Uses of magnetism — OCR GCSE Physics

This transformer equation links the two coil voltages to the two turn counts. Write it as Vp/Vs = Np/Ns, where Vp is the potential difference across the primary coil, Vs the potential difference across the secondary coil, Np the number of turns in the primary coil and Ns the number of turns in the secondary coil. A step-down transformer has fewer secondary turns, so Vs is smaller than Vp. Example: Np = 400, Ns = 20, Vp = 230 V gives Vs = 230 × 20/400 = 11.5 V. Rearrange by cross-multiplying: Vp × Ns = Vs × Np. Keep voltages in volts and turns as whole counts; the ratio is dimensionless. This content appears only in the separate physics qualification and is Higher tier only.

A current-carrying conductor creates its own magnetic field around it. When this field interacts with the field of a nearby magnet, the two fields combine and the conductor experiences a force; by Newton's third law the magnet experiences an equal and opposite force. The force is greatest when the conductor is at right angles to the magnetic field and is zero when it is parallel to the field. Reversing the current or reversing the magnet's poles reverses the direction of the force. This is the motor effect, exploited in loudspeakers and electric motors. This content is Higher tier only.

Fleming's left-hand rule is a memory aid for the motor effect: a current-carrying conductor in a magnetic field can experience a force. Hold the left hand with thumb, first finger and second finger mutually at right angles. The first finger points along the magnetic field (from north to south pole), the second finger points along the conventional current, and the thumb then points along the force on the conductor. Because the three directions are perpendicular, rotating the hand rotates all three together, so the rule shows their relative orientations rather than a fixed compass direction. To show the rule works, take a known field direction and a known current direction, set the fingers accordingly, and read off the force direction; then check that reversing the current reverses the force while reversing the field also reverses it.

The force on a straight conductor carrying current at right angles to a uniform magnetic field is given by F = B × I × L, where F is the force in newtons (N), B is the magnetic flux density in tesla (T), I is the current in amperes (A) and L is the length of conductor in the field in metres (m). The equation applies when the conductor is perpendicular to the field; if it lies parallel to the field the force is zero. To calculate, convert length to metres and current to amperes, then multiply the three values, keeping units consistent. For example, B = 0.20 T, I = 3.0 A and L = 0.50 m give F = 0.20 × 3.0 × 0.50 = 0.30 N. Rearranging gives B = F ÷ (I × L), I = F ÷ (B × L) and L = F ÷ (B × I).

A current-carrying conductor placed in a magnetic field experiences a force, because the field of the current interacts with the field of the magnet. The direction of that force is given by Fleming's left-hand rule: first finger points along the field from north to south, second finger along the conventional current, and the thumb shows the direction of the force. In a motor, a coil carrying a current sits between the poles of a magnet. The two sides of the coil carry current in opposite directions, so the force on one side acts upwards while the force on the other acts downwards. This pair of forces on opposite sides of the coil produces a turning effect, so the coil rotates about its axis. A commutator reverses the current every half turn so the forces keep driving rotation in the same direction.

When the magnetic field around a conductor changes, a potential difference is induced across its ends; this is electromagnetic induction. The change can come from moving a magnet near a coil, moving the coil in a field, or changing the current in a nearby circuit. If the conductor forms part of a complete circuit, the induced potential difference drives a current. That induced current generates its own magnetic field, and the direction of this field opposes the original change, in line with Lenz's law. So moving a magnet towards a coil produces a current whose field repels the approaching magnet, resisting the motion. A faster change or a stronger magnet induces a larger potential difference. This content appears only in the physics separate science qualification.

Both machines rely on electromagnetic induction: a conductor experiences an induced potential difference when the magnetic flux through it changes. In an alternator, a coil rotates between the poles of a magnet, so the flux linkage rises, falls, reverses and rises again each half-turn. The induced p.d. therefore reverses direction every half rotation, giving an alternating current. Slip rings and brushes connect the spinning coil to the external circuit without swapping connections, so the output polarity follows the coil. In a dynamo, the same rotating coil is joined to a split-ring commutator; the commutator swaps the coil-to-circuit connections every half-turn, exactly when the induced p.d. would reverse, so the external current keeps one direction and is direct. Learners should link each structural feature to the resulting output, and may sketch or describe the p.d.

A transformer has two coils wound on a shared iron core. An alternating current in the primary coil creates a magnetic field that continually grows, collapses and reverses, so the magnetic flux linking the secondary coil changes continuously. This changing flux induces an alternating potential difference across the secondary, and if the secondary circuit is closed an alternating current flows. The iron core channels the flux to link the coils efficiently. Because induction needs change, a steady direct current in the primary gives no changing flux and no output, which is why transformers work only with a.c. The ratio of turns sets the voltage: a step-up transformer has more secondary turns, a step-down transformer fewer. For an ideal transformer the power is conserved, so Vp × Ip = Vs × Is, and the turns ratio satisfies Vp/Vs = Np/Ns.

A transformer has a primary coil of Np turns and a secondary coil of Ns turns. The alternating potential difference across the primary, Vp, produces a changing magnetic flux in the iron core, and this induces an alternating potential difference Vs across the secondary. For an ideal transformer the ratio of the potential differences equals the ratio of the turns: Vp/Vs = Np/Ns. So a step-up transformer has more turns on the secondary (Ns > Np) and gives a larger Vs; a step-down transformer has fewer secondary turns and gives a smaller Vs. Doubling Ns while keeping Np and Vp fixed doubles Vs. Rearranged, Vs = Vp × Ns/Np.

The linking equation for an ideal transformer is Vp/Vs = Np/Ns, where Vp and Vs are the potential differences across the primary and secondary coils and Np and Ns are their numbers of turns. Rearranged, Vs = Vp × Ns/Np and Ns = Np × Vs/Vp. To apply it, list the three known quantities, rearrange for the unknown, substitute values with consistent units (potential differences in volts, turns as plain counts) and calculate. For example, with Vp = 12 V, Np = 100 and Ns = 400, Vs = 12 × 400/100 = 48 V. Check that a step-up result is larger than Vp and a step-down result is smaller.

A dynamic microphone turns sound into a current by electromagnetic induction. Sound waves are pressure variations that make a diaphragm vibrate. The diaphragm is attached to a coil sitting in the gap of a permanent magnet. As the coil moves, the magnetic field lines it cuts change, so an e.m.f. is induced across the coil and a varying current flows in the circuit. Louder sounds give larger vibrations and bigger induced currents; higher-pitched sounds give faster vibrations and a higher frequency of current. A loudspeaker or headphone reverses this: an alternating current from the circuit flows through a coil in a magnetic field, the coil experiences a force (the motor effect) that changes direction as the current alternates, and the attached cone vibrates, producing pressure variations heard as sound.