Title: InkPhysics / Magnetic fields
On every note of this deck, unless a note says otherwise:
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Front: DEFINITION
Define: Half-wave rectification.
Back: Rectification using a single diode, so only one half of each cycle passes and the other half is blocked.

Front: SPOT THE ERROR
Say what is wrong with this claim: Fields always push along their field lines.
Back: The magnetic force does not act along the field lines: it acts at right angles to both the field and the current, as Fleming's left-hand rule shows.

Front: RECALL
State the unit of rate of change of flux linkage.
Back: The volt: 1 Wb s^−1 is 1 V.

Front: RECALL
State the time between peaks of a full-wave rectified 50 Hz supply.
Back: 10 ms: full-wave rectification gives a peak every half-cycle.

Front: DEFINITION
Define: Rectification.
Back: Conversion of alternating current to direct current.

Front: RECALL
State what the oscilloscope timebase sets.
Back: The time represented by each horizontal division of the screen, in seconds per division.

Front: DEFINITION
Define: Fleming's left hand rule.
Back: First finger along the field, second finger along the conventional current, thumb gives the direction of the force, all three at right angles.

Front: RECALL
Explain why Lenz's law follows from energy conservation.
Back: A current aiding the change would create electrical energy from nothing, so the induced current must oppose the change.

Front: RECALL
State the period of a charge circling in a magnetic field.
Back: T = 2πm/BQ, independent of the particle's speed, provided that speed stays well below the speed of light.

Front: DEFINITION
Define: The weber.
Back: The flux through one square metre held at right angles to a field of one tesla: 1 Wb = 1 T m^2.

Front: DEFINITION
Define: Ripple.
Back: The small periodic variation left in a smoothed output as the capacitor discharges between peaks.

Front: SPOT THE ERROR
Say what is wrong with this claim: Magnetic fields can speed particles up.
Back: A magnetic field cannot speed a particle up: the force is always perpendicular to the velocity, so it does no work and only changes the direction of motion.

Front: DEFINITION
Define: Full-wave rectification.
Back: Rectification using a bridge of four diodes, which reverses the negative half-cycles so both halves of the cycle drive current one way through the load.

Front: SPOT THE ERROR
Say what is wrong with this claim: θ is the angle between the field and the coil.
Back: θ in Φ = BA cos θ is the angle between the field and the normal to the coil's plane, not the plane itself; field straight through the coil gives θ = 0 and maximum flux.

Front: SPOT THE ERROR
Say what is wrong with this claim: A transformer can step up power.
Back: A transformer cannot step up power: stepping the voltage up steps the current down in the same ratio, and a real transformer delivers slightly less power than it takes in.

Front: SPOT THE ERROR
Say what is wrong with this claim: The emf is biggest when the most flux passes through the coil.
Back: Flux itself induces nothing: the emf equals the rate of change of flux linkage, so it is greatest where the flux is changing fastest and zero at maximum flux.

Front: RECALL
State how mean power compares with peak power in a resistor.
Back: For a sinusoidal supply the mean power dissipated is half the peak power.

Front: DEFINITION
Define: Root-mean-square value.
Back: The value of the steady direct current that would dissipate the same average power in a resistor. For a sinusoid, and only for a sinusoid, I_rms = I₀/√2.

Front: EQUATION · NOT ON THE DATA SHEET
Radius of a charge's circular path in a field:
r = ?\(r = \, ?\)r = ?
Back: r = mvBQ\(r = \frac{mv}{BQ}\)r = mv/BQ

Front: RECALL
State the condition for zero magnetic force on a charged particle.
Back: The particle is stationary or moves parallel to the field.

Front: DEFINITION
Define: Reservoir capacitor.
Back: The capacitor across the load that charges near each peak of the rectified supply and discharges through the load between the peaks.

Front: RECALL
State the purpose of laminating a transformer core.
Back: Thin insulated layers break eddy-current loops into small ones, cutting the energy lost heating the core.

Front: DEFINITION
Define: Eddy currents.
Back: Currents induced in the iron core itself by the changing flux, which waste energy by heating the core.

Front: RECALL
State the condition for effective smoothing.
Back: The time constant RC must be long compared with the time between peaks of the rectified supply.

