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Flashcards · J249 · Topic 4

Magnetism and magnetic fields flashcards

19 cards on Topic 4, subtopics P4.1 to P4.2. Pick a subtopic or study them all.

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P4.1 Magnets and magnetic fields

Magnetic field
The space around a magnet or a current in which magnetic materials and other magnets experience a force.
Permanent vs induced magnet
Permanent: keeps its magnetism, always has two poles, can attract or repel. Induced: a magnetic material that is a magnet only while in another magnet’s field; always attracted.
What is the only sure test that an object is a magnet?
Repulsion. Attraction could just mean it is a magnetic material (iron, steel, cobalt or nickel).
Rules for magnetic field lines
Outside a magnet they go from north to south, never cross, and are closest (strongest field) at the poles.
What does a dipping compass show about the Earth?
The needle is nearly horizontal near the equator and nearly vertical near the magnetic poles, like the field of a bar magnet, so the Earth’s core must be magnetic.
Magnetic field around a straight current-carrying wire
Concentric circles at right angles to the wire. Right-hand grip rule: thumb along the conventional current, fingers curl along the field.
Ways to make a solenoid’s field stronger
More current, more turns, turns closer together, and an iron core (which makes it an electromagnet).

P4.2 Uses of magnetism

Motor effectHigher
A current-carrying wire in a magnet’s field experiences a force, at right angles to both the wire and the field. There is no force if the wire is parallel to the field.
Fleming’s left-hand ruleHigher
First finger: Field (north to south). Second finger: Current (positive to negative). Thumb: force (Motion). Left hand only, all three at right angles.
Force on a conductor in a magnetic field: equationHigher
F = B × I × l (N, T, A, m), for a conductor at right angles to the field. B is the magnetic flux density.
A wire with 0.15 m in a 0.40 T field carries 5.0 A at right angles. Force?Higher
F = 0.40 × 5.0 × 0.15 = 0.30 N
How does a d.c. motor keep turning one way?Higher
Opposite forces on the two sides of the coil turn it. The current in the coil is reversed every half turn, so each side keeps being pushed the same way round.
When is a potential difference induced in a conductor?Higher
When the magnetic field around it changes (moving a magnet, moving the conductor across field lines, switching the field). A current flows only if the circuit is complete.
Direction of an induced current’s magnetic fieldHigher
It opposes the change that caused it.
Alternator vs dynamoHigher
Alternator: slip rings, output a.c. Dynamo: split-ring commutator, output d.c. (pulsing, two pulses per rotation).
How does a transformer work?Higher
Alternating current in the primary coil gives a changing magnetic field in the core. This changing field passes through the secondary coil and induces an alternating pd across it. A steady d.c. supply induces nothing.
A transformer has 500 primary and 125 secondary turns, Vp = 80 V. Find Vs.Higher
Vs = Vp × Ns ÷ Np = 80 × 125 ÷ 500 = 20 V (step-down)
How does a microphone turn sound into a current?Higher
Sound makes the diaphragm and attached coil vibrate in a permanent magnet’s field, so a pd is induced and an alternating current flows with the frequency of the sound.
How does a loudspeaker make sound?Higher
A varying current in the coil, in a permanent magnet’s field, gives a force that keeps reversing, so the coil and cone vibrate and make pressure variations in the air.

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