Flashcards · 9702 · A Level Topic 20

Magnetic fields (A Level) flashcards

24 cards on A Level Topic 20, subtopics 20.1 to 20.5. Pick a subtopic or study them all.

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Magnetic field: A region of space in which a magnet, a current-carrying conductor or a moving charge experiences a force. It is produced by moving charges (currents) or permanent magnets.
Magnetic field lines outside a bar magnet: They run from the north pole to the south pole, form closed loops, and are closer together where the field is stronger.
Magnetic flux density B: unit: The tesla (T). B is a measure of the strength of the magnetic field.

Force on a current-carrying wire in a magnetic field: F = BIL sin θ, where θ is the angle between the wire (current) and the field. Maximum at 90°, zero when the wire is parallel to the field.
The tesla: The flux density that gives a force of 1 N on each metre of a conductor carrying a current of 1 A at right angles to the field.
Fleming’s left-hand rule: First finger: field. Second finger: current (conventional). Thumb: motion (force). All at right angles to each other.
A 0.20 m wire carries 5.0 A at right angles to a 0.30 T field. Find the force.: F = BIL = 0.30 × 5.0 × 0.20 = 0.30 N.
Forces between two parallel current-carrying wires: Currents in the same direction: the wires attract. Currents in opposite directions: they repel. Each wire is in the other’s magnetic field.

Force on a charge moving in a magnetic field: F = BQv sin θ, where θ is the angle between the velocity and the field. For a negative charge, the conventional current is opposite to its velocity.
Path of a charged particle moving at right angles to a uniform magnetic field: A circle. The force is always perpendicular to the velocity, so it does no work: the speed and kinetic energy stay constant.
Radius of a charged particle’s circular path in a magnetic field: BQv = mv² ÷ r, so r = mv ÷ (BQ). Larger mass or speed: larger radius. Stronger field or larger charge: tighter circle.
Hall voltage: V_H = BI ÷ (ntq): B flux density, I current, n charge-carrier number density, t thickness of the slice (in the direction of B), q carrier charge. It is proportional to B, so a Hall probe can measure B.
Why is a semiconductor used in a Hall probe?: It has a small charge-carrier number density n, so V_H = BI ÷ (ntq) is larger and easier to measure.
Velocity selector: Crossed electric and magnetic fields. Only particles for which the forces balance, qE = Bqv, pass undeflected: v = E ÷ B.

Magnetic field of a long straight wire: Concentric circles around the wire. The field is stronger with a larger current and weaker further from the wire.
Magnetic field of a flat circular coil: Similar to that of a short bar magnet, with the field lines passing through the centre of the coil.
Magnetic field of a long solenoid: Strong and uniform inside (parallel, equally spaced lines); outside it is like the field of a bar magnet.
Effect of a ferrous (iron) core in a solenoid: The core becomes magnetised and greatly increases the flux density for the same current: the basis of an electromagnet.

Magnetic flux and flux linkage: Flux Φ = BA cos θ, θ measured from the normal to the area (Wb). Flux linkage = NΦ for a coil of N turns.
Faraday’s law: The magnitude of the induced e.m.f. is proportional to (equal to) the rate of change of magnetic flux linkage: E = −d(NΦ)/dt.
Lenz’s law: The direction of the induced e.m.f. is such that it opposes the change producing it. It is the minus sign in Faraday’s law.
Why is Lenz’s law a consequence of conservation of energy?: If the induced current helped the change, energy would be created from nothing. Because it opposes it, work must be done (for example, pushing a magnet in) to produce the electrical energy.
A magnet is pushed into a coil connected to a centre-zero galvanometer, held still, then pulled out. What is seen?: In: a brief deflection. At rest: no deflection (no change in flux linkage). Out: a deflection the other way.
Three ways to increase the e.m.f. induced by moving a magnet into a coil: Move the magnet faster, use a stronger magnet, use more turns. Each increases the rate of change of flux linkage.