Magnetism & transformers

Like poles repel, unlike poles attract. Field lines run from north to south and are densest where the field is strongest. Soft magnetic materials (iron) magnetise and demagnetise easily; hard ones (steel) keep their magnetism. A magnet can induce magnetism in a nearby magnetic material.

A current creates a magnetic field around it: circular field lines around a straight wire, and a bar-magnet-like field through a solenoid (coil). The field is stronger with more current or more turns. Unlike a permanent magnet, an electromagnet can be switched off.

A current-carrying wire in a magnetic field experiences a force. For a wire at right angles to a uniform field, $F=BIL$. Fleming's left-hand rule gives the direction (thumb = force/motion, first finger = field, second finger = current). This is the basis of the dc motor.

A voltage is induced when a wire cuts magnetic field lines, or when the field through a coil changes. A bigger/faster change gives a bigger voltage; reversing the motion reverses the voltage. An ac generator spins a coil in a field, producing an alternating output.

A transformer changes voltage using the turns ratio: $\frac{V_p}{V_s}=\frac{n_p}{n_s}$. A step-up transformer has more turns on the secondary. Assuming 100% efficiency, power in = power out, so $V_p{I}_p=V_s{I}_s$.

The grid transmits power at high voltage so the current is small, and therefore so are the $I^{2}R$ heating losses in the cables.

• Confusing the two ratios (voltage scales with the turns, current scales inversely).
• Forgetting that induction needs a *changing* field, not just a field.
• Reversing Fleming's left-hand rule fingers.