0625 · 4.5Electromagnetic effects

Electromagnetic induction and generator rule

0:00A generator produces electricity when its shaft turns, while a motor rotates when current passes through its coil. A transformer changes voltage without any moving parts. These devices seem different, but they connect electricity and magnetism in closely related ways. By the end of this video, you will understand how they work, including why generators need movement and transformers require alternating current. You need a few familiar ideas before we begin.

0:25Current is a flow of electric charge, and its conventional direction is the way positive charges would move. Electromotive force means energy supplied per unit charge, measured in volts. Magnetic field lines show direction, running from north to south outside a magnet. Their spacing represents strength, with closer lines showing a stronger field. Connect a coil to a sensitive meter, then push a bar magnet into it. The meter deflects, showing that a current flows during the movement.

0:53Hold the magnet still, and the reading returns to zero. Pull it out, and the pointer deflects in the opposite direction. The important difference is not the presence of a magnet, but whether its field through the coil changes. Electromagnetic induction is the production of an electromotive force when a conductor cuts magnetic field lines, or when the field linking it changes. Moving a wire across a magnetic field can therefore produce a voltage between its ends.

1:17An induced current flows only if there is a complete conducting circuit. An open circuit can have an induced electromotive force without a current flowing. Moving the coil towards a stationary magnet also produces induction, because the field linking it changes. Sliding a straight wire parallel to field lines does not cut across them, so that movement produces no induced electromotive force. Simply placing a wire beside a magnet, with neither moving, is not enough.

1:43Induction requires the appropriate movement or a changing magnetic field. Repeat the magnet experiment with faster movement, and the meter shows a larger deflection. A stronger magnet also produces a greater induced electromotive force under otherwise identical conditions. Increasing the number of turns raises the voltage because the contributions from each turn add together. These factors increase either the rate at which the magnetic field changes or the number of turns experiencing that change.

2:11For Extended students, the direction follows Lenz’s law. The induced electromotive force acts in a direction that opposes the change causing it. In a complete circuit, the induced current produces a magnetic field with this opposing effect. When a north pole approaches a coil, the near face becomes north and repels it. When that pole moves away, the near face becomes south and attracts it. Notice that the coil opposes the change, not necessarily the original magnetic field.

2:39During withdrawal, its attraction tries to keep the magnet nearby, opposing the decreasing field through the coil. You must do work against this magnetic effect to keep the magnet moving. That mechanical work becomes electrical energy, so induction does not create energy from nothing. Another Extended skill is using Fleming’s right-hand rule for a moving conductor. Hold your thumb, first finger and second finger mutually perpendicular. Your thumb represents motion, your first finger represents magnetic field direction, and your second finger represents induced conventional current.

3:11The thumb gives the direction of the force moving the conductor. This is the generator rule, not the one used for motors. Consider a wire moving upwards through a magnetic field directed to the right. Point your right thumb upwards and your first finger rightwards. Your second finger points into the screen, giving the induced conventional current direction in a complete circuit. Reversing either movement or field reverses this direction.

3:34Reversing both leaves the induced current direction unchanged.

Alternating current generators

3:37The alternating current generator is Supplement content, so this section develops the Extended requirements. A simple generator rotates a coil between magnetic poles, making its sides cut field lines. Mechanical work supplied to the shaft becomes electrical energy delivered to the external circuit. After half a turn, each side moves across the field in the opposite direction. The induced electromotive force therefore reverses. Each coil end connects to a separate slip ring that rotates with it.

4:06Stationary conducting brushes press against the rings, maintaining contact with the external circuit. The rings do not swap the connections during rotation, so the reversing voltage reaches the output. Another simple generator rotates a magnet beside a stationary coil. Its changing magnetic field also induces an alternating electromotive force. For a coil rotating steadily in a uniform field, the electromotive force follows a smooth alternating graph. At the zeros, the plane of the coil is perpendicular to the field.

4:36The field passing through the coil is greatest in magnitude, but momentarily changes at the slowest rate. Induction depends on the rate of change, not just the amount of field passing through. At a positive peak or negative trough, the plane of the coil is parallel to the magnetic field. Here the field through the coil changes most rapidly, giving the greatest voltage magnitude. The trough represents the opposite polarity, not a weaker generating effect.

