Electromagnetic effects

Key points

4.5.1 Electromagnetic induction

• Electromagnetic induction: an e.m.f. is induced (produced) in a conductor when:
  • the conductor moves across a magnetic field, so it cuts the field lines, or
  • the magnetic field linking the conductor changes, for example a magnet moves in or out of a coil.
• If the conductor is part of a complete circuit, the induced e.m.f. makes a current flow. This is an induced current.
• Experiment to show induction:
  1. Connect a coil of wire to a sensitive centre-zero meter. (The needle of this meter can move either way from zero.)
  2. Push a bar magnet into the coil. The needle moves one way.
  3. Hold the magnet still inside the coil. The needle reads zero.
  4. Pull the magnet out. The needle moves the other way.
• In this experiment, moving the magnet into or out of the coil changes the magnetic field linking the coil. This change induces the e.m.f.
• When the magnet is still, the field linking the coil does not change, so no e.m.f. is induced. Reversing the motion reverses the e.m.f.
• The same experiment works with a straight wire moved up and down between the poles of a U-shaped magnet.
• The induced e.m.f. is larger when:
  • the magnet or conductor moves faster
  • the magnetic field is stronger
  • the coil has more turns.
• The direction of the induced e.m.f. always opposes the change that causes it. Extended
• Example: push the N pole of a magnet into a coil. The induced current makes the end of the coil nearest the magnet an N pole. This N pole repels the magnet, so it opposes the magnet moving in. Extended
• Pull the magnet out. Now the near end of the coil becomes an S pole. It attracts the magnet, so it opposes the magnet moving out. Extended
• So you must do work to move the magnet. This work is the source of the electrical energy that is produced. Extended
• Relative directions of force (motion), field and induced current: the induced current is at right angles to both the motion and the field. When the conductor moves at right angles to the field, all three are at right angles to each other. For this case, use Fleming’s right-hand rule: Extended
  • thuMb = Motion of the conductor (the direction of the force that moves it)
  • First finger = Field (from N to S)
  • seCond finger = induced Current (conventional current).
• Example: the field points from left to right (N pole on the left). A wire is moved upwards through the field. The induced current flows into the page. If the wire is moved downwards, the current flows out of the page. Extended

4.5.2 The a.c. generator

• A simple a.c. generator has a coil of wire that rotates between the poles of a magnet. Extended
• As the coil turns, its sides cut the magnetic field lines. So an e.m.f. is induced in the coil. Extended
• Each side of the coil moves up through the field for half a turn, then down for the next half turn. So the induced e.m.f. reverses every half turn. The output is alternating. Extended
• Each end of the coil is joined to its own slip ring: a complete metal ring that turns with the coil. Extended
• Two fixed brushes (usually carbon) press against the slip rings. They connect the turning coil to the outside circuit. Extended
• Slip rings and brushes keep the contact while the coil turns, so the wires do not twist. Extended
• In another kind of generator, a magnet rotates inside a fixed coil. The changing magnetic field linking the coil induces an alternating e.m.f. The coil does not move, so this kind needs no slip rings or brushes. Extended
• Graph of e.m.f. against time for a rotating coil: a smooth wave that goes above and below zero. One full turn of the coil gives one full cycle of the wave. Extended
• The e.m.f. is zero when the plane of the coil is at right angles to the field (the coil faces the poles). At this moment, the sides of the coil move along the field lines, so they do not cut them. Extended
• The e.m.f. has its largest value (a peak or a trough) when the plane of the coil is parallel to the field. At this moment, the sides of the coil cut the field lines fastest. Extended
• In one turn, starting with the coil at right angles to the field: zero → peak (quarter turn) → zero (half turn) → trough (three-quarter turn) → zero (full turn). Extended
• The trough is a peak in the opposite direction. The sides of the coil have swapped places, so the e.m.f. is reversed. Extended
• If the coil turns faster, the peaks are higher and there are more cycles each second (each cycle takes less time). Extended

