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IGCSE Physics 0625 · Chapter 10 notes · ~11 min read

Magnetism & electromagnetism

Magnets and their fields, electromagnets, the motor effect, the d.c. motor, the a.c. generator and transformers — and how the grid delivers power.

Magnetic poles & materialsFieldsElectromagnetsRelays & loudspeakersForce on a currentd.c. motorInductiona.c. generator
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Like poles repel, unlike poles attract. Magnets attract iron, steel, nickel and cobalt.
  • A current makes a magnetic field. A coil with a current is an electromagnet.
  • A wire carrying a current in a field feels a force — this is how the motor works.
  • A conductor moving in a field (or a changing field) induces an e.m.f. — this is how generators and transformers work.
  • Transformers: Vp ÷ Vs = Np ÷ Ns. High-voltage transmission wastes less energy.

By the end you can

  • Test materials for magnetism and describe permanent and temporary magnets.
  • Draw field patterns for a bar magnet, a wire and a solenoid.
  • Describe the motor effect, the d.c. motor and Fleming's left-hand rule. EXT
  • Explain electromagnetic induction and the a.c. generator.
  • Use the transformer equation and explain why the grid uses high voltage.

1Magnets and magnetic fields

Poles, materials and induced magnetism

NSNSunlike poles ATTRACTNSSNlike poles REPELa magnet attracts iron, steel, nickel and cobalt only
Unlike poles attract. Like poles repel.
  • Repulsion is the only true test of a magnet. A magnet will attract an unmagnetised piece of iron too, so attraction alone proves nothing.
  • Magnetic materials: iron, steel, nickel, cobalt. Non-magnetic: copper, aluminium, plastic, wood.
  • Induced magnetism: a piece of iron close to a magnet becomes a magnet itself, with the end nearest the magnet becoming the opposite pole. That is why a magnet picks up a chain of paper clips.
Soft ironSteel
magnetiseseasilywith difficulty
keeps magnetism?no — temporaryyes — permanent
used forelectromagnets, relays, transformer corescompass needles, permanent magnets
  • Permanent magnets are used in loudspeakers, motors, fridge doors and compasses. Electromagnets are used in scrapyard cranes, relays and doorbells.

Magnetic fields

Magnetic field — a region where a magnetic pole feels a force.

NS
Field lines leave the N pole and enter the S pole. Lines never cross. Closer lines mean a stronger field.
  • The direction of the field is the direction of the force on a free north pole.
  • Plot the pattern with a small plotting compass, or sprinkle iron filings on paper over the magnet.
  • EXT The spacing of the lines shows the relative strength: strongest at the poles. Magnetic forces arise when the fields of two magnets interact.

2Currents and magnetic fields

The field around a wire

current out of pagefield anticlockwisecurrent into pagefield clockwise
Straight wire: circles around the wire. Use the right-hand grip rule — thumb along the current, fingers show the direction of the field.
  • The field gets weaker with distance from the wire. A bigger current means a stronger field. Reversing the current reverses the field.

Solenoids and electromagnets

NScurrent in coil → field like a bar magnetgrip rule: curl fingers with the current, thumb points to N
A solenoid (coil) has a field like a bar magnet. Curl your fingers with the current and your thumb points to the N end.
  • To make an electromagnet stronger: more turns, more current, or a soft iron core.
  • Switch off the current and a soft iron electromagnet loses its magnetism, which makes it useful for lifting scrap.
  • EXT Inside the solenoid the field is strong and uniform. Changing the current changes the field strength, and reversing the current reverses the field direction.

Relay and loudspeaker

  • Relay: a small current in a coil magnetises a soft iron core. This pulls a lever (the armature) which closes the contacts of a second circuit carrying a large current. It lets a thin, safe switch control a powerful device.
  • Loudspeaker: a coil attached to a paper cone sits in the field of a permanent magnet. The alternating current in the coil makes a changing force, so the cone vibrates and makes sound waves.

3Force on a current

The motor effect

  • A wire carrying a current in a magnetic field feels a force, because the field of the wire interacts with the field of the magnet.
  • Experiment: a wire rests on rails between the poles of a magnet. When the current is switched on the wire rolls along the rails. Reversing the current or the field reverses the force.
  • The force is bigger with a bigger current and a stronger magnet.
FORCE / motionTHUMBFIELD N → SFIRST fingerCURRENT + → −SECOND fingeruse your LEFT hand · thumb, first and second fingers at right angles
EXT Fleming's left-hand rule: thumb = force (motion), first finger = field (N to S), second finger = current (+ to −).
  • EXT A beam of charged particles is also a current. Electrons are negative, so the conventional current points opposite to their motion.

The d.c. motor

NSfield N → Sforce ↑force ↓the coil turns clockwisecurrent out of page on the left · into page on the right
The coil has current out of the page on one side and into the page on the other, so the forces are opposite and the coil turns.
  • The turning effect is bigger with more turns on the coil, a bigger current and a stronger magnetic field.
  • EXT Split-ring commutator: every half turn the two halves of the ring swap brushes, so the current in the coil reverses. The force on each side then keeps turning the coil the same way.
−+brushbrushsplit-ring commutator + carbon brushesevery half turn the two halves swap brushes,so the current in the coil reverses→ the force always pushes the coil the same way round(the brushes stay still; the ring turns with the coil)
EXT The brushes stay still. The ring turns with the coil.