5:01Starting at zero, successive quarter turns give a peak, another zero, a trough, and the original zero again. Faster rotation increases the maximum induced electromotive force because the coil cuts field lines more rapidly. It also produces more complete cycles each second, increasing frequency. A stronger field or more coil turns increases the maximum voltage without necessarily changing frequency. Keep the distinction clear between voltage magnitude, shown by graph height, and frequency, indicated by how closely the cycles are spaced.

Magnetic fields from currents

5:32Induction connects a changing magnetic field to electricity, but an electric current also creates its own field. Around a straight wire, the field lines form concentric circles centred on it. They are not straight lines travelling along it. Use the right-hand grip rule to find direction, pointing your thumb along conventional current. Your curled fingers follow the circular field. To investigate this pattern, pass a straight wire through horizontal card and connect it to a suitable low voltage supply.

5:59Sprinkle iron filings on the card and tap it gently while current flows. The filings reveal the circular pattern, but do not identify its direction. Place plotting compasses at several positions instead, using their north-seeking ends to mark field arrows. Disconnect between observations to limit heating. A solenoid is a long coil containing many turns of wire. The fields produced by individual turns combine into a pattern like that of a bar magnet.

6:25One end acts as north and the other as south. Inside a long solenoid, the field is strong and approximately uniform, represented by nearly parallel, equally spaced lines. Outside, the lines curve from north towards south. Use plotting compasses around and, where accessible, inside a solenoid to map its field direction. Mark the direction of each compass’s north-seeking end, then connect the observations into field lines. Iron filings can help reveal the pattern around the coil, but compass readings supply the arrows.

6:55For solenoid polarity, curl your right fingers in the current direction around its turns. Your thumb points towards its north end. The qualitative changes in field strength and direction are Extended content. The field around a straight wire becomes weaker farther away and stronger when current increases. Increasing current also strengthens a solenoid’s field, as do more turns per unit length. A soft-iron core strengthens the solenoid’s magnetic effect further.

7:21Reversing current reverses field direction and swaps the solenoid’s poles. A relay uses this magnetic effect to operate a switch. Current in its coil magnetises a soft-iron core, which attracts an iron armature and closes contacts in another circuit. When the coil current stops, a spring returns the armature and opens the contacts. A small control current can therefore switch a separate circuit carrying a much larger one. Applications include operating a car starter motor or automatic street lighting.

7:50A loudspeaker combines the magnetic effect of a current with the field of a permanent magnet. Its voice coil sits in the magnet’s field and is attached to a cone. An alternating current produces a force that keeps reversing, making the coil and cone move backwards and forwards. The vibrating cone produces sound in the surrounding air. This arrangement is used in headphones, televisions and public address speakers.

Motor effect and motors

8:13To demonstrate the force directly, place a wire between magnetic poles and connect it through flexible leads to a low voltage supply. Arrange the wire perpendicular to the field, with freedom to move. Switch on briefly, and the wire moves because a force acts on it. Reverse the current while keeping the field unchanged, and the movement reverses. Reversing only the magnetic field also reverses the force. This is called the motor effect, and the force acts perpendicular to both current and magnetic field.

8:41The effect is greatest when the conductor is perpendicular to the field. A conductor parallel to the field experiences no magnetic force from that arrangement. Reversing both current and field leaves the force direction unchanged, because the two reversals cancel. For Extended students, Fleming’s left-hand rule determines the force direction. Point your first finger along the magnetic field and your second finger along conventional current. Your thumb then gives the direction of force.

9:07For a rightward field and current into the screen, the force is downwards. The left hand predicts motor force, whereas the right hand gives generator current. The same direction relationship applies to beams of charged particles, another Extended requirement. A beam of positive particles corresponds to conventional current in the direction of travel. For negative particles, conventional current is opposite to their movement. Positive particles travelling rightwards through a field into the screen experience an upward force.

9:36Electrons travelling in that same direction experience a downward force, while particles moving parallel to the field are not deflected. Place a current-carrying coil in a magnetic field, and its two sides can experience forces in opposite directions. These forces act at different positions, producing a turning effect rather than simply moving the whole coil sideways. Increasing current or magnetic field strength increases the forces. Adding coil turns increases the total turning effect because more sections contribute.

10:05A motor uses electrical energy to produce rotation. The detailed operation of the motor is Supplement content. After half a turn, the coil sides have exchanged positions, so an unchanged current would reverse the turning effect. A split-ring commutator prevents this by reversing current in the coil every half-turn. Each half of the ring connects to one coil end. Fixed brushes connect the rotating commutator to the direct current supply.

