Deck 26: Current and Resistance

Question 1

Current has units:

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

Answer 1

ampere

Question 2

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

Answer 2

much less than the average electron speed

Question 3

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

Answer 3

their cross-sectional areas are the same

Question 4

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

Question 5

Current has units:

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

Question 6

This figure shows a junction. What is true of the currents?

A) i₁ = i₀ + i₂
B) i₂= i₀ + i₁
C) i₀ = i₁ - i₂
D) i₁ = i₀ - i₂
E) i₀ = i₂- i₁

Question 7

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

Question 8

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 9

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 10

The units of resistivity are:

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

Question 11

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 12

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 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) 20 A
D) 120 A
E) 300 A

Question 14

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 15

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

A) 0.63 A
B) 2.0 A
C) 5.9 A
D) 300 A
E) 26000 A

Question 16

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

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

Question 17

Current in a conductor is normally due to:

A) the collective motion of all electrons in the conductor
B) the thermal motion of the conduction electrons
C) the drift motion of the conduction electrons due to electric fields
D) the drift motion of both electrons and positrons
E) the drift motion of both electrons and protons

Question 18

In schematic diagrams, currents are indicated using arrows. What do the arrows indicate?

A) the direction of motion of the electrons
B) the direction of the current vector
C) the direction of motion of the charge carriers
D) the direction that positive charge carriers would move
E) nothing; they are just a convenient drawing tool

Question 19

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 20

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 21

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 22

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 23

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 24

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 25

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 26

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 27

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) 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 28

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 29

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

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 30

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-sectional area of the sample

Question 31

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 32

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

Question 33

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 34

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

Question 35

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 36

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 37

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?

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

Question 38

Energy may be measured in:

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

Question 39

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 2 V
C) has a resistance of 2 $\Omega$ at 2 V
D) has a resistance of 25 $\Omega$ at 2 V
E) does not have a resistance

Question 40

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 41

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?

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

Question 42

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 43

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 44

The mechanical equivalent of heat is 1 cal = 4.18 J. The specific heat of water is 1 cal/g·K, and its mass is 1 g/cm³. 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) 12 s
B) 50 s
C) 3.5 min
D) 15 min
E) 45 min

Question 45

Compared to a conductor, a semiconductor

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

Question 46

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

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

Question 47

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?

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

Question 48

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

A) 5600 W
B) 560 W
C) 26 W
D) 11.4 W
E) 0.088 W

Question 49

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

Question 50

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 51

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 52

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

A) 0.5 $\Omega$
B) 120 $\Omega$
C) 180 $\Omega$
D) 240 $\Omega$
E) 300 $\Omega$

Question 53

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 54

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) 2.0 A
C) 3.5 A
D) 500 A
E) 2100 A

Question 55

A 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) 0.056 W
B) 0.167 W
C) 0.50 W
D) 1.5 W
E) none of these