure shows a step down electric transformer l. Why is the iron core made of thin sheets insulated from each other ? 2.1f the number of turns in the primary coil is 640turns and the efficiency of the transformer is 80% , find the number of turns in the secondary coil. ( Ans. 80 turns ) ……………………………………………………………………………………………………………………………………………………………………………………………… 33- A step-down transformer of efficiency 80% and the number of turns of its primary coil is 4000 turns, connected to the end of A.C. power lines to step down the voltage from 3000 volts to 120 volts. If the electric power produced from the transformer is 15 kilowatt. Calculate : 1- The number of turns of the secondary coil . 2- The current intensity in each of primary and secondary coils . ……………………………………………………………………………………………………………………………………………………………………………………………… 34- A step down transformer, the number of turns of its primary coil is 5000 turn and the number of turns of its secondary coil is 250 turns. If the primary coil voltage is 240 volt. 1- Calculate induced electromotive force generated between the two ends of its secondary coil. 2- If a back induced electromotive force 4 volt is generated in the secondary coil when the current passing through the primary coil changes by a rate of 5 Ampere / second, calculate the coefficient of mutual induction between the two coils. ( Ans. 12 Volt & 0.8 Henry ) ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 35- A simple A.C. generator consists of a coil of cross section area ( 2/π m2) rotates in a magnetic field of flux density 10-3 Tesla with constant frequency ( υ Hz). The following table illustrates the relation between the number of turns (N) of the coil and the maximum value of the induced e.m.f. in ,the coil (e.m.fmax ) Emax 80 160 320 B 480 N 10 20 A 50 60

Draw a graphical relation between (N) on the x-axis and (e.m.fmax ) on the (y)- axis, from the graph find : 1-The values of both a and b . 2- The frequency (υ) with which the coil rotates in the magnetic field .

Chapter 3 Work book

19 | P a g e

36- A conducting rod of length , L=35 cm is free to slide on two parallel conducting bars as shown in the shown Figure. A resistors R1 =2 ohm is connected across the ends of the bars to form a loop. A constant magnetic field B = 2.5 T is directed perpendicularly into the page. An external agent pulls the rod to the left with a constant speed of v = 8 m/s. Find (a) the direction and magnitude of current through R1. (b) The total power delivered to R1. (c) The magnitude of the applied force that is needed to move the rod with this constant velocity. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 37- The flexible loop in Figure has a radius of 12.0 cm and is in a magnetic field of magnitude 0.150 T. The loop is grasped at points A and B and stretched until its area is nearly zero. If it takes 0.200 s to close the loop, what is the magnitude of the average induced emf in it during this time interval?

……………………………………………………………………………………………………………………………………………………………………..…. …………………………………………………………………………... 38- A coil of 15 turns and radius 10.0 cm surrounds a long solenoid of radius 2.00 cm and 1.x 103 turns/meter. The current in the solenoid changes from I =5A to zero A, in time t = 0.1 seconds. Find the induced emf in the 15-turn coil

…………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 39- A circular loop of wire of resistance R = 0.5 ohm and radius r =8 cm is in a uniform magnetic field directed out of the page as in the shown figure. If a clockwise current of I =2.5 mA is induced in the loop, (a) is the magnetic field increasing or decreasing in time? (b) Find the rate at which the field is changing with time.

Chapter 3 Work book

20 | P a g e

41- A coil formed by wrapping 50 turns of wire in the shape of a square is positioned in a magnetic field so that the normal to the plane of the coil makes an angle of 30.08 with the direction of the field. When the magnetic field is increased uniformly from 200 mT to 600 mT in 0.400 s, an emf of magnitude 80.0 mV is induced in the coil. What is the total length of the wire in the coil? ………………………………………………………………….. ……………………………………………………………………… 42- An Ideal transformer is used to operate a TV (50W – 220 volt). The primary coil is connected with an A.C. generator of number of turns 2000 turns and its coil of area 30 cm x 20 cm rotates with velocity 50 rotation/sec through a magnetic field lines B= 583.8x10-3 T. 1-Is the used transformer step up or step down? 2-Find the ratio between the numbers of turns of transformer coils (NP :NS). 3-Find the current intensity through the primary coil IP. …………………………………………………………………………………………………………………………………………………………………………… 43- If the magnetic flux density cutting a coil of 400 turns and 4 cm2 changes with time according to the shown graph : Calculate the induced e.m.f. in each part a- From A to B b- From B to C 0.6 c- From C to D

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 44- A step down transformer is used to operate an electric device of power 55 watt, which works on a potential difference of 22 volts. If an electric source of 110 volts is used and the number of turns of the primary coil is 600 turns calculate: 1- The electric current intensity that passes through each of the primary and secondary coils. 2-The number of turns of the secondary coil. …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..……………………………………………………………………………………………

Chapter 3 Work book

21 | P a g e

45- If the intensity of A.C. is given by I = 200 Sin (18000 t) Find: a- maximum current intensity. a- Angular velocity. b- Frequency. c- Effective value of current intensity. d- Power through a resistor of 20 Ohm during one complete cycle

……………………………………………………………………………………………………………………………………………………………………………………………………………..………………………………………………………………………

46- A step-down transformer of efficiency 80% and the number of turns of its primary coil are 3000 turns, connected to the end of A.C. power lines to step down the voltage from 4000 volts to 200 volts. If the electric power connected to the transformer is 30 Kilowatt, calculate: a- The number of turns of the secondary coil. b- The current intensity in each of primary and secondary coils. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 47- Given that in an ideal step-up transformer, the ratio between the its number of turns is 1:100, it was connected to an A.C. source of 200 volts. Calculate each of the following: 1. The E.M.F. induced in the secondary coil. 2. The ratio between the current intensity of primary and that of secondary coil (Ip/Is). 3. The power produced from the secondary coil, if the current flowing in it is 2A. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 48- A simple dynamo has coil of 70 turns rotate in magnetic field of density 0.4 T . Following table illustrate the relation between max. emf and frequency .Draw a graphical relation bet. emf on Y-axis and frequency on X-axis

emf max. of coil (volt) 10 20 30 40 Frequency (Hz) 20 40 60 80

From the graph find 1-the area of the coil 2-Angulare velocity when emfmax. is 25 volt

Chapter 3 Work book

22 | P a g e

49-

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 50- In the following figure: A wire of length 20 cm is perpendicular to magnetic field of flux density 2.5 Tesla and moved by velocity 8 m/sec , Calculate a) The current intensity passing through 6Ω b) The force needed to move the wire AB c) Which is higher in potential A or B …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..…… 51- the drawing x show coil rotate bet. two poles of magnet in motor , two ends T1 , T2 connected with external circuit while the other drawing y show the change of emf in same motor 1- what point in the drawing y indicate the emf in the coil when the coil is normal in the magnetic flux , give reason 2-Find the time take by the coil to change emf from 45 v to 22.5 v in first time

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..…………

Chapter 3 Work book

23 | P a g e

52-The drawing shows the change of AC produced from dynamo with the time Find 1- the angular velocity 2-the effective value of AC 3-Explain how can from AC produce current of two drawing X ,Y

……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………..…… 53- if maximum voltage of A.C.is 100 V and maximum current is 10 A ,then the produced power is ( 100 – 1000 – 50 – 500 ) watt 54- From the graph Find its maximum value of e.m.f and its effective value 55 - If R = 20 Ω a- find Iinst after 5 mS b- find Eavrage during 15 mS c- find the energy consumed during one cycle.

……………………………………………………………………………………………… 56-Figure is a graph of the induced emf versus time for a coil of N turns rotating with angular speed v in a uniform magnetic field directed perpendicular to the coil’s axis of rotation. What If? Copy this sketch (on a larger scale) and on the same set of axes show the graph of emf versus t (a) if the number of turns in the coil is doubled, (b) if instead the angular speed is doubled, and (c) if the angular speed is doubled while the number of turns in the coil is halved.

Chapter 3 Work book

24 | P a g e

57- An electric power station generates 100 Kilowatt at P.D. 200V. It is desired to transfer such power through a wire whose resistance is 4 ohm. calculate the efficiency of the transfer process, given that the ratio between the no of turns of the transformer used at the power station 1) 5: 1 and 2) 10:1 and which is preferred. ……………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………… 58 - In the shown figure a step up transformer used to transmit an electric power of an a.c source to an appliance of power 5800 Watt and works at 200 volt using a transmission power line of ohmic resistance 2 Ω, as the current intensity of the line is 10 Ampere, as the efficiency of such transformer is 60%, calculate: a. The power at beginning of transmission line. b. The voltage at the secondary coil. c. The intensity of current in primary coil. d. The number of turns in the primary coil as the number in the secondary coil is 1200 turns ……………………………………………………………………………………………………………………………………………………………………………………..…………………………………………………………………………………………………………………………………………………………………………………………………… 59- Figure shows four configurations of a wire loop placed near a long, straight wire with a time-varying current. Determine the direction of (a) the induced magnetic field in the wire loop (into or out of the page) and (b) the induced current (clockwise or counterclockwise) for each configuration (1) through (4).

………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

Chapter 3 Work book

25 | P a g e

60- the induced current flow as shown in figure. Which choice of the following is correct? (Knowing that: Each raw is a choice )

Direction of magnet motion

Polarity at A

A Down ward North B Down ward South C Up ward North D Up ward South

61- In the opposite figure, when the magnet is moving away, the reading of ammeter: ( Increases – Decreases - Remains unchanged - Equals zero ) 62- A constant magnetic field is directed into the page in the shaded rectangular region. discusses what is the direction of induced current in a copper ring that slides through the region from position 1 to position 5. 63- What is meant by

(a)Induced emf in a coil when the current changes by rate 1A/sec is 0.1 volt ……………………………………………………………………………………………………………………………………………………………………….. (b)Ratio between output power produced in secondary coil of transformer to input power of primary coil is 80% ……………………………………………………………………………………………… 64- What happens when: a- Passing AC current in motor's coil. ……………………………………………………………………………………………………………………………………………………………………….

Chapter 3 Work book

26 | P a g e

b- Opening the secondary coil circuit of electric transformer while closing primary coil connected with AC source. ………………………………………………………………………………………………………………………………………………………………………. c- Replacing the two slip rings with a commutator consisting of two halves. ………………………………………………………………………………………………………………………………………………………………………. d- Opening the secondary coil of a transformer where its primary coil is joined with an a.c source. ………………………………………………………………………………… 65- Choose: a- At the instant when the coil of an AC dynamo is parallel to the direction of the magnetic flux, the value of the magnetic flux through the coil (Ø) and the induced electromotive force (E) would be:

b- When the magnetic flux (φm) intercepted by a number of turns is changed , due to variation of current (∆ I), thus the ratio ( 𝐍𝐍 ∆ 𝛗𝛗𝒎𝒎

𝜟𝜟 𝑰𝑰 ) is equal to:

A- The total magnetic flux. B- The magnetic flux density. C- The self-inductance of the coil. D- The Induced (emf) in the coil c- The graph that represents the current generated by a dynamo consisting of a number of coils with equal small angles between their planes is

d-The graph that represents the current generated by a dynamo consisting of two metallic rings connected to plane of coil :

e-The graph that represents the current generated by a dynamo consisting of two commutators connected to plane of coil:

Ø E A Maximum Zero B Zero Maximum C Maximum Maximum D Zero Zero

Chapter 3 Work book

27 | P a g e

66- The diagram illustrates an ideal transformer. Its secondary coil is connected to a device (x). A current of value 2A passes through the device. First :What is the type of this transformer? Second :Find the resistance of the device (x) that is connected to the secondary coil ,given. That Ns = 𝟏𝟏

𝟐𝟐 Np

67- The diagram shows a simple electric motor First : Name the rule that can be used to determine the magnetic flux direction due to the flowing of current through the side (ab). Second: Determine the direction of the coil rotation. …………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………………

68- In an AC dynamo, the two metal rings at the coil terminals are replaced by a commutator. Plot a curve to represent the generated current in this case. Where coil starting rotation from parallel position 69- In an AC dynamo, the two metal rings at the coil terminals are replaced by a commutator. Plot a curve to represent the generated current in this case. Where coil starting rotation from vertical position 70- The graph represents the electromotive force (emf) generated in the dynamo. Use the given data to find the average electromotive force through a 1/4 revolution of its coil.

…………………………………………………………………………………………………………………………………………………………………………………………………………………………..