Lesson test: Current and circuits

1.[3p]
A copper wire of cross section 4.0 mm² carries 10 A, with $n = 8.49 \times 10^{28}$ carriers per cubic metre. What is the drift speed, in units of $10^{- 4}$ metres per second?

CorrectNot quite: 1.84

2.[3p]
A lamp comes on almost instantly when the switch is thrown, even though electrons drift at a fraction of a millimetre per second, because

the wire is already full of charge everywhere, and only the field has to propagate

Correct

the electrons speed up enormously once the circuit is closed

The answer is: the wire is already full of charge everywhere, and only the field has to propagate

current is carried by protons, which move faster

The answer is: the wire is already full of charge everywhere, and only the field has to propagate

the drift speed calculation applies only to thick wires

The answer is: the wire is already full of charge everywhere, and only the field has to propagate

3.[2p]
A copper wire 25 m long has a cross section of 2.5 mm². Taking $ρ = 1.68 \times 10^{- 8}$ Ω m, what is its resistance, in ohms?

CorrectNot quite: 0.168

4.[3p]
Which of these do not obey Ohm's law?

Select all that apply

A semiconductor diode

Correct

A filament lamp at its working temperature

Correct

An ionised gas in a fluorescent tube

Correct

A carbon resistor at constant temperature

The answer is: A semiconductor diode, A filament lamp at its working temperature, An ionised gas in a fluorescent tube

5.[3p]
A 9.0 V battery of internal resistance 1.2 Ω drives a 6.0 Ω load. What is the terminal voltage, in volts?

CorrectNot quite: 7.5

6.[2p]
Kirchhoff's loop rule states that the potential changes round a closed loop sum to zero. What is it really an expression of?

That the electrostatic potential is a function of position, so returning to a point returns to its potential

Correct

Conservation of charge at a junction

The answer is: That the electrostatic potential is a function of position, so returning to a point returns to its potential

Ohm's law applied to every element in turn

The answer is: That the electrostatic potential is a function of position, so returning to a point returns to its potential

That no capacitor may appear in the loop

The answer is: That the electrostatic potential is a function of position, so returning to a point returns to its potential

7.[2p]
A 22 μF capacitor is discharged through a 470 kΩ resistor. What is the time constant, in seconds?

CorrectNot quite: 10.34

8.[3p]
A capacitor charged to 6.0 V discharges with a time constant of 2.5 s. What is its voltage after 5.0 s, in volts?

CorrectNot quite: 0.81

9.[2p]
Two neutral parallel wires carrying currents attract each other. Why can this not be an electrostatic effect?

Both wires hold exactly as many protons as electrons, so Coulomb's law gives no force between them

Correct

Electrostatic forces are always repulsive between parallel objects

The answer is: Both wires hold exactly as many protons as electrons, so Coulomb's law gives no force between them

The wires are too far apart for Coulomb's law to apply

The answer is: Both wires hold exactly as many protons as electrons, so Coulomb's law gives no force between them

Coulomb's law does not hold inside metals

The answer is: Both wires hold exactly as many protons as electrons, so Coulomb's law gives no force between them

1.[3p] A copper wire of cross section 4.0 mm² carries 10 A, with n=8.49×1028 carriers per cubic metre. What is the drift speed, in units of 10−4 metres per second?

2.[3p] A lamp comes on almost instantly when the switch is thrown, even though electrons drift at a fraction of a millimetre per second, because

3.[2p] A copper wire 25 m long has a cross section of 2.5 mm². Taking ρ=1.68×10−8 Ω m, what is its resistance, in ohms?

4.[3p] Which of these do not obey Ohm's law?

5.[3p] A 9.0 V battery of internal resistance 1.2 Ω drives a 6.0 Ω load. What is the terminal voltage, in volts?

6.[2p] Kirchhoff's loop rule states that the potential changes round a closed loop sum to zero. What is it really an expression of?

7.[2p] A 22 μF capacitor is discharged through a 470 kΩ resistor. What is the time constant, in seconds?

8.[3p] A capacitor charged to 6.0 V discharges with a time constant of 2.5 s. What is its voltage after 5.0 s, in volts?

9.[2p] Two neutral parallel wires carrying currents attract each other. Why can this not be an electrostatic effect?

1.[3p]
A copper wire of cross section 4.0 mm² carries 10 A, with $n = 8.49 \times 10^{28}$ carriers per cubic metre. What is the drift speed, in units of $10^{- 4}$ metres per second?

2.[3p]
A lamp comes on almost instantly when the switch is thrown, even though electrons drift at a fraction of a millimetre per second, because

3.[2p]
A copper wire 25 m long has a cross section of 2.5 mm². Taking $ρ = 1.68 \times 10^{- 8}$ Ω m, what is its resistance, in ohms?

4.[3p]
Which of these do not obey Ohm's law?

5.[3p]
A 9.0 V battery of internal resistance 1.2 Ω drives a 6.0 Ω load. What is the terminal voltage, in volts?

6.[2p]
Kirchhoff's loop rule states that the potential changes round a closed loop sum to zero. What is it really an expression of?

7.[2p]
A 22 μF capacitor is discharged through a 470 kΩ resistor. What is the time constant, in seconds?

8.[3p]
A capacitor charged to 6.0 V discharges with a time constant of 2.5 s. What is its voltage after 5.0 s, in volts?

9.[2p]
Two neutral parallel wires carrying currents attract each other. Why can this not be an electrostatic effect?