Lesson test: Inductance and magnetic energy

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Inductance and magnetic energy

1.[3p]
A solenoid 40 cm long is wound with 1000 turns over a cross section of 5.0 cm². What is its inductance, in millihenries?

CorrectNot quite: 1.57

2.[3p]
Why does an inductor with no current in it behave momentarily like a break in the circuit?

Its back EMF exactly cancels the source at the first instant, so no current can flow yet

Correct

Its resistance is infinite until current starts

The answer is: Its back EMF exactly cancels the source at the first instant, so no current can flow yet

The magnetic field has to be established before charge can move

The answer is: Its back EMF exactly cancels the source at the first instant, so no current can flow yet

Because the energy stored is zero

The answer is: Its back EMF exactly cancels the source at the first instant, so no current can flow yet

3.[2p]
An inductor of 0.80 H is in series with 200 Ω. What is the time constant, in milliseconds?

CorrectNot quite: 4.0

4.[2p]
A 0.25 H inductor carries 3.0 A. How much energy is stored in its field, in joules?

CorrectNot quite: 1.125

5.[3p]
What is the practical limit on the strongest steady magnetic fields?

The magnetic pressure $B^{2} / 2 μ_{0}$ approaches the strength of the material holding the coil together

Correct

The current required exceeds what any power supply can deliver

The answer is: The magnetic pressure $B^{2} / 2 μ_{0}$ approaches the strength of the material holding the coil together

Magnetic fields saturate above about 45 T and cannot be increased

The answer is: The magnetic pressure $B^{2} / 2 μ_{0}$ approaches the strength of the material holding the coil together

The energy density becomes larger than that of chemical fuel

The answer is: The magnetic pressure $B^{2} / 2 μ_{0}$ approaches the strength of the material holding the coil together

6.[2p]
A transformer works equally well on direct current, provided the current is large enough.

True

The answer is: False

False

Correct

7.[3p]
Tripling the transmission voltage for a fixed power reduces the resistive loss in the line by what factor?

Nine, since the current falls by three and the loss goes as the square of the current

Correct

Three, in proportion to the voltage

The answer is: Nine, since the current falls by three and the loss goes as the square of the current

Nine, since the resistance falls by three squared

The answer is: Nine, since the current falls by three and the loss goes as the square of the current

It makes no difference, since the power delivered is the same

The answer is: Nine, since the current falls by three and the loss goes as the square of the current

8.[3p]
An inductor of 40 mH is connected to a capacitor of 250 nF. What is the oscillation frequency, in hertz?

CorrectNot quite: 1592

9.[3p]
Match each electrical quantity to its mechanical counterpart in the LC oscillator.

Inductance
One over capacitance
Current
Charge on the capacitor

velocity
displacement
mass
spring constant

Show the answer

Inductance: mass One over capacitance: spring constant Current: velocity Charge on the capacitor: displacement

1.[3p] A solenoid 40 cm long is wound with 1000 turns over a cross section of 5.0 cm². What is its inductance, in millihenries?

2.[3p] Why does an inductor with no current in it behave momentarily like a break in the circuit?

3.[2p] An inductor of 0.80 H is in series with 200 Ω. What is the time constant, in milliseconds?

4.[2p] A 0.25 H inductor carries 3.0 A. How much energy is stored in its field, in joules?

5.[3p] What is the practical limit on the strongest steady magnetic fields?

6.[2p] A transformer works equally well on direct current, provided the current is large enough.

7.[3p] Tripling the transmission voltage for a fixed power reduces the resistive loss in the line by what factor?

8.[3p] An inductor of 40 mH is connected to a capacitor of 250 nF. What is the oscillation frequency, in hertz?

9.[3p] Match each electrical quantity to its mechanical counterpart in the LC oscillator.

1.[3p]
A solenoid 40 cm long is wound with 1000 turns over a cross section of 5.0 cm². What is its inductance, in millihenries?

2.[3p]
Why does an inductor with no current in it behave momentarily like a break in the circuit?

3.[2p]
An inductor of 0.80 H is in series with 200 Ω. What is the time constant, in milliseconds?

4.[2p]
A 0.25 H inductor carries 3.0 A. How much energy is stored in its field, in joules?

5.[3p]
What is the practical limit on the strongest steady magnetic fields?

6.[2p]
A transformer works equally well on direct current, provided the current is large enough.

7.[3p]
Tripling the transmission voltage for a fixed power reduces the resistive loss in the line by what factor?

8.[3p]
An inductor of 40 mH is connected to a capacitor of 250 nF. What is the oscillation frequency, in hertz?

9.[3p]
Match each electrical quantity to its mechanical counterpart in the LC oscillator.