4.5.3 Magnetic effect of a current

• A current in a wire produces a magnetic field around the wire.
• Straight wire: the field lines are circles around the wire, with the wire at the centre. The circles are in a plane at right angles to the wire.
• Find the direction with the right-hand grip rule: grip the wire with your right hand, thumb pointing along the conventional current. Your fingers curl the way the field goes.
• Solenoid (a long coil of wire): outside the coil, the field is like the field of a bar magnet. One end acts as an N pole and the other as an S pole.
• Inside the solenoid, the field lines are straight, parallel and close together.
• For a solenoid, curl the fingers of your right hand the way the current goes round the coil. Your thumb points to the N pole end.
• Experiment to show the field pattern:
  1. Pass a straight wire vertically through a hole in a horizontal card. (For a solenoid, fix the card so that the coil passes through it.)
  2. Switch on a large current. Sprinkle iron filings on the card and tap it gently. The filings line up to show the pattern.
  3. Place small plotting compasses on the card. The N ends of the needles show the direction of the field.
  4. Reverse the current. The compass needles turn round, so the field direction has reversed. Extended
• Relay: a switch worked by an electromagnet.
  • A small current flows in the relay coil, which has a soft-iron core. The coil becomes an electromagnet.
  • It attracts an iron armature (a movable piece of iron). The armature closes the switch contacts in a second circuit.
  • When the small current stops, the soft iron loses its magnetism. A spring pulls the armature back and the contacts open.
• Why use a relay: a small, safe current can switch on a large current in a separate circuit. Examples: the starter motor of a car; a light sensor circuit switching on mains lights.
• Loudspeaker: a coil is fixed to a paper or plastic cone. The coil sits in the field of a permanent magnet.
  • A varying (alternating) current from the amplifier flows in the coil. The coil becomes an electromagnet whose field changes all the time.
  • The force between the coil and the magnet changes in size and direction. So the coil and the cone vibrate in and out.
  • The cone pushes the air and makes sound waves with the same frequency as the current.
• Examples of loudspeakers: phones, headphones, radios and a school’s public address system.
• Straight wire: the field is strongest close to the wire. It gets weaker further away. The field lines are drawn further apart as the distance increases. Extended
• Solenoid: the field is strongest inside the coil. It is weaker outside. Extended
• A larger current makes the field stronger everywhere, for both a straight wire and a solenoid. The pattern stays the same. Extended
• Reversing the current reverses the direction of the field. For a solenoid, the N and S poles swap ends. The pattern and strength stay the same. Extended

4.5.4 Force on a current-carrying conductor

• A wire that carries a current in a magnetic field can feel a force. This is the motor effect.
• Experiment:
  1. Lay a short, bare copper rod across two horizontal metal rails.
  2. Place a U-shaped magnet so that the rod is between its poles.
  3. Connect the rails to a d.c. supply. A current flows through the rod.
  4. The rod rolls along the rails. A force acts on it.
• Reverse the current: the rod moves the opposite way.
• Reverse the field (turn the magnet over so the poles swap): the rod moves the opposite way.
• Reverse both the current and the field: the rod moves the same way as at the start.
• Relative directions of force, field and current: the force is at right angles to both the current and the field. When the current is at right angles to the field, all three are at right angles to each other. For this case, use Fleming’s left-hand rule: Extended
  • thuMb = Motion (the force)
  • First finger = Field (from N to S)
  • seCond finger = Current (conventional current, + to −).
• Example: the field points from left to right (N pole on the left). The current in a wire flows into the page. The force on the wire is downwards. Extended
• Extra detail: the force is largest when the current is at right angles to the field. There is no force when the current is parallel to the field.
• Beams of charged particles: a moving charged particle is a current. So a beam of charged particles moving across a magnetic field feels a force. Extended
• For positive particles, the current is in the same direction as their motion. For negative particles such as electrons, the current is in the opposite direction to their motion. Extended
• Use the left-hand rule with the direction of the current, not the direction of the electrons. Extended
• The force is always at right angles to the motion. So the beam curves. Extended
• Example: a beam of electrons moves to the right. The field points into the page. The current is to the left. The force on the electrons is downwards, so the beam curves downwards. Extended

4.5.5 The d.c. motor

• A coil that carries a current in a magnetic field can feel a turning effect.
• The turning effect is increased by:
  • more turns on the coil
  • a larger current
  • a stronger magnetic field.
• How a d.c. motor works: Extended
  1. A rectangular coil sits between the poles of a magnet. It can turn on an axle.
  2. The current goes one way along one side of the coil and the other way along the opposite side.
  3. So the forces on the two sides are in opposite directions: one side is pushed up and the other is pushed down. This gives a turning effect, and the coil rotates.
  4. The ends of the coil are joined to a split-ring commutator. This is a ring cut into two halves. It turns with the coil.
  5. Two fixed brushes (usually carbon) press on the commutator. They connect the coil to the d.c. supply.
  6. Every half turn, the gaps in the split ring pass the brushes. Each half of the ring then touches the other brush. This reverses the current in the coil.
  7. Reversing the current keeps the force on the side nearest each pole in the same direction. So the coil keeps turning the same way.
• The turning effect is largest when the plane of the coil is parallel to the field. It is zero when the coil is at right angles to the field. The coil’s momentum carries it past this position. Extended
• Without the commutator, the forces would turn the coil back after each half turn. The coil would just rock to and fro. Extended

