Flashcards · 4PH1 · Topic 6

Magnetism and electromagnetism flashcards

25 cards on Topic 6, subtopics 6a to 6d. Pick a subtopic or study them all.

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6a Units

Units of current, voltage and power used in electromagnetism: Current: ampere (A). Voltage: volt (V). Power: watt (W).
Express 1 watt in terms of amperes and volts: 1 W = 1 A × 1 V, since power = current × voltage.
Units in the ideal transformer relation Vp × Ip = Vs × Is: Voltages in volts (V), currents in amperes (A); each side is a power in watts (W).

6b Magnetism

What do magnets attract and repel?: Magnets repel and attract other magnets (like poles repel, unlike attract), and attract magnetic substances.
Magnetically hard vs soft materials: Hard (steel): keeps its magnetism; used for permanent magnets. Soft (iron): loses it easily; used for electromagnet cores.
Magnetic field lines: Run from north to south outside the magnet, never cross, and are closer together where the field is stronger.
Induced magnetism: Magnetism produced in a magnetic material placed in a magnetic field. It is temporary: it appears only in the field.
How can a uniform magnetic field be produced?: With two permanent magnets, opposite poles facing each other: the field between them is uniform.

6c Electromagnetism

Magnetic field of a current-carrying wire: Circular field lines around the wire. Right-hand grip rule: thumb along the current, fingers curl in the field direction.
Field patterns of a straight wire, a flat coil and a solenoid: Straight wire: concentric circles. Flat coil: like a short bar magnet. Solenoid: like a bar magnet, strong and nearly uniform inside. (Paper 2 only)
How is an electromagnet made, and made stronger?: A coil of wire (solenoid) carrying a current, usually around a soft iron core. Stronger with more current, more turns, or a soft iron core. (Paper 2 only)
Fleming's left-hand rule: For the force on a current-carrying wire in a magnetic field: first finger = field, second finger = current, thumb = motion (force).
When is the force on a current-carrying wire in a field zero, and how is it reversed?: Zero when the wire is parallel to the field (maximum when perpendicular). Reversing either the current or the field reverses the force; reversing both leaves it unchanged.
How can the force on a current-carrying wire be increased?: Increase the current or use a stronger magnetic field.
What does the split-ring commutator in a d.c. motor do?: It reverses the current every half turn, so the force on each side keeps acting in the same rotational sense and the coil keeps turning.
How does a loudspeaker work?: An alternating current in a coil in a magnetic field produces a changing force (motor effect), making the cone vibrate and produce sound.
Force on a charged particle moving in a magnetic field: It experiences a force, unless it moves parallel to the field. (Paper 2 only)

6d Electromagnetic induction

When is a voltage induced in a conductor or coil?: When it moves through a magnetic field (cuts field lines), or when the magnetic field through it changes.
Three ways to increase an induced voltage: Move faster, use a stronger magnet, use more turns on the coil.
How is electricity generated by rotation?: By rotating a magnet inside a coil, or rotating a coil in a magnetic field; the changing field induces an alternating voltage.
Lenz's law, and why it must be true: The induced current opposes the change producing it. If it helped the motion, energy would be created from nothing, breaking conservation of energy.
Structure of a transformer, and why it needs a.c.: Two coils (primary and secondary) on a soft iron core. a.c. makes a continuously changing field in the core, inducing a voltage in the secondary; steady d.c. induces none. (Paper 2 only)
A transformer has 100 primary turns, 500 secondary turns and 12 V input. Output voltage?: Vp/Vs = Np/Ns: 12/Vs = 100/500, so Vs = 60 V (step-up). (Paper 2 only)
Why are step-up transformers used for transmission?: For the same power, a higher voltage means a lower current. Cable losses are I²R, so halving the current quarters the loss. Step-down transformers then lower the voltage for consumers. (Paper 2 only)
Power in a 100% efficient transformer: Input power = output power: Vp × Ip = Vs × Is. (Paper 2 only)