Flashcards · 8463 · Topic 7

Magnetism and electromagnetism flashcards

23 cards on Topic 7, subtopics 4.7.1 to 4.7.3. Pick a subtopic or study them all.

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Magnetic pole: The place on a magnet where the magnetic forces are strongest.
Permanent vs induced magnet: Permanent: produces its own magnetic field all the time; can attract or repel. Induced: becomes a magnet only when placed in a magnetic field, loses most or all of it quickly when removed, and always attracts.
Magnetic field, and the direction of field lines: The region around a magnet where a force acts on another magnet or a magnetic material. Field lines point from the north pole to the south pole.
The four magnetic materials: Iron, steel, cobalt and nickel.
What evidence does a compass give about the Earth’s core?: A compass needle lines up with a magnetic field wherever it is, which shows the Earth has a magnetic field, produced by its magnetic core.
Where is a magnet’s field strongest, and how do field lines show it?: At the poles. The closer together the field lines, the stronger the field.

Magnetic field around a straight current-carrying wire: Concentric circles around the wire. Stronger with a larger current; weaker further from the wire. Direction from the right-hand grip rule.
Field of a solenoid, and how to make it stronger: Strong and uniform inside, shaped like a bar magnet’s outside. Adding an iron core makes it stronger: this is an electromagnet.
Motor effectHigher: A conductor carrying a current in a magnetic field and the magnet producing the field exert a force on each other.
Fleming’s left-hand ruleHigher: First finger = Field; seCond finger = Current; thuMb = Motion (force). Used to find the direction of the force on a wire.
Force on a wire in a magnetic field: equationHigher: F = BIl (F in N, B magnetic flux density in T, I in A, l length in the field in m), for a wire at right angles to the field. The force is zero if the wire is parallel to the field.
A 0.30 m wire carries 2.5 A at right angles to a 0.40 T field. Force?Higher: F = 0.40 × 2.5 × 0.30 = 0.30 N
How does an electric motor keep turning?Higher: Current flows in opposite directions in the two sides of the coil, so the forces on them are in opposite directions and the coil rotates. A split-ring commutator keeps it turning one way.
How does a loudspeaker work?Higher: Alternating current in a coil in a permanent magnet’s field produces a force that reverses as the current reverses, so the cone vibrates and makes pressure variations: sound.

Generator effectHigher: A potential difference is induced across a conductor that moves relative to a magnetic field, or that is in a changing field. In a complete circuit it drives an induced current.
Direction of the field from an induced currentHigher: It opposes the change that induced it.
Three ways to increase the potential difference induced by moving a magnet in a coilHigher: Move it faster, use a stronger magnet, use more turns on the coil.
Alternator vs dynamoHigher: Alternator: slip rings, gives ac (the output graph crosses the axis). Dynamo: split-ring commutator, gives dc (the graph stays on one side).
MicrophoneHigher: Sound moves a diaphragm and coil in a magnet’s field, inducing a pd, so the current varies like the sound (generator effect).
How does a transformer work?Higher: Alternating current in the primary coil makes a changing magnetic field in the iron core. This changing field passes through the secondary coil and induces an alternating pd across it.
Transformer equationsHigher: Vp / Vs = np / ns Vs × Is = Vp × Ip (for a 100% efficient transformer).
A transformer has 200 primary and 50 secondary turns; Vp = 240 V. Find Vs.Higher: Vs = 240 × 50 ÷ 200 = 60 V. Fewer secondary turns, so it is a step-down transformer.
Why is the core of a transformer made of iron?Higher: Iron is easily magnetised.