Deck 25: Electric Current and Direct-Current Circuits

Question 1

The forward current for a pn-diode is given by where I₀ and V₀ are constants.If I₀ = 4 $\mu$ A and V₀ = 1.5 V,what is the ratio of the current for V = 2 V to 0.5 V?

A)2.0
B)4.0
C)5.1
D)6.2
E)7.1

Answer 1

7.1

Question 2

Wire A,which is of the same length and material as wire B,has twice the diameter of wire B.If the resistance of wire B is R,what is the resistance of wire A?

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

Answer 2

R/4

Question 3

The current in a wire varies with time according to the relation where I is in amperes and t is in seconds.The number of coulombs that pass a cross section of the wire in the time interval between t = 0 and t = 1/20 s is

A)2 $\pi$
B)4 $\pi$
C)40 $\pi$
D)4
E)zero

Answer 3

4

Question 4

The current in a wire varies with time according to the relation where I is in amperes and t is in seconds.The number of coulombs of charge that pass a cross section of the wire between t = 3 s and t = 5 s is

A)12
B)48
C)52
D)106
E)305

Question 5

Wire A,which is of the same material and cross-sectional area as wire B,is twice as long as wire B.If the resistance of wire B is R,what is the resistance of wire A?

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

Question 6

Which of the following statements is false?

A)An electric field is needed to produce an electric current.
B)A potential difference between two points is needed to produce an electric current.
C)For a steady current to flow in a wire,the wire must be part of a closed circuit.
D)The electric field is constant along all parts of the circuit when a steady current is flowing.
E)The electric current in a wire is proportional to the drift velocity of the charges.

Question 7

For copper, $\rho$ = 8.93 g/cm³ and M = 63.5 g/mol.Assuming one free electron per copper atom,what is the drift velocity of electrons in a copper wire of radius 0.435 mm carrying a current of 5 A?

A)5.43 $\times$ 10^-4 m/s
B)2.11 $\times$ 10^-4 m/s
C)4.67 $\times$ 10^-4 m/s
D)6.21 $\times$ 10^-4 m/s
E)8.37 $\times$ 10^-4 m/s

Question 8

The seemingly instantaneous propagation of electric current in a wire when a switch is closed can be understood in terms of

A)the propagation of an electric field down the wire with nearly the speed of light.
B)the acquisition of the drift velocities of the free electrons almost immediately.
C)a very large number of charges slowly drifting down the wire.
D)the replacement of charge flowing out of the wire at one end by charge entering the wire at the other end.
E)all of the above

Question 9

The same potential difference is applied across two wires.Wire A carries twice the current of wire B.If the resistance of wire B is R,what is the resistance of wire A?

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

Question 10

In a certain particle accelerator,a current of 300 $\mu$ A is carried by a 4.00-MeV proton beam that has a radius of 1.30 mm.The mass of a proton is 1.67 $\times$ 10^-27 kg.If the beam hits a target,how many protons hit the target in 3 s?

A)1.55 $\times$ 10¹⁵
B)8.64 $\times$ 10¹⁵
C)3.42 $\times$ 10¹⁵
D)4.29 $\times$ 10¹⁵
E)None of these is correct.

Question 11

In a certain particle accelerator,a current of 300 $\mu$ A is carried by a 4.00-MeV proton beam that has a radius of 1.30 mm.The mass of a proton is 1.67 $\times$ 10^-27 kg.What is the number of protons per unit volume in this beam?

A)3.64 $\times$ 10¹³ protons/m³
B)2.76 $\times$ 10¹³ protons/m³
C)1.27 $\times$ 10¹³ protons/m³
D)5.98 $\times$ 10¹³ protons/m³
E)4.17 $\times$ 10¹³ protons/m³

Question 12

The drift velocity of an electron in a wire varies

A)directly with the number of charge carriers per unit volume.
B)directly with the cross-sectional area of the conducting wire.
C)directly with the charge carried by it.
D)directly with the current flowing in the conducting wire.
E)inversely with the current flowing in the conducting wire.

Question 13

For copper, $\rho$ = 8.93 g/cm³ and M = 63.5 g/mol.Assuming one free electron per copper atom,what is the drift velocity of electrons in a copper wire of radius 0.625 mm carrying a current of 3 A?