4Electromagnetic induction

Making electricity from movement

GSNpush inmove the magnet in →needle flicks one wayhold it still →needle reads zeropull it out →needle flicks the other wayfaster · stronger magnet · more turns → bigger induced e.m.f.
Move a magnet into a coil connected to a meter and the needle flicks. Hold it still and the reading is zero. Pull it out and the needle flicks the other way.
  • An e.m.f. is induced when a conductor cuts magnetic field lines, or when the field through a coil changes. If the circuit is complete, a current flows.
  • A bigger induced e.m.f. needs: faster movement, a stronger magnet, more turns on the coil.
  • Reversing the direction of the movement (or the magnet) reverses the induced e.m.f.
  • EXT The induced e.m.f. opposes the change that causes it. Pushing a N pole into a coil makes that end of the coil a N pole, which repels the magnet — so you must do work, and energy is conserved.

The a.c. generator

NSslip rings + brushescoil seen end-one.m.f.timemax0max (reversed)one turn = one cyclemax when the sides cut the field fastestzero when they move along it
A coil spins in a magnetic field. The e.m.f. is largest when the sides cut across the field lines fastest, and zero when they move along them.
  • A coil (or a magnet) rotates, so the field through the coil keeps changing and an e.m.f. is induced. Slip rings and brushes connect the spinning coil to the outside circuit.
  • The output is an alternating e.m.f. (a sine wave). It is zero when the coil is perpendicular to the field and at a peak (positive or negative) when the coil is parallel to the field.
  • Spinning faster gives a bigger peak and a higher frequency. More turns or a stronger magnet also give a bigger e.m.f.

5Transformers

How a transformer works

primarycoilNₚ turnssecondarycoilNₛ turnssoft iron core: carries the changing fieldVₚ ÷ Vₛ = Nₚ ÷ Nₛmore turns on the secondary → step-UP (higher voltage)fewer turns on the secondary → step-DOWNa.c. in → changing field → a.c. out
Two coils wound on a soft iron core. An a.c. in the primary makes a changing magnetic field in the core, which induces an a.c. in the secondary.
  • Step-up: more turns on the secondary, higher voltage out. Step-down: fewer turns on the secondary, lower voltage out.
  • It only works with a.c., because the magnetic field must keep changing to induce an e.m.f.
VpVs = NpNs
p = primary, s = secondary, V = voltage, N = number of turns
EXT Ip Vp = Is Vs
power in = power out for a 100% efficient transformer
Worked example

A transformer has 1150 turns on the primary, connected to 230 V. How many turns does the secondary need to give 10 V?

  1. Vp ÷ Vs = Np ÷ Ns
  2. 230 ÷ 10 = 1150 ÷ Ns
  3. Ns = 1150 × 10 ÷ 230 = 50 turns (a step-down transformer)
Worked example

EXT A 100% efficient transformer has 240 V across its primary. The secondary gives 12 V to a lamp, with a current of 2.0 A. Find the primary current.

  1. Power out = IsVs = 2.0 × 12 = 24 W
  2. Power in = 24 W, so Ip = P ÷ Vp = 24 ÷ 240
  3. = 0.10 A

The power grid

PowerstationStep-uptransformer400 kVcablesStep-downtransformerHomes230 Vhigh voltage → low current → little energy lost as heat (P = I²R)transformers only work with a.c. (a changing field is needed)power stations make a.c. at ~25 kV; the grid carries 400 kV;homes receive 230 V
Power stations generate a.c. Step-up transformers raise the voltage for the cables, and step-down transformers lower it again for homes.
  • Transmitting at high voltage means a low current for the same power (P = IV), so much less energy is lost as heat in the cables.
  • EXT The power lost in a cable is P = I²R. If the voltage is ten times higher, the current is ten times smaller and the power lost is one hundred times smaller.
Worked example

EXT 100 kW is sent through a cable of resistance 2.0 Ω. Compare the power lost at 1000 V and at 10 000 V.

  1. At 1000 V: I = P ÷ V = 100 000 ÷ 1000 = 100 A
  2. loss = I²R = 100² × 2.0 = 20 000 W
  3. At 10 000 V: I = 10 A
  4. loss = 10² × 2.0 = 200 W
  5. 100 times smaller
  • High-voltage transmission also lets you use thinner, cheaper cables.

6Test yourself

Test yourself — cover the answers first

Quick check

  1. What is the only sure test for a magnet?
  2. Name two ways of making an electromagnet stronger.
  3. Which wires of a motor are swapped every half turn, and by what?
  4. Why can a transformer not work with d.c.?
  5. Name three ways of increasing the e.m.f. induced in a coil.
  6. Where does the fuse go in a plug?

Exam-style

  1. A transformer has 500 turns on the primary and 100 turns on the secondary, and 240 V on the primary. Find the secondary voltage.
  2. Describe how a relay lets a small current switch a large current. [3]
  3. EXT An electron beam moves to the right through a magnetic field pointing into the page. State the direction of the force on the beam and explain. [2]
  4. Explain why power is transmitted at high voltage. [3]
Show answers
  1. Repulsion with another magnet.
  2. more turns, a bigger current, or a soft iron core
  3. The coil connections are swapped between the brushes by the split-ring commutator.
  4. The field in the core must keep changing to induce an e.m.f.; a steady d.c. gives a steady field.
  5. move the magnet faster, use a stronger magnet, use more turns
  6. the live wire
  7. Vs = Vp × Ns ÷ Np = 240 × 100 ÷ 500 = 48 V
  8. The small current magnetises the soft iron core of a coil
    which pulls the iron armature
    closing the contacts of a second circuit that carries the large current
  9. Electrons moving right are a conventional current to the left
    using Fleming's left-hand rule, with the field into the page and the current to the left, the force is downwards
  10. High voltage means a low current for the same power
    the heating loss in the cables is I²R
    so much less energy is wasted
Practise this chapter
MagnetismMagnetic effect of a current & motorsElectromagnetic inductionTransformers
Next chapterNuclear physics →
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