Ch17 Lecture

7/27/2019 Ch17 Lecture

1/30

Chapter 17Current and Resistance

Conceptual questions: 3,6,10,14

Quick quizzes: 1,3,4,5,6,7

Problems: 12,27,30,47


2/30

Electric Current

The current is the rate at which thecharge flows through a surface

The direction of current flow is the

direction positive charge would flow The SI unit of current is Ampere (A)

1 A = 1 C/s


3/30

QUICK QUIZ 17.1

Consider positive and negative charges movinghorizontally through the four regions in Figure17.2. Rank the currents in these four regions,from lowest to highest.


4/30

Charge Carrier Motion in aConductor

The zig-zag black linerepresents the motion of chargecarrier in a conductor

The net drift speed is small

The sharp changes in directionare due to collisions

The net motion of electrons is

opposite the direction of theelectric field


5/30

Electrons in a Circuit

The drift speed is much smaller thanthe average speed between collisions

Although the drift speed is on the orderof 10-4 m/s the effect of the electricfield is felt on the order of 108 m/s


6/30

Meters in a Circuit -- Ammeter

An ammeter is used to measure current

In line with the bulb, all the charge passingthrough the bulb also must pass through themeter


7/30

Meters in a Circuit --Voltmeter

A voltmeter is used to measure voltage

(potential difference) Connects to the two ends of the bulb


8/30

QUICK QUIZ 17.3

Look at the four circuits shown below and

select those that will light the bulb.


9/30

Resistance

The resistanceof the conductor

Units of resistance are ohms()

1 = 1 V / A

Resistance in a circuit arises due tocollisions between the electronscarrying the current with the fixed

atoms inside the conductor

VR=

I


10/30

Ohms Law; ohmic materialsV = I R

Ohms Law is an empirical relationshipMaterials that obey Ohms Law are said to be

ohmic


11/30

Non-ohmic materials

The current-voltagerelationship is

nonlinearA diode is a

common example ofa non-ohmic device


12/30

QUICK QUIZ 17.4In the figure below, does the resistance of

the diode(a) increase or(b) decrease as the positive voltage V

increases?


13/30

Resistivity

The resistance of an ohmic conductor isproportional to its length, L, andinversely proportional to its cross-sectional area, A

is the constant of proportionality and is

called the resistivityof the material See table 17.1

A

LR


14/30


15/30

Problem 17-12

Suppose that you wish to fabricate auniform wire out of 1 g of copper. If

the wire is to have a resistance ofR=0.500 Q, and if all of the copper is tobe used, what will be

A. the lengthB. the diameter of that wire?


16/30

QUICK QUIZ 17.5

Aliens with strange powers visited Earth anddoubled every linear dimension of every object onthe surface of the Earth. A wire has the length

and diameter twice their original values.Does the wire now have(a) more resistance than before,(b) less resistance,(c) the same resistance?

(Assume the resistivities of materials remain the

same before and after the doubling.)


17/30

Temperature Variation ofResistivity

For most metals, resistivity increasesapproximately linearly with temperature

o is the resistivity at some referencetemperature To

To is usually taken to be 20 C is the temperature coefficient of resistivity

see Table 17.1

)]TT(1[ oo


18/30

Temperature Variation ofResistance

Since the resistance of a conductor withuniform cross sectional area is

proportional to the resistivity, the effectof temperature on resistance is

)]TT(1[RRoo


19/30

Problem 17-27

A 34.5 m length of copper wire at20.0oC has a radius of 0.25 mm. If a

potential difference of 9.0 V is appliedacross the length of the wire, determinethe current in the wire.

If the wire is heated to 30.0oC while thepotential difference is maintained, whatis the resulting current in the wire?


20/30

A platinum resistance thermometer has resistances of 200.0 when

placed in a 0C ice bath and 253.8 when immersed in a crucible

containing melting potassium. What is the melting point of potassium?(Hint: First determine the resistance of the platinum resistance

thermometer at room temperature, 20C.)