A)3.54 $\times$ 10^-4 m/s
B)1.80 $\times$ 10^-4 m/s
C)4.26 $\times$ 10^-4 m/s
D)7.52 $\times$ 10^-4 m/s
E)2.46 $\times$ 10^-4 m/s

Question 14

In a certain particle accelerator,a current of 800 $\mu$ A is carried by a 6.00-MeV proton beam that has a radius of 1.50 mm.The mass of a proton is 1.67 $\times$ 10^-27 kg.How many protons per unit volume are in this beam?

A)3.24 $\times$ 10¹³ protons/m³
B)7.76 $\times$ 10¹³ protons/m³
C)1.57 $\times$ 10¹³ protons/m³
D)2.09 $\times$ 10¹³ protons/m³
E)4.34 $\times$ 10¹³ protons/m³

Question 15

The resistivity of any given metal

A)depends on its temperature.
B)varies linearly with temperature.
C)is the proportionality constant between the resistance,R,and the ratio of the length,L,to the cross-sectional area,A,of a wire made of the metal.
D)has units of ohm-meter.
E)is described by all of the above.

Question 16

If 4.0 $\times$ 10¹⁸ electrons and 1.5 $\times$ 10¹⁸ protons move in opposite directions past a given cross section of a hydrogen discharge tube every second,the average current in the tube is

A)0.40 A
B)0.56 A
C)0.88 A
D)1.5 A
E)4.0 A

Question 17

If 4.7 $\times$ 10¹⁶ electrons pass a particular point in a wire every second,what is the current in the wire?

A)4.7 mA
B)7.5 A
C)2.9 A
D)7.5 mA
E)0.29 A

Question 18

In a certain particle accelerator,a current of 800 $\mu$ A is carried by a 6.00-MeV proton beam that has a radius of 1.50 mm.The mass of a proton is 1.67 $\times$ 10^-27 kg.If the beam hits a target,how many protons hit the target in 5 s?

A)2.50 $\times$ 10¹⁶
B)6.78 $\times$ 10¹⁶
C)5.68 $\times$ 10¹⁶
D)1.37 $\times$ 10¹⁶
E)3.71 $\times$ 10¹⁶

Question 19

Which of the following statements is false?

A)When there is no electric field in a wire,the free electrons move in random directions with speeds of the order of 10⁶ m/s.
B)By convention,electrons move in the direction opposite to the direction of current.
C)In the absence of an electric field,the average velocity of the electrons in a wire is very large.
D)The drift velocity of electrons in a typical metal is very small.
E)The motion of the free electrons in a metal is similar to that of the molecules of a gas.

Question 20

You want to use a metal bar as a resistor.Its dimensions are 2 by 4 by 10 units.To get the smallest resistance from this bar,you should attach leads to the two opposite sides that have the dimensions of

A)2 by 4 units.
B)2 by 10 units.
C)4 by 10 units.
D)any number of units because all connections give the same resistance.
E)None of these is correct.

Question 21

By what percentage does the resistance of a copper wire ( $\alpha$ = 3.9 $\times$ 10^-3/K)increase when its temperature increases from 20ºC to 100ºC?

A)11%
B)14%
C)23%
D)31%
E)57%

Question 22

A 10-gauge copper wire (diameter = 2.588 mm, $\rho$ = 1.7 $\times$ 10^-8 $\Omega$ · m)has a total resistance of 0.32 $\Omega$ .How long is the wire?

A)4.0 $\times$ 10² m
B)1.6 km
C)99 m
D)31 m
E)65 m

Question 23

Consider a conductor for which Which of the following is an expression for $\rho$ ?

A)RL/A
B)RLA
C)R/(LA)
D)LA/R
E)AR/L

Question 24

A wire 10 m long with a diameter of 1.0 mm has a resistance of 5.0 $\Omega$ .The resistance of a second wire made of the same material,but 3.0 m long with a diameter of 4.0 mm,is approximately

A)0.38 $\Omega$
B)0.094 $\Omega$
C)0.75 $\Omega$
D)1.7 $\Omega$
E)0.27 k $\Omega$

Question 25

A wire of length L,cross sectional area A and resistance R is now drawn out by pulling such that its new resistance is 25R.What is its new cross sectional area? (Assume the density remains constant.)

