Deck 26: Current and Resistance

Question 1

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

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

Answer 1

thick, short and cool

Question 2

Current has units:

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

Answer 2

coulomb/second

Question 3

Conduction electrons move to the right in a certain wire. This indicates that:

A) the current density and electric field both point right
B) the current density and electric field both point left
C) the current density points right and the electric field points left
D) the current density points left and the electric field points right
E) the current density points left but the direction of the electric field is unknown

Answer 3

the current density and electric field both point left

Question 4

Current has units:

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

Question 5

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

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

Question 6

Copper contains 8.4 * 10²⁸ free electrons/m³. 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) 3 * 10⁸ m/s
B) 10³ m/s
C) 1 m/s
D) 10^-4 m/s
E) 10^-23 m/s

Question 7

A certain wire has resistance R. Another wire, of the same material, has half the length and half the diameter of the first wire. The resistance of the second wire is:

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

Question 8

The resistance of a rod does NOT depend on:

A) its temperature
B) its material
C) its length
D) its conductivity
E) shape of its (fixed) cross-sectional area

Question 9

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

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

Question 10

Current is a measure of:

A) force that moves a charge past a point
B) resistance to the movement of a charge past a point
C) energy used to move a charge past a point
D) amount of charge that moves past a point per unit time
E) speed with which a charge moves past a point

Question 11

Five cylindrical wires are made of the same material. Their lengths and radii are Rank the wires according to their resistances, least to greatest.

A) 1, 2, 3, 4, 5
B) 5, 4, 3, 2, 1
C) 1 and 2 tie, then 5, 3, 4
D) 1, 3, 4, 2, 5
E) 1, 2, 4, 3, 5

Question 12

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

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

Question 13

A 10-ohm resistor has a constant current. If 1200 C of charge flow through it in 4 minutes what is the value of the current?

A) 3.0 A
B) 5.0 A
C) 11 A
D) 15 A
E) 20 A

Question 14

A wire with a length of 150 m and a radius of 0.15 m carries a current with a uniform current density of 2.8 *10⁷ A/m². The current is:

A) 0.63 A²
B) 2.0 A
C) 5.9 A²
D) 296 A
E) 400 A²

Question 15

The units of resistivity are:

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

Question 16

Two wires made of different materials have the same uniform current density. They carry the same current only if:

A) their lengths are the same
B) their cross-sectional areas are the same
C) both their lengths and cross-sectional areas are the same
D) the potential differences across them are the same
E) the electric fields in them are the same

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:

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

The current is zero in a conductor when no potential difference is applied because:

A) the electrons are not moving
B) the electrons are not moving fast enough
C) for every electron with a given velocity there is another with a velocity of equal magnitude and opposite direction
D) equal numbers of electrons and protons are moving together
E) otherwise Ohm's law would not be valid

Question 19

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

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

Question 20

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

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

Question 21

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

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

Question 22

A certain sample carries a current of 4 A when the potential difference is 2 V and a current of 10 A when the potential difference is 4 V. This sample:

A) obeys Ohm's law
B) has a resistance of 0.5 $\Omega$ at 1 V
C) has a resistance of 2.5 $\Omega$ at 2 V
D) has a resistance of 25 $\Omega$ at 2 V
E) does not have a resistance

Question 23

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

A) 150
B) 3,600
C) 7,200
D) 18,000
E) 21,600

Question 24

For an ohmic substance, the electron drift velocity is proportional to:

A) the cross-sectional area of the sample
B) the length of the sample
C) the mass of an electron
D) the electric field in the sample
E) none of the above

Question 25

Two substances are identical except that the electron mean free time for substance A is twice the electron mean free time for substance B. If the same electric field exists in both substances the electron drift speed in A is:

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

Question 26

If the potential difference across a resistor is doubled:

A) only the current is doubled
B) only the current is halved
C) only the resistance is doubled
D) only the resistance is halved
E) both the current and resistance are doubled

Question 27

For an ohmic substance the resistivity is the proportionality constant for:

A) current and potential difference
B) current and electric field
C) current density and potential difference
D) current density and electric field
E) potential difference and electric field