The resistance at 20.0C is

Solving forTgives the temperature of the

melting potassium as

01

-30

200.0217

11+ 3.92 10 C 0 C 20.0 C

RR

T T

0 01R R T T

00

10

3

253.8 21720.0 C

3.92 10 C 217

R RT T

R

63.2 C

Problem 17-30


21/30

Superconductors

A class of materials andcompounds whoseresistances fall to

virtually zero below acertain temperature, TC

TC is called the criticaltemperature

The graph is the sameabove TC, but suddenlydrops to zero at TC


22/30

Electrical Energy and Power

The rate at which the energy is lost isthe power

From Ohms Law, alternate forms ofpower are

VIVt

Q

P

R

)V(RIP

22


23/30

Electrical Energy and Power

The SI unit ofpower is Watt (W) The unit ofenergy used by electric

companies is the kilowatt-hour

This is defined in terms of the unit ofpower and the amount of time it issupplied

1 kWh = 3.60 x 106 J


24/30

QUICK QUIZ 17.6A voltage Vis applied across the ends of

a nichrome heater wire having a cross-sectional areaA and length L. The samevoltage is applied across the ends of asecond heater wire having a cross-

sectional areaA and length 2L. Which wiregets hotter?(a) the shorter wire,

(b) the longer wire,(c) not enough information to say.

Q C Q


25/30

QUICK QUIZ 17.7

For the two resistorsshown here, rank thecurrents at points a

through f, from largestto smallest.

P bl 17 47


26/30

The heating coil of a hot water heater has a resistance of 20 andoperates at 210 V. If electrical energy costs $0.080/kWh, what does it

cost to raise the 200 kg of water in the tank from 15 C to 80 C?

The kilowatt-hour is a measure of energy equal to

The energy needed to raise the temperature of 200 kg ofwater from 15 C to 80 C is

and the cost of operating the heater to produce this quantity ofthermal energy is

(cost)= (energy used) (rate) = (15 kWh) ($0.080/kWh) = $1.20

Problem 17-47


27/30

3. Why dont the free electrons in a metal fall to the bottom of

the metal due to gravity? Charges in a conductor are supposeto reside on the surfacewhy dont the free electrons all go

to the surface?

6. Two lightbulbs are each connected to a voltage of 120V. One

has a power of 25 W, the other 100 W. Which bulb has a

higher resistance? Which bulb carries more current?

10. Some homes have light dimmers that operate by rotating a

knob. What is being changed in the electric circuit when the

know is being rotated?

14. Use the atomic theory of matter to explain why the resistanceof a metal should increase as its temperature increases.

Conceptual questions


28/30

In the two cases shown here a battery is connected up to a

box containing some resistors. The battery provides an

EMF =V0.

Which box will draw more current from the battery?

1. They will draw the same amount.

2. #1 will draw twice as much as #2.

3. #1 will draw more than #2 but not twice.

4. #2 will draw twice as much as #1

5. #2 will draw more than #1 but not twice.

6. You cant tell from the information given

MCAT


29/30

MCAT A positive charge q=1.1x10-11 C is located 0.01 m

away from a negative charge of equal magnitude.

Point P is exactly between them. What is themagnitude of the electric field at point P?

a. 1000 N/C b. 2000 N/C

c. 4000 N/C d. 8000 N/C

If a water molecule is placed between two platesof a capacitor, which orientation would it take tominimize its energy? The electric field is up.

a. H b. H

O OH H

c. H H d. O

O H H

E


30/30

MCAT, cont. The ratio of the potential difference across a

conductor and the current moving through it iscalled the

a. resistance b. capacitance

d. conductance d. electric potential

In a solid metal conductor, electric current is themovement ofa. electrons only b. protons only

c. nuclei d. protons and electrons

A 12 load is connected across a 6.0 V battery. Howmuch energy does the load use in an hour?a. 1.5 x 10-3 kWh b. 2.0 x 10-3 kWh

c. 3.0 x 10-3 kWh d. 12 x 10-3 kWh

Documents

Ch17 Lecture