A)1.00 $\times$ 10^-1 A
B)6.67 $\times$ 10^-1 A
C)4.00 $\times$ 10^-1 A
D)2.50 $\times$ 10^-1 A
E)2.00 $\times$ 10^-1 A

Question 26

Since you cannot afford expensive cable to hook up between your loudspeaker and music amplifier,you try some common 14-gauge (diameter = 1.628 mm)copper wire.If the total length of the wire (there and back)used for each speaker to amplifier hookup was 10 m,then calculate the resistance in the wire for one channel.( $\rho$ (Cu)= 1.7 $\times$ 10^-8 $\Omega$ /m)

A)2.6 $\times$ 10^-2 $\Omega$
B)2.0 $\times$ 10^-2 $\Omega$
C)4.1 $\times$ 10^-3 $\Omega$
D)8.2 $\times$ 10^-2 $\Omega$
E)1.6 $\times$ 10^-1 $\Omega$

Question 27

Two pieces of copper wire have the same length,but wire A has a square cross section of width s whereas wire B has a circular cross section of diameter s.Which of the following statements is true?

A)The resistance of both wires is the same.
B)The resistivity of both wires is the same.
C)Both the resistance and the resistivity of A and B are the same.
D)The resistance of A is greater than that of B.
E)The resistivity of A is greater than that of B.

Question 28

A wire of length L and resistance R is now drawn out by pulling such that its new resistance is 15R.What is its new length? (Assume the density remains constant.)

A)1.50 L
B)6.67 $\times$ 10^-1 L
C)2.25 L
D)3.87 L
E)2.58 $\times$ 10^-1 L

Question 29

A potential difference of 120 V produces a current of 8.0 A in the heating element of a toaster.What is the resistance of this heating element while the toaster is carrying this current?

A)6.7 $\times$ 10^-2 $\Omega$
B)15 $\Omega$
C)0.96 k $\Omega$
D)67 $\Omega$
E)30 $\Omega$

Question 30

A wire of length L = 5.0 m and uniform diameter has an electric field of E = 4.0 V/m throughout the wire.The potential difference across the wire is

A)1.25 V
B)0.80 V
C)20.0 V
D)3.20 V
E)zero

Question 31

By what percentage does the resistance of a copper wire ( $\alpha$ = 3.9 $\times$ 10^-3/K)increase when its temperature increases from 20 to 75ºC?

A)11%
B)14%
C)22%
D)31%
E)57%

Question 32

Use the diagram below for the next problem:
Two copper wires have the same volume,but wire 2 is 10% longer than wire 1.The ratio of the resistances of the two wires R₂/R₁ is

A)1.2
B)1.1
C)0.82
D)0.91
E)1.0

Question 33

A wire of length L and resistance R is now drawn out by pulling such that its new length is 1.5L.What is its new resistance? (Assume the density remains constant.)

A)1.00 R
B)1.50 R
C)2.25 R
D)5.06 R
E)4.44 $\times$ 10^-1 R

Question 34

A potential difference V = 25 V is applied across a wire of length L = 10.0 m and uniform diameter.The electric field throughout the wire is

A)250 V/m
B)2.5 V/m
C)0.4 V/m
D)0.25 V/m
E)zero

Question 35

Use the diagram below for the next problem:
The diagram shows the current versus voltage for a diode.The region which has the highest resistance is

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

Question 36

A wire of length L = 6.0 m has a uniform diameter d = 3.0 mm.When an electric field of E = 0.25 V/m is applied to the wire,the current is 1.5 A.The resistivity of the wire is

A)1.18 $\times$ 10^-6 $\Omega$ .m
B)3.53 $\times$ 10^-6 $\Omega$ .m
C)2.81 $\times$ 10^-7 $\Omega$ .m
D)7.03 $\times$ 10^-8 $\Omega$ .m
E)2.21 $\times$ 10^-7 $\Omega$ .m

Question 37

Use the diagram below for the next problem:
Two copper wires have the same volume,but wire 2 is 20% longer than wire 1.The ratio of the resistances of the two wires R₂/R₁ is

A)1.2
B)0.83
C)1.1
D)0.91
E)1.4

Question 38

A wire of length L and resistance R is cut into 4 equal pieces.If the 4 pieces are now twisted together to make a wire of length L/4,what is the new resistance of the shorter wire combination?

A)R/16
B)R/4
C)R
D)4R
E)16R

Question 39

A wire of length L = 3.0 m.When an electric field of E = 0.5 V/m is applied to the wire,the current is 1.5 A.The resistance of the wire is

A)0.53 $\Omega$
B)8.0 $\Omega$
C)5.33 $\Omega$
D)0.13 $\Omega$
E)1.0 $\Omega$

Question 40

Use the diagram below for the next problem:
The diagram shows the current versus voltage for a diode.The region which has the least resistance is

A)I
B)II
C)III
D)IV
E)A diode does not have any resistance.