Question 28

Conductivity is:

A) the same as resistivity, it is just more convenient to use for good conductors
B) expressed in $\Omega$ ^-1
C) equal to 1/resistance
D) expressed in ( $\Omega$ .m)^-1
E) not a meaningful quantity for an insulator

Question 29

Two conductors are made of the same material and have the same length. Conductor A is a solid wire of diameter 1 m. Conductor B is a hollow tube of inside diameter 1 m and outside diameter 2 m. The ratio of their resistance, R_A/R_B, is:

A) 1
B)
C) 2
D) 3
E) 4

Question 30

A current of 0.5 ampere exists in a 60-ohm lamp. The applied potential difference is:

A) 15 V
B) 30 V
C) 60 V
D) 120 V
E) none of these

Question 31

A nichrome 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 nichrome is:

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

Question 32

An certain resistor dissipates 0.5 W when connected to a 3 V potential difference. When connected to a 1 V potential difference, this resistor will dissipate:

A) 0.5 W
B) 0.167 W
C) 1.5 W
D) 0.056 W
E) none of these

Question 33

Energy may be measured in:

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

Question 34

You wish to triple the rate of energy dissipation in a heating device. To do this you could triple:

A) the potential difference keeping the resistance the same
B) the current keeping the resistance the same
C) the resistance keeping the potential difference the same
D) the resistance keeping the current the same
E) both the potential difference and current

Question 35

For an ohmic substance, the resistivity depends on:

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

Question 36

For a cylindrical resistor made of ohmic material, the resistance does NOT depend on:

A) the current
B) the length
C) the cross-sectional area
D) the resistivity
E) the electron drift velocity

Question 37

A flat iron is marked "120 volt, 600 watt". In normal use, the current in it is:

A) 2 A
B) 4 A
C) 5 A
D) 7.2 A
E) 0.2 A

Question 38

The rate at which electrical energy is used may be measured in:

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

Question 39

Which of the following graphs best represents the current-voltage relationship of an incandescent light bulb?

A) I
B) II
C) III
D) IV
E) V

Question 40

Which of the following graphs best represents the current-voltage relationship for a device that obeys Ohm's law?

A) I
B) II
C) III
D) IV
E) V

Question 41

Which one of the following quantities is correctly matched to its unit?

A) power - kW·h
B) energy - kW
C) potential difference - J/C
D) current - A/s
E) resistance - V/C

Question 42

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

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

Question 43

It is better to send 10,000 kW of electric power long distances at 10,000 V rather than at 220 V because:

A) there is less heating in the transmission wires
B) the resistance of the wires is less at high voltages
C) more current is transmitted at high voltages
D) the insulation is more effective at high voltages
E) the iR drop along the wires is greater at high voltage

Question 44

The mechanical equivalent of heat is 1 cal = 4.18 J. A heating coil, connected to a 120-V source, develops 60,000 calories in 10 minutes. The current in the coil is:

A) 0.83 A
B) 2 A
C) 3.5 A
D) 20 A
E) 50 A

Question 45

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

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 46

Suppose the electric company charges 10 cents per kW.h. How much does it cost to use a 125 watt lamp 4 hours a day for 30 days?

A) $1.20
B) $1.50
C) $1.80
D) $7.20
E) none of these

Question 47

An ordinary light bulb is marked "60 watt, 120 volt". Its resistance is:

A) 60 $\Omega$
B) 120 $\Omega$
C) 180 $\Omega$
D) 240 $\Omega$
E) 15 $\Omega$

Question 48

The mechanical equivalent of heat is 1 cal = 4.18 J. The specific heat of water is 1 cal/g·K. An electric immersion water heater, rated at 400 W, should heat a liter of water from 10 $\degree$ C to 30 $\degree$ C in about:

A) 3.5 min
B) 1 min
C) 15 min
D) 45 min
E) 15 s

Question 49

A certain x-ray tube requires a current of 7 mA at a voltage of 80 kV. The rate of energy dissipation (in watts) is:

A) 560
B) 5600
C) 26
D) 11.4
E) 87.5