10:31The reversal makes the forces change direction just as the sides exchange positions, maintaining rotation in the same sense. The coil’s momentum carries it through positions where the turning effect is zero and contact may briefly break. Slip rings would not perform this switching action, so they are unsuitable for this simple direct current motor. Remember the functional distinction: continuous connections for the generator, switched connections for the motor.

Transformers and power transmission

10:55A transformer changes an alternating voltage using two coils wound on a soft-iron core. The primary coil receives the input, while the secondary supplies the output. The coils are electrically separate, but the core links their magnetic fields. A step-up transformer raises voltage, and a step-down type lowers it. Unlike a motor or generator, a simple transformer does not need rotating parts. The explanation of its operation is Extended content.

11:22Alternating current in the primary produces a changing magnetic field in the core, which links the secondary turns. This changing field induces an electromotive force in the secondary coil. Soft iron is suitable because it magnetises and demagnetises readily. Direct current produces a constant field once switched on, so it cannot maintain an induced secondary voltage. For voltage transformations, primary voltage divided by secondary voltage equals primary turns divided by secondary turns.

11:50Equivalently, secondary voltage divided by primary voltage equals secondary turns divided by primary turns. Each turn experiences approximately the same changing magnetic field, so additional turns contribute extra voltage. More secondary turns therefore mean a step-up transformer. Fewer secondary turns mean a step-down transformer. Consider an ideal transformer with one thousand primary turns, two hundred secondary turns, and an input of two hundred and thirty volts. Secondary voltage equals primary voltage multiplied by secondary turns divided by primary turns.

12:23Substitute 230 × 200 ÷ 1000. The output is forty-six volts, making this a step-down transformer because its secondary has fewer turns. For Extended calculations, an ideal transformer has one hundred per cent efficiency, so input power equals output power. Primary current multiplied by primary voltage therefore equals secondary current multiplied by secondary voltage. A voltage increase must be accompanied by a current decrease for the same power transfer. Real transformers lose some energy, so their output power is lower than the input.

12:58Suppose the transformer in our example supplies a secondary current of two amperes at forty-six volts. Output power equals 46 × 2, giving ninety-two watts. For an ideal transformer, input power is also ninety-two watts. Primary current equals input power divided by primary voltage, so 92 ÷ 230 gives zero point four amperes. The lower secondary voltage allows a larger output current without creating extra energy. Electricity transmission uses step-up transformers to raise voltage before electrical energy travels along long cables.

13:31Step-down transformers then reduce it for consumers. For the same transmitted power, increasing voltage reduces current. A smaller current wastes less energy heating the cables, so transmission is more efficient and thinner cables can be used. Extended students explain this using the equation: power loss equals current squared multiplied by cable resistance. Suppose cables transmit one hundred thousand watts at one thousand volts, and their total resistance is two ohms.

13:59Current equals power divided by voltage, giving one hundred amperes. Cable power loss equals 100² × 2, giving twenty thousand watts. At ten thousand volts, the same transmitted power requires only ten amperes. The loss becomes 10² × 2, or two hundred watts.

Exam recap and key distinctions

14:17An exam might ask you to describe the observation when a magnet approaches a connected coil. State that the meter deflects while the magnet moves. Explain that the changing magnetic field linking the coil induces an electromotive force, causing current in the complete circuit. To state one change producing a larger deflection, give faster movement, a stronger magnet, or more coil turns. Each sentence supplies a distinct creditworthy point. For an Extended question about direction, state that the induced current opposes the change causing it.

14:48An approaching north pole makes the near coil face north, so repulsion opposes the approach. Do not say that induction always opposes the magnetic field, because withdrawal produces a different response. In graph questions, distinguish the coil’s plane from a line perpendicular to that plane when identifying peaks and zeros. Keep the main distinctions secure across all six sections. Induction requires cutting field lines or a changing field, while the motor effect requires current in a magnetic field.

15:15Slip rings maintain connections in an alternating current generator, whereas a split-ring commutator reverses coil current in a direct current motor. A transformer needs a changing field, not merely an iron core. High transmission voltage reduces cable losses by reducing current, not resistance. The connecting idea is that electric currents and magnetic fields interact. A changing field can induce voltage, and a current creates its own magnetic field. In an external field, that current can experience force and produce movement.

15:45Generators turn movement into electrical energy, motors do the reverse, and transformers link coils at different voltages. Understanding those relationships makes the devices much easier to explain.