Electromagnetism

Question 1

What is the definition of electric field strength?

Answer 1

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

Question 2

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

Answer 2

Question 3

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

Answer 3

Question 4

What is the definition of electrical potential?

Answer 4

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

Question 5

An electric field is an example of a conservative field. What is meant by a conservative field?

Answer 5

The energy required to move a charge
between two points in an electric field is
independent of the path taken

Question 6

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.

Answer 6

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

Question 7

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

Answer 7

Question 8

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.

Answer 8

• 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.

Question 9

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

Answer 9

Question 10

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

Answer 10

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

Question 11

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

Answer 11

Question 12

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

Answer 12

Question 13

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

Answer 13

• 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

Question 14

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

Answer 14

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.

Question 15

What is meant by the term capacitive reactance?

Answer 15

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 X_c

Question 16

What is meant by the term inductive reactance?

Answer 16

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>}

Question 17

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

Answer 17

• 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 X_c

Question 18

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.

Answer 18

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.

Question 19

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.

Answer 19

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

Question 20

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?

Answer 20

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

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

Question 21

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?

Answer 21

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

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

Question 22

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

Answer 22

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.

Question 23

Explain the process of electromagnetic induction in a conductor.

Answer 23

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.)

Question 24

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.

Answer 24

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.

Question 25

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

Answer 25

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.

Question 26

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

Answer 26

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.

Question 27

In terms of fields, describe the nature of electromagnetic radiation

Answer 27

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

Question 28

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?

Answer 28

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

Question 29

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

Answer 29

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