Question 41

A heating element made of nichrome wire (temperature coefficient of resistivity, $\alpha$ = 0.400 $\times$ 10^-3 K^-1)is connected to a source.When it has just been turned on and its temperature is 20ºC,it dissipates 960 W of power.What power is dissipated when its temperature has risen to 300ºC?

A)1.1 $\times$ 10³ W
B)9.6 $\times$ 10² W
C)8.6 $\times$ 10² W
D)8.6 $\times$ 10³ W
E)1.1 $\times$ 10² W

Question 42

A 6-V storage battery supplies energy to a simple circuit at the constant rate of 48 W.The resistance of the circuit is

A)8 $\Omega$
B) $\Omega$
C)3/2 $\Omega$
D)4/3 $\Omega$
E)3/4 $\Omega$

Question 43

You can pay a lot of money for fancy connecting cables between your audio/video equipment components.Some of the more expensive ones use silver wire rather than copper.Calculate the difference in resistance between a silver and a copper cable with the same 12-gauge diameter of 2.05 mm and length of 3 m (hence total length = 6m).(Use $\rho$ (Cu)= 1.7 $\times$ 10^-8 $\Omega$ /m and $\rho$ (Ag)= 1.6 $\times$ 10^-8 $\Omega$ /m.Unfortunately (or fortunately),there are many other factors that go into making an interconnect cable than simple resistance considerations.)

A)4.1 $\times$ 10^-3 $\Omega$
B)1.8 $\times$ 10^-3 $\Omega$
C)2.9 $\times$ 10^-2 $\Omega$
D)9.0 $\times$ 10^-4 $\Omega$
E)3.1 $\times$ 10^-2 $\Omega$

Question 44

An immersion heater of negligible mass draws a current of 7.5 A from a source with a potential difference of 120 V.The heater is in 0.950 m³ of water.The initial temperature of the water is 12.5ºC.How much time does it take to increase the temperature of the water to 27.5ºC? Neglect any heat loss.One calorie is equivalent to 4.18 J.

A)4.7 $\times$ 10⁴ s
B)3.3 $\times$ 10⁴ s
C)1.1 $\times$ 10⁴ s
D)1.6 $\times$ 10⁴ s
E)None of these is correct.

Question 45

A motor running from a 220-V line is lifting a mass of 35 kg against Earth's gravity at a constant speed of 6.0 m/s.If we assume 100% efficiency,the current required is

A)0.27 A
B)9.4 A
C)7.7 A
D)3.3 A
E)4.7 A

Question 46

A storage battery with emf 6.4 V and internal resistance 0.080 $\Omega$ is being charged by a current of 15 A.The power loss in internal heating of the battery is

A)18 W
B)96 W
C)1.2 W
D)80 W
E)0.11 kW

Question 47

An electric motor drives a pump that lifts water from a well that is 11.9 m deep and then delivers the water through a hose at a rate of 180 L/min and a speed of 21.0 m/s.If we assume 100% efficiency and ignore viscosity,the current drawn by the motor from a 220-V source is

A)4.6 A
B)7.2 A
C)6.0 A
D)1.0 $\times$ 10³ A
E)2.3 A

Question 48

The total charge flowing per hour past a given point in a piece of electrical equipment rated at 550 kW and 110 V is

A)1.8 $\times$ 10⁷ C
B)3.0 $\times$ 10⁴ C
C)3.3 $\times$ 10⁴ C
D)2.4 $\times$ 10³ C
E)impossible to determine without more information.

Question 49

Suppose instead of using a 12-gauge (diameter = 2.05 mm)solid copper cable you made a hollow copper cable of outer diameter 2.59 mm (10-gauge)and inner diameter 1.29 mm (16-gauge).Calculate the length of hollow cable that would give the same resistance as 1 m of 12-gauge solid cable.Use $\rho$ (Cu)= 1.7 $\times$ 10^-8 $\Omega$ /m.

A)0.402 m
B)2.48 m
C)0.833 m
D)1.44 m
E)1.20 m

Question 50

An energy efficient light bulb uses 15 W of power for an equivalent light output of a 60 W incandescent light bulb.How much money do you save each month by using the energy efficient light bulb instead of the incandescent light bulb for 6 hours a day? Assume that 1 kW.hr costs 14 cents and there are 30 days in one month.

