Question 1

If   is the current density and   is a vector element of area then the integral   over an area represents:

Accepted Answer

A) the electric flux through the area 
B) the average current density at the position of the area 
C) the resistance of the area 
D) the resistivity of the area 
E) the current through the area 
A) the electric flux through the area 
B) the average current density at the position of the area 
C) the resistance of the area 
D) the resistivity of the area 
E) the current through the area E

Question 2

A wire has an electric field of 6.2 V/m and carries a current density of 2.4 x 108 A/m2.What is its resistivity?

Accepted Answer

A) 6.7 x 10-10 $\Omega$.m 
B) 1.5 x 10-8 $\Omega$.m 
C) 2.6 x 10-8 $\Omega$.m 
D) 3.9 x 107 $\Omega$.m 
E) 1.5 x 109 $\Omega$.m 
A) 6.7 x 10-10 $\Omega$.m 
B) 1.5 x 10-8 $\Omega$.m 
C) 2.6 x 10-8 $\Omega$.m 
D) 3.9 x 107 $\Omega$.m 
E) 1.5 x 109 $\Omega$.m C

Question 3

Compared to a conductor, a semiconductor

Accepted Answer

A) has larger conductivity 
B) has smaller resistivity 
C) has a larger charge carrier density 
D) has a negative temperature coefficient of resistivity 
E) has zero resistivity 
A) has larger conductivity 
B) has smaller resistivity 
C) has a larger charge carrier density 
D) has a negative temperature coefficient of resistivity 
E) has zero resistivity D

Question 4

A cylindrical copper rod has resistance R.It is reformed to twice its original length with no change of volume.Its new resistance is:

Accepted Answer

A) R/2 
B) R 
C) 2R 
D) 4R 
E) 8R 
A) R/2 
B) R 
C) 2R 
D) 4R 
E) 8R

Question 5

Energy may be measured in:

Accepted Answer

A) kilowatt 
B) joule·second 
C) watt 
D) watt·second 
E) volt/ohm 
A) kilowatt 
B) joule·second 
C) watt 
D) watt·second 
E) volt/ohm

Question 6

Of the following the copper conductor that has the least resistance is:

Accepted Answer

A) thin, long and hot 
B) thick, short and cool 
C) thick, long and hot 
D) thin, short and cool 
E) thin, short and hot 
A) thin, long and hot 
B) thick, short and cool 
C) thick, long and hot 
D) thin, short and cool 
E) thin, short and hot

Question 7

You buy a "75 W" light bulb.The label means that:

Accepted Answer

A) no matter how you use the bulb, the power will be 75 W 
B) the bulb was filled with 75 W at the factory 
C) the actual power dissipated will be much higher than 75 W since most of the power appears as heat 
D) the bulb is expected to "burn out" after you use up its 75 watts 
E) none of the above 
A) no matter how you use the bulb, the power will be 75 W 
B) the bulb was filled with 75 W at the factory 
C) the actual power dissipated will be much higher than 75 W since most of the power appears as heat 
D) the bulb is expected to "burn out" after you use up its 75 watts 
E) none of the above

Question 8

Current has units:

Accepted Answer

A) kilowatt·hour 
B) ampere 
C) coulomb 
D) volt 
E) ohm 
A) kilowatt·hour 
B) ampere 
C) coulomb 
D) volt 
E) ohm

Question 9

For an ohmic substance, the resistivity depends on:

Accepted Answer

A) the electric field 
B) the potential difference 
C) the current density 
D) the electron mean free time 
E) the cross-sectional area of the sample 
A) the electric field 
B) the potential difference 
C) the current density 
D) the electron mean free time 
E) the cross-sectional area of the sample

Question 10

A 60-watt light bulb carries a current of 0.5 ampere.The total charge passing through it in one hour is:

Accepted Answer

A) 3600 C 
B) 3000 C 
C) 2400 C 
D) 1800 C 
E) 120 C 
A) 3600 C 
B) 3000 C 
C) 2400 C 
D) 1800 C 
E) 120 C

Question 11

A car battery is rated at 80 A.h.An ampere-hour is a unit of:

Accepted Answer

A) power 
B) energy 
C) current 
D) charge 
E) force 
A) power 
B) energy 
C) current 
D) charge 
E) force

Question 12

A current of 0.3 A passes through a lamp for 2 minutes using a 6 V power supply.The energy dissipated by this lamp during the 2 minutes is:

Accepted Answer

A) 1.8 J 
B) 12 J 
C) 20 J 
D) 36 J 
E) 216 J 
A) 1.8 J 
B) 12 J 
C) 20 J 
D) 36 J 
E) 216 J

Question 13

A 1.5-V battery is connected to a circuit and puts out a current of 0.45 A.How much power is the battery putting out?