4.5.6 The transformer

• A transformer changes the size of an alternating voltage.
• Construction: two coils of insulated wire are wound on a soft-iron core. The core is a closed loop of iron. The two coils are not connected to each other by any wire.
• The primary coil is the input coil. It is connected to the a.c. supply.
• The secondary coil is the output coil.
• A step-up transformer makes the output voltage larger than the input voltage. The secondary coil has more turns than the primary.
• A step-down transformer makes the output voltage smaller. The secondary coil has fewer turns than the primary. A phone charger contains a step-down transformer.
• Transformer equation: V_p / V_s = N_p / N_s. The ratio of the voltages is equal to the ratio of the numbers of turns.
• High-voltage transmission: at the power station, a step-up transformer makes the voltage very high. The electricity travels through the transmission cables at this high voltage. Near homes, step-down transformers reduce the voltage to a safer value for use.
• Advantages of transmitting at a high voltage:
  • the current in the cables is smaller for the same power
  • so less energy is wasted as thermal energy in the cables
  • so thinner, cheaper cables can be used.
• How a transformer works: Extended
  1. An alternating current in the primary coil makes an alternating (changing) magnetic field in the soft-iron core.
  2. The core carries this changing field through the secondary coil.
  3. The changing magnetic field linking the secondary coil induces an alternating e.m.f. in it.
• A transformer does not work with steady d.c. A steady current makes a magnetic field that does not change, so no e.m.f. is induced in the secondary. Extended
• Soft iron is used for the core because it is easily magnetised and demagnetised. So it follows the changes in the current. Extended
• If a transformer is 100% efficient, the power in = the power out. Power = current × voltage, so I_pV_p = I_sV_s. Extended
• So when a transformer steps the voltage up, it steps the current down by the same factor. Extended
• The power lost as thermal energy in the cables is P = I²R. Here I is the current in the cables and R is their resistance. Extended
• For a fixed power sent, P = IV. A higher voltage means a smaller current. The power loss depends on I². So making the current 10 times smaller makes the power loss 100 times smaller. Extended

Common mistakes

• Students write that a magnet held still inside a coil induces an e.m.f. / The mark scheme wants: an e.m.f. is induced only while the field linking the coil is changing, for example by moving the magnet into or out of the coil.
• Students write that a transformer works with d.c. / The mark scheme wants: a transformer needs a.c., because only a changing field induces an e.m.f. in the secondary coil. Extended
• Students write that a step-up transformer increases the power. / The mark scheme wants: it increases the voltage. At 100% efficiency, the power stays the same and the current goes down. Extended
• Students mix up the two hand rules. / The mark scheme wants the left hand for the force on a current (motor), and the right hand for the induced current (generator). Extended
• Students use the direction the electrons move as the current in the left-hand rule. / The mark scheme wants conventional current, which is opposite to the electrons’ motion. Extended
• Students write that the generator e.m.f. is largest when the coil faces the poles. / The mark scheme wants: largest when the plane of the coil is parallel to the field, because the sides cut the field lines fastest. Extended
• Students call the d.c. motor’s commutator a “slip ring”. / The mark scheme wants: a split-ring commutator in a d.c. motor; slip rings in an a.c. generator. Extended

Exam tips

• State the three factors for a bigger induced e.m.f.: faster motion, stronger field, more turns.
• Describe an experiment: name the apparatus (coil, magnet, sensitive centre-zero meter), say what you do and what you see, and say what happens when you reverse the motion or hold the magnet still.
• Describe the field around a straight wire as circles centred on the wire. Give the direction with the right-hand grip rule.
• Add that reversing the current reverses the field, and that a larger current makes the field stronger. Extended
• Describe a relay as a sequence: small current → electromagnet → armature pulled → contacts close → second circuit switched on.
• Calculate with the transformer equation: write it out, substitute, then rearrange. Say whether the transformer is step-up or step-down.
• Explain why high voltage is used in steps: same power → smaller current → smaller I²R loss → less energy wasted. Extended
• Explain how a transformer works using all three links: a.c. in primary → changing field in the core → induced e.m.f. in secondary. Extended
• Sketch a generator graph: a smooth wave. Mark where the coil is parallel to the field (peaks and troughs) and at right angles to it (zeros). Extended
• Explain the commutator’s job: it reverses the current every half turn, so the coil keeps turning the same way. Extended
• A typical 1-mark answer: “A transformer core is made of soft iron.”
• A typical 2-mark answer to “Give two ways to increase the turning effect on the coil of a motor”: “Use a larger current (1). Use a stronger magnet (1).”
• A typical 2-mark answer to “Why is electricity transmitted at a high voltage?”: “The current in the cables is smaller (1). So less energy is lost as thermal energy in the cables (1).”
• A typical 3-mark answer to “Explain the action of a transformer”: “The alternating current in the primary coil makes a changing magnetic field (1). The soft-iron core carries this field to the secondary coil (1). The changing field induces an alternating e.m.f. in the secondary coil (1).” Extended