A)$0.39
B)$0.76
C)$1.51
D)$0.57
E)$1.13

Question 51

The power dissipated in each of two resistors is the same.The potential drop across resistor A is twice that across resistor B.If the resistance of resistor B is R,what is the resistance of A?

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

Question 52

A current of 0.50 A passes through a lamp with a resistance of 600 $\Omega$ .The power supplied to this lamp is

A)2.0 W
B)0.10 kW
C)0.12 kW
D)0.15 kW
E)0.30 kW

Question 53

If electric energy costs 10 cents per kilowatt-hour,how many cents does it cost to keep a 660-W toaster in steady operation for 30 min?

A)15 cents
B)12 cents
C)6.9 cents
D)3.3 cents
E)1.7 cents

Question 54

An electric generator supplies power as a function of time according to the expression where the units are SI.The electric energy provided by the generator between t = 0 and t = 1 s is

A)1.8 J
B)2.0 J
C)2.5 J
D)5.0 J
E)8.0 J

Question 55

A 12-V battery has an internal resistance of 0.20 $\Omega$ .If the battery is being charged by a current of 30 A,the magnitude of the potential difference between its terminals is

A)18 V
B)6.0 V
C)42 V
D)0
E)12 V

Question 56

Which of these five terms is most unlike the others?

A)electromotive force
B)voltage
C)potential difference
D)current
E)electric potential

Question 57

A resistor carries a current I.The power dissipated in the resistor is P.What is the power dissipated if the same resistor carries current 3I?

A)P
B)3P
C)P/3
D)9P
E)P/P

Question 58

You connect two similar heating coils in series on a constant-voltage line.When you later connect them in parallel to the same line,the heat developed per minute,compared with the former rate,is approximately

A)the same.
B)twice as great.
C)one-half as much.
D)four times as great.
E)one-fourth as much.

Question 59

A resistor develops heat at the rate of 20 W when the potential difference across its ends is 30 V.The resistance of the resistor is approximately

A)45 $\Omega$
B)5.5 $\Omega$
C)30 $\Omega$
D)13 $\Omega$
E)2 $\Omega$

Question 60

An energy efficient light bulb uses 15 W of power for an equivalent light output of a 60 W incandescent light bulb.How much energy is saved each month by using the energy efficient light bulb instead of the incandescent light bulb for 4 hours a day? Assume that there are 30 days in one month.

A)5.4 kW.hr
B)1.35 kW.hr
C)21.6 kW.hr
D)1.8 kW.hr
E)7.2 kW.hr

Question 61

When two identical resistors are connected in series across a battery,the total power dissipated by them is 20 W.If these resistors are connected in parallel across the same battery,the total power dissipated is

A)5 W
B)10 W
C)20 W
D)40 W
E)80 W

Question 62

Battery A has a relatively high internal resistance,whereas battery B has a low internal resistance.If both batteries provide the same current to some external circuitry,which of the following statements is likely to be true?

A)Battery A is warmer to the touch than battery B.
B)Battery B is warmer to the touch than battery A.
C)Battery A is cooler to the touch than battery B.
D)Both batteries are equally warm to the touch.
E)The emf of both batteries must be known to predict their warmth to the touch.

Question 63

When two identical resistors are connected in parallel,the equivalent resistance,compared with that of the same two resistors connected in series,is

A)exactly the same.
B)twice as great.
C)one-half as much.
D)four times as great.
E)one-fourth as much.

Question 64

When two resistors are connected in series,the equivalent resistance is R_s = 10 $\Omega$ .When they are connected in parallel,the equivalent resistance is R_p = 2.4 $\Omega$ .What are the values of the two resistances?

A)4.0 $\Omega$ and 6.0 $\Omega$
B)5.0 $\Omega$ for both
C)1.0 $\Omega$ and 1.4 $\Omega$
D)1.2 $\Omega$ for both
E)2.5 $\Omega$ and 7.5 $\Omega$

Question 65

Two resistors are connected in series across a potential difference.If the current carried by resistor A is I,what is the current carried by B?

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

Question 66

In a parallel circuit,

A)the current is the same in every branch.
B)the potential drop is the sum of those in all branches.
C)the potential drop is the same for each element of the parallel circuit.
D)the heat generated is the same in all branches.
E)the resistance is the sum of the resistances of the branches.