Front: DEFINITION
Define: Flux linkage.
Back: The flux through a coil multiplied by its number of turns: NΦ.

Front: RECALL
State the general force on a current-carrying wire, with the angle defined.
Back: F = BIl sin θ, with θ the angle between the wire and the magnetic field. θ = 90° returns the perpendicular case F = BIl, and θ = 0 gives zero force. CIE, Edexcel and OCR all set it; OCR and Edexcel print it and CIE makes you recall it. AQA examines only the perpendicular form.

Front: DEFINITION
Define: Faraday's law.
Back: The magnitude of the induced emf equals the rate of change of flux linkage.

Front: DEFINITION
Define: Lenz's law.
Back: The induced emf acts in the direction that opposes the change of flux producing it.

Front: RECALL
State the general magnetic force on a moving charge, with the angle defined.
Back: F = BQv sin θ, with θ the angle between the velocity and the magnetic field. θ = 90° returns the perpendicular case F = BQv, and θ = 0 gives zero force on a charge flying along the field. CIE and Edexcel both set it; Edexcel prints it on its sheet and CIE makes you recall it. AQA and OCR examine only the perpendicular form.

Front: SPOT THE ERROR
Say what is wrong with this claim: Mains is 230 volts at its peak.
Back: 230 V is the rms value of the mains, not the peak: the peak voltage is √2 times larger, about 325 V.

Front: RECALL
State the speed selected by a velocity selector.
Back: v = E/B, the one speed at which the electric and magnetic forces balance so the particle passes undeflected.

Front: RECALL
State what a search coil's oscilloscope amplitude shows.
Back: In an alternating field the amplitude of the induced trace is proportional to the flux linkage through the coil.

Front: DEFINITION
Define: Magnetic flux density.
Back: The force per unit current per unit length on a wire carrying current at right angles to the field: B = F/IL, in tesla.

Front: RECALL
State the unit of flux linkage.
Back: The weber-turn (Wb turns).

Front: RECALL
Explain why a transformer needs ac.
Back: Only an alternating current produces a changing core flux, and only changing flux linkage induces an emf in the secondary.

Front: DEFINITION
Define: Smoothing.
Back: Reducing the variation of a rectified supply using a reservoir capacitor across the load.

Front: RECALL
State the emf across a conductor moving through a magnetic field.
Back: ε = Blv for a conductor of length l moving at speed v at right angles to the field.

Front: SPOT THE ERROR
Say what is wrong with this claim: Rectified ac is direct current, so it is steady.
Back: Rectified current is one-directional but not steady: it still rises and falls each half-cycle. A reservoir capacitor smooths it towards steady dc, and some ripple always remains.

Front: DEFINITION
Define: Peak-to-peak value.
Back: The full span from minimum to maximum of the waveform, twice the peak value for a sinusoid.

Front: RECALL
State the condition for zero force on a current-carrying wire.
Back: The wire lies parallel to the magnetic field, so sin θ = 0 in F = BIl sin θ.

Front: RECALL
State the magnetic force on a moving charge, with its condition.
Back: F = BQv, for a charge moving at right angles to the field; only the component of velocity perpendicular to the field counts, and the force acts at right angles to both.

Front: RECALL
Explain why grid transmission uses high voltage.
Back: For a fixed power a higher voltage means a smaller current, and the cable loss I^2R falls with the square of the current.

Front: DEFINITION
Define: Magnetic flux.
Back: The product of flux density and the area it passes through at right angles: Φ = BA, in webers.

Front: DEFINITION
Define: Describe the magnetic field pattern of a flat circular coil.
Back: The field runs through the centre of the coil, straightest and strongest there, then spreads out and loops back round the outside, so one face is north and the other south, like a flattened bar magnet. The right-hand grip rule fixes which face is which. (CIE only.)

Front: RECALL
State the diagram symbols for field into and out of the page.
Back: A circled cross shows a field into the page; a circled dot shows a field out of the page.

Front: RECALL
State the condition for the root-two rms relations.
Back: I_rms = I₀/√2 and V_rms = V₀/√2 hold for sinusoidal waveforms only.