# Electromagnetism Flashcards Preview

## Harris Academy AH Physics > Electromagnetism > Flashcards

Flashcards in Electromagnetism Deck (28)
1
Q

What is the definition of electric field strength?

A

Electric field strength is the force exerted on a unit positive charge (by an electric field).

2
Q

What would the electric field pattern be around a single point negative charge?

A
3
Q

What would the electric field pattern be around a single point positive charge?

A
4
Q

What is the definition of electrical potential?

A

The work done in moving a unit positive charge from infinity to a point in a field.

5
Q

A few metals, including iron, nickel and cobalt, are classed as ferromagnetic materials. Applying an external magnetic field to one of these materials results in it becoming magnetised.

Explain how this external magnetic field causes the material to become magnetised.

A

The external field aligns the magnetic dipoles in the material, magnetising it.

6
Q

What would the the electric field pattern between two oppositely charged parallel plates look like? (ignore end effects).

A
7
Q

Three similar looking relationships are shown below for E, F and V.

Define what E, F and V are in terms of the charges (Q) in each relationship.

A
• E is the electric field strength at a distance r from point charge Q.
• F is the electrostatic force that exists between point charges Q1 and Q2, separated by a distance r. (This can be attractive or repulsive.)
• V is the electrostatic potential of an electric field at a distance r from point charge Q.
8
Q

Sketch a current against time graph for a charging capacitor in a d.c.“RC” circuit.

A
9
Q

Sketch a current against time graph for a discharging capacitor in a d.c.“RC” circuit.

A

(note the ‘negative’ current indicating a change in direction of current flow)

10
Q

Sketch a voltage against time graph for a discharging capacitor in a d.c.“RC” circuit.

A
11
Q

Sketch a voltage against time graph for a charging capacitor in a d.c.“RC” circuit.

A
12
Q

What is meant by the ‘time constant’ of a d.c. “RC” circuit?

A
• When the charge stored in a capacitor has increased by 63% of the difference between initial charge and full charge, one time constant has passed.
• The time constant can be calculated using the relationship τ = RC
13
Q

How could you determine the time constant from a voltage-time graph such as this?

A

Work out what 63% of the supply voltage is, and read off the graph how long it taks to reach this value from when it started charging.

14
Q

What is meant by the term capacitive reactance?

A

In an a.c. circuit a capacitor will oppose the flow of current (similarly to resistance).

We call this opposition to a.c. current capacitive reactance Xc

15
Q

What is meant by the term inductive reactance?

A

In an a.c. circuit an inductor will oppose the flow of current (similarly to resistance).

We call this opposition to a.c. current inductive reactance X<span>L</span>

16
Q

Using the circuit shown, describe how you could determine graphically the capacitive reactance of the capacitor.

A
• Vary the a.c. power supply voltage taking readings of V and I.
• Plot V (y-axis) against I (x-axis)
• The gradient of the straight line gives Xc
17
Q

In the circuit shown, the frequency of the a.c. current is increased steadily. Describe what happens to the value of the a.c. current in the circuit.

A

The value of the a.c. current decreases as the frequency of the supply increases.

The a.c. current is inversely proportional to the frequency of the supply.

18
Q

In the circuit shown, the frequency of the a.c. current is increased steadily. Describe what happens to the value of the a.c. current in the circuit.

A

The value of the a.c. current increases in direct proportion to the frequency of the supply.

19
Q

It can be shown that the value of the a.c. current in a inductive circuit is inversely proportional to the frequency of the supply.

What does this tell us about the inductive reactance of the inductor as freqnecy increases?

A

As the frequency of the supply is increased, the current decreases. This must mean X<span>L</span> is increasing.

This means that XL is directly proportional to the frequency of the supply.

20
Q

It can be shown that the value of the a.c. current in a capactive circuit is direclty proportional to the frequency of the supply.

What dos this tell us about the capacitive reactance of the capacitor as freqnecy increases?

A

As the frequency of the supply is increased, the current increases. This must mean Xc is decreasing.

This means that Xc is inversely proportional to the frequency of the supply.

21
Q

What is meant by the self-inductance of a coil (inductor)?

A

When the current in the coil changes, the magnetic field around the coil also changes and an e.m.f. is induced in the coil. This e.m.f. is caused by a change in its own magnetic field. This is known as self-inductance.

22
Q

Explain the process of electromagnetic induction in a conductor.

A

If a conductor is moved through a changing magnetic field, it causes charges in the conductor to move. There is an induced e.m.f. in the conductor.

The induced e.m.f is dependant on:

• The relative speed of motion
• The strength of the magnetic field
• The number of ‘turns’ (in a coil).

(This e.m.f. is different to an e.m.f. produced by a power supply.)

23
Q

Explain why in a d.c. circuit which contains an inductor, there is a small delay in the current reaching its maxmum value when it is switched on.

A

As the current increases rapidly from zero, it induces an e.m.f across the coil which is in the opposite direction to the e.m.f. of the supply, opposing the flow of current. This is known as a ‘back e.m.f.’ or ‘induced e.m.f’. (this has the symbol ε)

The instant the circuit is switched on, ε = - e.m.f. As the current in the circuit increases to it’s maximum value, ε reduces to zero.

24
Q

State what is meant by the ‘induced e.m.f.’ or ‘back e.m.f.’ in an inductor.

A

A rapidly changing magnetic field (caused by a rapidly changing current) can cause a large induced e.m.f. in an inductor which is in the opposite direction to the e.m.f. of the supply.

25
Q

How can you determine the rate of change of current from a current-time graph such as this?

A

Draw a tangent to the curve at a point on the graph.

The gradient of this tangent is equal to the rate of change of current.

In the example shown, the rate of change of current at t = 0 is 20 A s-1.

26
Q

In terms of fields, describe the nature of electromagnetic radiation

A

All EM waves are oscillating electric and magnetic fields at right angles to, and in phase with, each other.

27
Q

James Clerk Maxwell derived an equation relating the two physical constants for E and B fields, ε0 and µ0 respectively.

What does this relationship tell us about the speed of EM radiation?

A

This relationship proved that the speed of EM radiation is constant at 3x108 ms-1 (in a vacuum)

28
Q

List the four fundamental forces in order of their relative strengths from strongest to weakest.

A
1. Strong Nuclear Force
2. Weak Nuclear Force
3. Electromagnetism
4. Gravity.