PhysicsCore26 min read

Electromagnetic Effects

The motor effect, electromagnetic induction, generators and transformers

This topic appears in:

10th Class Physics · ch.1710th Class Physics · ch.181st Year Physics · ch.10O Level Physics (5054) · ch.4
01

The force on a current in a magnetic field

Definition

Fleming's left-hand rule — First finger Field, seCond finger Current, thuMb Motion — for the force on a current-carrying conductor in a magnetic field.

Put a wire carrying a current into a magnetic field and the wire experiences a force. The field of the magnet and the field around the current interact, and the wire is pushed.

The direction is given by Fleming's left-hand rule. Hold the thumb and first two fingers of the left hand at right angles to each other: the First finger points along the Field, from north to south; the seCond finger points along the Current, conventional current; and the ThuMb gives the Motion.

The force is largest when the wire is at right angles to the field, and falls to zero when the wire lies along the field. It can be increased by increasing the current or using a stronger magnet, and it reverses if you reverse either the current or the field. Reverse both and it stays the same.

The field runs left to right, from the north face to the south. The wire runs into the page — the circle is the wire seen end-on, and the cross in it is the tail of the current going away from you. All three directions are then at right angles and all three can be drawn: field across, current into the page, force straight down. Turn the current round and the force turns with it. Lay the wire along the field instead and the force disappears entirely, however large the current — the current has to cut across the field, not run with it.

02

The d.c. motor

A motor is that force put to work. A rectangular coil sits in a magnetic field. Current flows up one side of the coil and down the other, so by the left-hand rule the two sides feel forces in opposite directions — one up, one down. That pair of forces is a couple, and it turns the coil.

A problem appears after half a turn. The side that was on the left is now on the right, so the force on it would now oppose the rotation, and the coil would simply oscillate.

The split-ring commutator solves this. It reverses the connections to the coil every half turn, so the current in each side reverses at exactly the moment it changes sides. The forces therefore stay in the same rotational sense and the coil keeps spinning.

A motor is made faster by increasing the current, using a stronger magnetic field, or adding more turns to the coil. Adding a soft iron core also helps, by concentrating the field through the coil.

Looking down the axis, so the coil is two conductors: one carrying current into the page, one out of it. Both forces are vertical, one up and one down, and the pair turns the coil. Now remove the split ring. Nothing else changes — the current simply stops reversing — and the coil rocks instead of turning, because past the upright the force on each side starts opposing the rotation. That is the entire reason the commutator exists.

Commutator or slip rings?

A split-ring commutator reverses the connections every half turn — used in a d.c. motor and a d.c. generator. Slip rings do not reverse anything and simply maintain contact — used in an a.c. generator, where the output is meant to alternate. Naming the wrong one reverses the whole answer.

03

Electromagnetic induction

The effect also works backwards. Move a conductor through a magnetic field, or change the field through a coil, and a voltage is induced across it. If the circuit is complete, a current flows.

The essential word is change. A stationary magnet inside a stationary coil induces nothing at all, however strong it is. It is the rate at which the field through the coil changes that determines the induced e.m.f.

So the induced voltage is increased by moving the magnet faster, using a stronger magnet, or adding more turns to the coil. Stop moving and the voltage falls immediately to zero.

The direction of the induced current always opposes the change producing it — push a north pole into a coil and the coil's near face becomes a north pole, repelling it. This is why a generator is harder to turn when it is supplying current, and it is conservation of energy in action: the work you do against that opposition is exactly what becomes electrical energy.

Worked example 14 marks

A magnet is pushed into a coil connected to a sensitive meter, then held still inside, then pulled out. Describe and explain the three meter readings.

  1. Pushing in: the meter deflects one way.The field through the coil is changing, so an e.m.f. is induced.
  2. Held still: the meter reads zero.No change in field means no induced e.m.f., even though the magnet is inside.
  3. Pulling out: the meter deflects the other way.The field is changing again, but in the opposite sense.
  4. Moving faster would give a larger deflection in each case.A faster rate of change gives a larger induced e.m.f.

deflection, zero, opposite deflection — induction needs a changing field

Move the magnet and the meter answers; hold it still and the needle falls to zero, even with the magnet sitting inside the coil. That is the whole of it: a field induces nothing, a changing field induces a voltage. Move it faster or add turns and the deflection grows; lead with the other pole and the needle swings the other way.

04

Transformers and the grid

A transformer has two coils wound on a soft iron core. Alternating current in the primary produces a continually changing magnetic field, the core carries that changing field to the secondary, and a voltage is induced there.

It works only on alternating current. Direct current produces a steady field, and a steady field induces nothing — this is a favourite exam question.

The core is soft iron because it magnetises and demagnetises easily, so it can follow a field reversing fifty times a second without loss. The voltages are in the same ratio as the numbers of turns, so more turns on the secondary steps the voltage up, and fewer steps it down.

The National Grid uses this to save an enormous amount of energy. Power is transmitted at very high voltage, which for a given power means a very low current — and since the heating loss in the cables is I²R, cutting the current by a factor of ten cuts the loss by a factor of a hundred. Transformers step the voltage up at the power station and back down before it reaches homes.