Question 67

The conservation of charge in an electric circuit is closely related to which of the following?

A)Ohm's law
B)Kirchhoff's junction rule
C)Kirchhoff's loop rule
D)Newton's laws
E)Ampère's law

Question 68

A resistor of 2 $\Omega$ and another of 3 $\Omega$ are connected in parallel,and this combination is joined in series with a 4- $\Omega$ resistor across a battery.If the current in the 2- $\Omega$ resistor is 2.0 A,the potential drop across the 3- $\Omega$ resistor is

A)2.4 V
B)1.2 V
C)6.0 V
D)4.0 V
E)8.0 V

Question 69

The conservation of energy in an electric circuit is closely related to which of the following?

A)Ohm's law
B)Kirchhoff's junction rule
C)Kirchhoff's loop rule
D)Newton's laws
E)Ampère's law

Question 70

A 12-V car battery may supply 60 A for 2.5 s in order for the starter motor to start the engine.Calculate how much electrical energy has been used.

A)1.8 $\times$ 10³ J
B)2.2 $\times$ 10⁴ J
C)7.2 $\times$ 10² J
D)3.6 $\times$ 10³ J
E)4.5 $\times$ 10³ J

Question 71

You leave your 120-W light bulb on in your room by mistake while you are at school for 8.5 hours.If the price of electrical energy is $0.085 per kilowatt-hour,how much did it cost you?

A)$0.082
B)$0.075
C)$0.065
D)$0.087
E)$0.078

Question 72

A galvanometer has an internal resistance of 200 $\Omega$ and requires a current of 5.0 mA for full-scale deflection.What resistance should be connected in parallel with this galvanometer to make an ammeter that reads 10 A at full-scale deflection?

A)0.10 $\Omega$
B)4.0 $\times$ 10⁵ $\Omega$
C)400 $\Omega$
D)0.20 $\Omega$
E)2.0 $\Omega$

Question 73

Two resistors R₁ and R₂ are connected in parallel,and this combination is connected in series with a third resistor R₃.If R₁ = 5.0 $\Omega$ ,R₂ = 2.0 $\Omega$ ,and R₃ = 6.0 $\Omega$ ,what is the equivalent resistance of this combination?

A)0.3l $\Omega$
B)3.2 $\Omega$
C)6.7 $\Omega$
D)7.4 $\Omega$
E)13 $\Omega$

Question 74

If it takes you 5 minutes to dry your hair using a 1200-W hairdryer plugged into a 120-V power outlet,how many Coulombs of charge pass through your hair dryer?

A)120 C
B)300 C
C)60.0 C
D)1.00 C
E)5.00 C

Question 75

Two resistors are connected in parallel across a potential difference.The resistance of resistor A is twice that of resistor B.If the current carried by resistor A is I,what is the current carried by B?

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

Question 76

In your four years at college you dry your hair everyday for 5 minutes with a 1200-W hairdryer.If the average price of electrical energy was $0.085 per kilowatt-hour,how much did it cost you during this time? (Assume 1 year = 365 days.)

A)$4.10
B)$8.90
C)$12.4
D)$22.8
E)$15.6

Question 77

Each of three identical electric heaters is rated at 1000 W when operated across 100-V lines.When the three are connected in series across a 120-V line,at what rate is electric energy converted to heat energy? (Neglect variation of resistance with temperature.)

A)0.48 kW
B)1.0 kW
C)1.2 kW
D)3.0 kW
E)3.6 kW

Question 78

You connect two identical resistors in series across supply lines maintained at a constant potential difference.When you later connect them in parallel across the same supply lines,the total current furnished by the supply lines,compared with the former current,is

A)exactly the same.
B)twice as great.
C)one-half as much.
D)four times as great.
E)one-fourth as much.

Question 79

A galvanometer has an internal resistance of 200 $\Omega$ and requires a current of 5.0 mA for full-scale deflection.What resistance should be connected in series with this galvanometer to make a voltmeter that reads 25 V at full-scale deflection?

A)2.4 k $\Omega$
B)4.8 k $\Omega$
C)5.0 k $\Omega$
D)5.2 k $\Omega$
E)9.6 k $\Omega$

Question 80

If two elements of a circuit are in parallel,they must have the same

A)charge.
B)potential difference across them.
C)resistance.
D)potential difference across them and the same charge.
E)current.