Magnetism and electromagnetism

Explanation

• Magnetic forces are strongest at a magnet's north and south poles; like poles repel and unlike poles attract without touching.
• A permanent magnet makes its own magnetic field, whereas an induced magnet becomes magnetic only while it is in another magnetic field.
• For example, either pole of a permanent magnet attracts an unmagnetised iron nail because the nail becomes an induced magnet with the nearer end as the opposite pole.
• A common error is to predict repulsion from an induced magnet: induced magnetism produces attraction, and most or all of it is lost quickly when the field is removed.

Explanation

• A magnetic field is the region where a magnet exerts a force on another magnet or on iron, steel, cobalt or nickel.
• Plot a field by placing a compass at successive positions, marking the direction of its north-seeking end, and joining the marks with smooth directed lines.
• Outside a bar magnet, field lines point from north to south and are closest at the poles; compass alignment with Earth's field is evidence that Earth's core must be magnetic.
• A common error is to draw crossing field lines or arrows from south to north outside the magnet; each point has one field direction, defined by the force on a north pole.

Explanation

• A current in a straight conductor creates concentric field lines; increasing current strengthens the field, increasing distance weakens it, and a nearby compass deflection demonstrates the effect.
• Use the right-hand grip rule for direction: point the thumb in the conventional-current direction and the curled fingers show the field direction.
• A solenoid's turns reinforce to make a strong, nearly uniform internal field and a bar-magnet-shaped external field; adding an iron core strengthens it and makes an electromagnet.
• When interpreting an electromagnetic-device diagram, trace current to magnetic field and then to force or motion; a common error is to name the electromagnet without explaining that causal chain.

Explanation

• The motor effect is the force produced when a current-carrying conductor lies in a magnetic field; the conductor and field-producing magnet exert forces on each other.
• For Fleming's left-hand rule, hold the thumb, first finger and second finger mutually perpendicular: thumb is force, first finger is field from north to south, and second finger is conventional current.
• For a conductor perpendicular to the field, calculate force with $F=BIl$; for example, $B=0.30\,\text{T}$, $I=2.0\,\text{A}$ and $l=0.40\,\text{m}$ give $F=0.24\,\text{N}$.
• A common error is to use electron flow for the current finger or to apply $F=BIl$ unchanged when the conductor is not at right angles to the field.

Explanation

• In a motor, opposite sides of a current-carrying coil experience forces in opposite directions because their currents run oppositely through the magnetic field.
• These forces form a turning effect; use Fleming's left-hand rule separately on each active side to predict the rotation direction.
• A split-ring commutator reverses the current every half-turn, so the forces swap sides and the turning effect continues in the same rotational direction.
• A common error is to say the coil turns because unlike poles attract; the required explanation is the motor-effect force on current-carrying conductors in a magnetic field.

Explanation

• A moving-coil loudspeaker uses the motor effect to convert variations in electrical current into pressure variations in a sound wave.
• The alternating current in the voice coil repeatedly reverses, so the motor-effect force reverses and the coil moves backwards and forwards in the permanent magnet's field.
• The coil is attached to a cone: larger current variations produce larger cone displacements and a louder sound, while faster variations produce a higher-frequency sound.
• A common error is to describe electromagnetic induction in a loudspeaker; induction is used by a microphone, whereas a loudspeaker requires force on a supplied current.

Explanation

• The generator effect induces a potential difference when a conductor moves relative to a magnetic field or when the magnetic field around it changes; a complete circuit then carries an induced current.
• Increase the induced potential difference by increasing relative speed, field strength or coil turns; reversing either the relative motion or the magnetic-field direction reverses the induced potential difference and current.
• The induced current creates its own magnetic field opposing the change that produced it, so pushing a magnet into a coil produces a magnetic effect that resists the push.
• A common error is to say a stationary magnet permanently induces current in a stationary coil: induction requires relative motion or a changing magnetic field.

Explanation

• An alternator rotates a coil in a magnetic field and generates alternating potential difference, so its output reverses every half-turn.
• A dynamo uses a split-ring commutator to exchange coil connections every half-turn, producing a direct potential difference of one polarity.
• For steady rotation, the alternator graph crosses zero when the coil sides move parallel to the field and reaches maximum magnitude when they cut field lines at the greatest rate.
• A common error is to draw a constant dc line for a dynamo: its output keeps one polarity but normally varies in magnitude as the coil rotates.

Explanation

• A moving-coil microphone uses the generator effect to convert pressure variations in sound into variations in current in an electrical circuit.
• Sound pressure makes a diaphragm vibrate; its attached coil moves relative to a permanent magnet's field, inducing a varying potential difference.
• The induced signal follows the diaphragm: greater sound amplitude gives greater coil speed and signal amplitude, while sound frequency sets the signal frequency.
• A common error is to supply a driving current to the microphone coil; the sound-driven motion induces the signal, whereas supplied current drives a loudspeaker.

Explanation

• A transformer has primary and secondary coils wound on an easily magnetised iron core; alternating current in the primary produces a changing core field that induces a potential difference in the secondary.
• Use $\dfrac{V_{\mathrm{p}}}{V_{\mathrm{s}}}=\dfrac{n_{\mathrm{p}}}{n_{\mathrm{s}}}$: more secondary turns than primary turns gives a step-up transformer, and fewer gives a step-down transformer.
• For an ideal transformer, input and output powers are equal, so $V_{\mathrm{p}}I_{\mathrm{p}}=V_{\mathrm{s}}I_{\mathrm{s}}$; raising transmission potential difference reduces current for the same power and therefore reduces heating losses.
• A common error is to connect a transformer to steady dc: without a changing primary current there is no continuously changing core field and therefore no continuous secondary potential difference.