V_s / V_p = N_s / N_pV_p I_p = V_s I_s(if 100% efficient)P_loss = I²Rthe transformer changes voltage and current together, keeping the power the same
V_p
primary voltageV
V_s
secondary voltageV
N
number of turns

Key points

  1. Fleming's left-hand rule gives the force; the right-hand rule is for induction.
  2. A split-ring commutator reverses the current every half turn to keep a motor spinning.
  3. Induction needs a changing field — a stationary magnet induces nothing.
  4. Transformers work on a.c. only, because d.c. gives no changing field.
  5. High-voltage transmission means low current, and loss goes as I².

Practice questions

6 questions · 28 marks · full working on every one

Try each one on paper first, then open the working. The marks are shown where they are actually awarded, because that is where they are actually lost.

Short questions

2 · 4 marks

Two marks each, in the style of the short-question section of the paper. Answer in two or three lines.

SQ1[2 marks]
State two ways of increasing the speed of a simple d.c. motor.
Model answer

Increase the current through the coil, or use a stronger magnetic field. (Also acceptable: increase the number of turns on the coil.)

Examiner tip. Two distinct changes. "Make it stronger" is not a method.

SQ2[2 marks]
Explain why a transformer core is made of soft iron.
Model answer

Soft iron is easily magnetised and demagnetised, so it follows the rapidly alternating field of the primary and carries the changing flux efficiently to the secondary.

Examiner tip. The word "alternating" matters — the core must reverse its magnetism many times a second.

Long questions

1 · 9 marks

Theory and numerical together, as they appear in the long-question section.

LQ1[9 marks]
A power station generates 20 MW of electrical power at 25 kV. This is transmitted through cables of total resistance 4.0 Ω.
  1. Calculate the current in the cables if the power is transmitted at 25 kV. [2]
  2. Calculate the power lost as heat in the cables at this voltage. [2]
  3. A transformer steps the voltage up to 400 kV. Calculate the new current and the new power loss. [4]
  4. State the turns ratio of the transformer used. [1]
Mark scheme
  1. Uses I = P/V = 20 × 10⁶ / 25 × 10³[1]
  2. I = 800 A[1]
  3. Uses P = I²R = 800² × 4.0[1]
  4. = 2.56 × 10⁶ W = 2.56 MWnearly 13% of the output[1]
  5. New current = 20 × 10⁶ / 400 × 10³ = 50 A[1]
  6. New loss = 50² × 4.0[1]
  7. = 1.0 × 10⁴ W = 10 kW[1]
  8. A reduction by a factor of 256, because the current fell by 16 and the loss depends on I²the point of the question[1]
  9. Turns ratio N_s : N_p = 400 : 25 = 16 : 1[1]

(a) 800 A (b) 2.56 MW (c) 50 A and 10 kW (d) 16 : 1

Examiner tip. The whole question exists to show that a 16-fold voltage increase cuts the loss 256-fold. Say that explicitly in part (c) — it is a mark, and it is the reason the grid exists.

Exam questions

3 · 15 marks

Multi-part questions with a full mark scheme.

Q1[6 marks]
A transformer has 200 turns on its primary coil and 5000 turns on its secondary. The primary is connected to a 230 V alternating supply.
  1. Calculate the secondary voltage.
  2. State whether this is a step-up or step-down transformer.
  3. Explain why the transformer would not work on a direct current supply.
Mark scheme
  1. Uses V_s/V_p = N_s/N_p[1]
  2. V_s = 230 × 5000/200[1]
  3. V_s = 5750 V[1]
  4. Step-upmore turns on the secondary[1]
  5. Direct current produces a constant magnetic field in the core[1]
  6. A changing field is required to induce an e.m.f., so nothing is induced in the secondarythe word "changing" is the mark[1]

(a) 5750 V (b) step-up (c) d.c. gives a constant field, and induction needs a changing one

Examiner tip. Check the ratio makes sense before writing the answer: more turns on the secondary must give a higher voltage. If your answer came out lower, you inverted the fraction.

Q2[5 marks]
A student moves a bar magnet into a coil connected to a sensitive centre-zero meter.
  1. State what is observed on the meter.
  2. State two changes that would increase the reading.
  3. State what happens if the magnet is held stationary inside the coil, and explain why.
Mark scheme
  1. The needle deflects to one side while the magnet is moving[1]
  2. Move the magnet faster[1]
  3. Use a stronger magnet, or a coil with more turnsany second valid change[1]
  4. The needle returns to zero — no deflection[1]
  5. There is no change in the magnetic field through the coil, and induction requires a change[1]

Examiner tip. The stationary-magnet part appears constantly. The answer is always "no change in field, so no induced e.m.f." — it is not about the magnet being weak.

Q3[4 marks]
Explain why electrical energy is transmitted across the country at very high voltage, and state the role of transformers at each end.
Mark scheme
  1. For a given power, a higher voltage means a smaller currentfrom P = VI[1]
  2. Power lost as heat in the cables is I²R, so a smaller current wastes much less energythe I² is the key point[1]
  3. A step-up transformer raises the voltage at the power station[1]
  4. A step-down transformer lowers it again for safe use in homes[1]

Examiner tip. Quote P = I²R explicitly. An answer that says only "less current means less loss" misses the point that the relationship is a square, which is why the saving is so large.