Accepted Answer

A) 0.30 W 
B) 0.68 W 
C) 3.3 W 
D) 5.0 W 
E) 7.4 W 
A) 0.30 W 
B) 0.68 W 
C) 3.3 W 
D) 5.0 W 
E) 7.4 W

Question 14

Copper contains 8.4 *1028 free electrons per cubic meter.A copper wire of cross-sectional area 7.4 *10-7 m2 carries a current of 1 A.The electron drift speed is approximately:

Accepted Answer

A) 3 * 108 m/s 
B) 103 m/s 
C) 1 m/s 
D) 10-4 m/s 
E) 10-23 m/s 
A) 3 * 108 m/s 
B) 103 m/s 
C) 1 m/s 
D) 10-4 m/s 
E) 10-23 m/s

Question 15

For an ohmic resistor, resistance is the proportionality constant for:

Accepted Answer

A) potential difference and electric field 
B) current density and electric field 
C) current and length 
D) current and cross-sectional area 
E) current and potential difference 
A) potential difference and electric field 
B) current density and electric field 
C) current and length 
D) current and cross-sectional area 
E) current and potential difference

Question 16

The units of resistivity are:

Accepted Answer

A) ohm 
B) ohm·meter 
C) ohm/meter 
D) ohm/meter2 
E) none of these 
A) ohm 
B) ohm·meter 
C) ohm/meter 
D) ohm/meter2 
E) none of these

Question 17

The current density is the same in two wires.Wire A has twice the free electron concentration of wire B.The drift speed of electrons in A is:

Accepted Answer

A) twice that of electrons in B 
B) four times that of electrons in B 
C) half that of electrons in B 
D) one-fourth that of electrons in B 
E) the same as that of electrons in B 
A) twice that of electrons in B 
B) four times that of electrons in B 
C) half that of electrons in B 
D) one-fourth that of electrons in B 
E) the same as that of electrons in B

Question 18

A wire is 1 m long and 1 *10-6 m2 in cross-sectional area.When connected to a potential difference of 2 V, a current of 4 A exists in the wire.The resistivity of this wire is:

Accepted Answer

A) 2 * 10-6 $\Omega$.m 
B) 8* 10-6 $\Omega$. m 
C) 1 * 10-7 $\Omega$.m 
D) 4 *10-7 $\Omega$.m 
E) 5 * 10-7 $\Omega$m 
A) 2 * 10-6 $\Omega$.m 
B) 8* 10-6 $\Omega$. m 
C) 1 * 10-7 $\Omega$.m 
D) 4 *10-7 $\Omega$.m 
E) 5 * 10-7 $\Omega$m

Question 19

A student kept her 60-watt, 120-volt study lamp turned on from 2:00 PM until 2:00 AM.How much charge went through it?

Accepted Answer

A) 720 C 
B) 3,600 C 
C) 7,200 C 
D) 18,000 C 
E) 21,600 C 
A) 720 C 
B) 3,600 C 
C) 7,200 C 
D) 18,000 C 
E) 21,600 C

Question 20

In a conductor carrying a current we expect the electron drift speed to be:

Accepted Answer

A) much greater than the average electron speed 
B) much less than the average electron speed 
C) about the same as the average electron speed 
D) less than the electron speed at low temperature and greater than the electron speed at high temperature 
E) less than the electron speed at high temperature and greater than the electron speed at low temperature 
A) much greater than the average electron speed 
B) much less than the average electron speed 
C) about the same as the average electron speed 
D) less than the electron speed at low temperature and greater than the electron speed at high temperature 
E) less than the electron speed at high temperature and greater than the electron speed at low temperature

Exam 26: Current and Resistance

If is the current density and is a vector element of area then the integral over an area represents:

A wire has an electric field of 6.2 V/m and carries a current density of 2.4 x 108 A/m2.What is its resistivity?

Compared to a conductor, a semiconductor

A cylindrical copper rod has resistance R.It is reformed to twice its original length with no change of volume.Its new resistance is:

Energy may be measured in:

Of the following the copper conductor that has the least resistance is:

You buy a "75 W" light bulb.The label means that:

Current has units:

For an ohmic substance, the resistivity depends on:

A 60-watt light bulb carries a current of 0.5 ampere.The total charge passing through it in one hour is:

A car battery is rated at 80 A.h.An ampere-hour is a unit of:

A current of 0.3 A passes through a lamp for 2 minutes using a 6 V power supply.The energy dissipated by this lamp during the 2 minutes is:

A 1.5-V battery is connected to a circuit and puts out a current of 0.45 A.How much power is the battery putting out?

Copper contains 8.4 *1028 free electrons per cubic meter.A copper wire of cross-sectional area 7.4 *10-7 m2 carries a current of 1 A.The electron drift speed is approximately:

For an ohmic resistor, resistance is the proportionality constant for:

The units of resistivity are:

The current density is the same in two wires.Wire A has twice the free electron concentration of wire B.The drift speed of electrons in A is:

A wire is 1 m long and 1 *10-6 m2 in cross-sectional area.When connected to a potential difference of 2 V, a current of 4 A exists in the wire.The resistivity of this wire is:

A student kept her 60-watt, 120-volt study lamp turned on from 2:00 PM until 2:00 AM.How much charge went through it?

In a conductor carrying a current we expect the electron drift speed to be:

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A wire has an electric field of 6.2 V/m and carries a current density of 2.4 x 10⁸ A/m².What is its resistivity?

Copper contains 8.4 10²⁸ free electrons per cubic meter.A copper wire of cross-sectional area 7.4 10^-7 m² carries a current of 1 A.The electron drift speed is approximately:

A wire is 1 m long and 1 *10^-6 m² in cross-sectional area.When connected to a potential difference of 2 V, a current of 4 A exists in the wire.The resistivity of this wire is: