REVIEW SESSION Midterm 2

Summary of Chapter 20

•  Magnets have north and south poles

•  Like poles repel, unlike attract

•  Unit of magnetic field: tesla

•  Electric currents produce magnetic fields

•  A magnetic field exerts a force on an electric current:

© 2014 Pearson Education, Inc.

Summary of Chapter 20

•  A magnetic field exerts a force on a moving charge:

•  Magnitude of the field of a long, straight current-carrying wire:

•  Parallel currents attract; antiparallel currents repel

© 2014 Pearson Education, Inc.

Summary of Chapter 20

•  Magnetic field inside a solenoid:

•  Ampère’s law:

•  Torque on a current loop:

© 2014 Pearson Education, Inc.

Ch. 20, multiple choice question

¨  A very long straight current-carrying wire produces a magnetic field of 20 mT at a distance d from the wire. To measure a field of 5 mT due to this wire, you would have to go to a distance from the wire of

¨  A) 16d.

¨  B) 8d. ¨  C) 4d.

¨  D) 2d.

¨  E) d

¨  Answer: C

Chapter 20, problem 1

¨  A proton travels at a speed of 5.0 × 107 m/s through a 1.0-T magnetic field. What is the magnitude of the magnetic force on the proton if the angle between the proton's velocity and the magnetic field vector is 30°? (e = 1.60 × 10−19 C)

¨  Answer: 4×10−12N

Summary of Chapter 21

•  Magnetic flux:

•  Changing magnetic flux induces emf: •  If the magnetic field is constant, then the flux is changing due to

the change of the area, and

•  If the magnetic field is changing, and area does not, then induced emf is

•  Induced emf produces current that opposes original flux change

•  Changing magnetic field produces an electric field

© 2014 Pearson Education, Inc.

€

E = −NBlv

€

E = −NA ΔBΔt

Summary of Chapter 21

•  Transformer uses induction to change voltage and current:

• 

•  Energy density stored in magnetic field:

© 2014 Pearson Education, Inc.

€

VS

VP

=NS

NP

; ISIP

=NP

NS

Chapter 21, multiple choice question

¨  A coil lies flat on a level tabletop in a region where the magnetic field vector points straight up. The magnetic field suddenly grows stronger. When viewed from above, what is the direction of the induced current in this coil as the field increases? ¤  A) counterclockwise ¤  B) clockwise ¤  C) clockwise initially, then counterclockwise before stopping ¤  D) There is no induced current in this coil.

Answer: B

Chapter 21 problem 1

¨  A conducting rod with a length ℓ = 25 cm is placed on a U-shaped metal wire that is connected to a lightbulb having a resistance of 8.0 Ω as shown in the figure. The wire and the rod are in the plane of the page. A constant uniform magnetic field of strength 0.40 T is applied perpendicular to and into the paper. An external applied force moves the rod to the right with a constant speed of 6.0 m/s. What are the magnitude and direction of the induced current in the circuit?

¨  Answer: 75mA counterclockwise

€

E = Blv; I =ER

Chapter 21, problem 2

¨  The output voltage of 95-W transformer is 12 V, and the input current is 25 A. a) Is this a step-up or a step-down transformer? b) By what factor is the voltage multiplied? c) What is the output current?

¨  Solution:

€

P = IPVP ⇒VP =PIP

= 3.8V;

VS

VP

=123.8

= 3.2; ISIP

=Vp

VS

⇒ IS = 25⋅ 13.2

=14.1 A

Summary of Chapter 22

•  Maxwell’s equations are the basic equations of electromagnetism

•  Electromagnetic waves are produced by accelerating charges; the propagation speed is given by:

•  The fields are perpendicular to each other and to the direction of propagation.

•  The wavelength and frequency of EM waves are related:

•  The electromagnetic spectrum includes all wavelengths, from radio waves through visible light to gamma rays.

© 2014 Pearson Education, Inc.

Summary of Chapter 22

•  Relation between amplitudes of the electric field and magnetic field in the EM wave:

•  Average intensity of the EM wave

© 2014 Pearson Education, Inc.

(22-8) €

c =EB

Chapter 22

¨  For an electromagnetic wave in free space having an electric field of amplitude E and a magnetic field of amplitude B, the ratio of B/E is equal to ¤  A) c ¤  B) c2

¤  C) 1/c ¤  D) 1/c2

¤  E) √c

Answer: C

Chapter 22 Problem 1

¨  What is the maximum value of the magnetic field at a distance of 2.5 m from a light bulb that radiates 100 W of single-frequency sinusoidal electromagnetic waves uniformly in all directions? ε0 = 8.85 × 10-12 C2/N ·∙ m2, μ0 = 4π × 10-7 T ·∙ m/A, c = 3.0 × 108 m/s)

¨  Answer: 0.1μT

€

P = I ⋅ A; I = 12

cµ0

B02 =; A = 4πr2

€

B0 =2µ0I

c

Ch. 22 problem 2

¨  Radiation from the Sun reaches the Earth with the intensity of 1350W/m2. Calculate the maximum values of E and B.

€

I = 12

cµ0

B02 =

12ε 0cE0

2 ⇒ E0 =2I ε 0c

=1.01⋅ 103 V / m

€

B0 =E0

c= 3.37⋅ 10−6T

Ch. 22 Problem 3

¨  What is the wavelength used by a radio station that broadcasts at a frequency of 920 kHz? (c = 3.00 × 108 m/s)

¨  Answer: 326 m

€

c = λ⋅ f ⇒ λ =cf