Title: InkPhysics / Electricity
On every note of this deck, unless a note says otherwise:
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Front: DEFINITION
Define: Kirchhoff's second law.
Back: Round any closed loop the sum of the emfs equals the sum of the pds: conservation of energy.

Front: DEFINITION
Define: Resistivity.
Back: The property of a material given by ρ = RA/L: the resistance of a piece of the material of unit length and unit cross-sectional area.

Front: RECALL
State the resistance of an ideal ammeter and an ideal voltmeter.
Back: An ideal ammeter has zero resistance and an ideal voltmeter has infinite resistance.

Front: RECALL
Explain how to find emf and internal resistance from a V-I graph.
Back: The intercept on the V axis is the emf and the gradient is −r.

Front: DEFINITION
Define: Resistance.
Back: The ratio of the potential difference across a component to the current through it: R = V/I.

Front: SPOT THE ERROR
Say what is wrong with this claim: A 1.5 V battery gives you 1.5 volts.
Back: A 1.5 V cell gives each coulomb 1.5 J, but some is dissipated in the cell's internal resistance, so the terminal pd V = ε − Ir is below 1.5 V whenever current flows.

Front: SPOT THE ERROR
Say what is wrong with this claim: A material has a resistance.
Back: Resistance belongs to a particular object and depends on its dimensions: R = ρL/A. Resistivity is the material's own property, the same for any sample of that material.

Front: RECALL
State the assumption behind the potential divider equation.
Back: That the output draws negligible current. A load across the output sits in parallel with that resistor and drags the output below the value the ratio promises, unless the load resistance is far larger than the divider resistances.

Front: DEFINITION
Define: The coulomb.
Back: The charge passing a point in one second when the current is one ampere: 1 C = 1 A s.

Front: EQUATION · NOT ON THE DATA SHEET
Electrical energy transferred in time t:
E = ?\(E = \, ?\)E = ?
Back: E = IVt\(E = IVt\)E = IVt

Front: RECALL
Explain why a metal's resistance rises when heated.
Back: The lattice ions vibrate with greater amplitude, so the conduction electrons collide with them more often.

Front: DEFINITION
Define: Semiconductor diode.
Back: A component that conducts in the forward direction only, with very high resistance in reverse.

Front: SPOT THE ERROR
Say what is wrong with this claim: Ohm's law applies to everything.
Back: Ohm's law is a special case, not a universal law: only ohmic conductors at constant temperature have current proportional to pd. Filament lamps and diodes do not, so their resistance varies.

Front: SPOT THE ERROR
Say what is wrong with this claim: The current arrows show which way the electrons go.
Back: Conventional current is defined from + to −, a convention fixed before the electron was known. In a metal the charge carriers are electrons, which drift the opposite way.

Front: RECALL
State the shape of a filament lamp I-V characteristic.
Back: A curve through the origin that flattens at larger pds as the heating filament raises its resistance.

Front: RECALL
State how the potential varies along a uniform current-carrying wire.
Back: It falls at a steady rate along the wire, so the pd across any part of it is proportional to the length of that part.

Front: DEFINITION
Define: Thermistor (NTC).
Back: A semiconductor component whose resistance falls steeply as temperature rises.

Front: RECALL
Explain why series resistors share the supply pd in the ratio of their resistances.
Back: The same current flows through both, so each pd V = IR is proportional to its resistance.

Front: DEFINITION
Define: Electromotive force.
Back: The energy transferred to each coulomb of charge passing through a source: ε = E/Q.

Front: RECALL
State the three forms of electrical power.
Back: P = IV = I^2R = V^2/R.

Front: DEFINITION
Define: Internal resistance.
Back: The resistance inside a source, in which energy is dissipated whenever the source drives a current.

Front: RECALL
State the range of a potential divider output.
Back: From zero up to the full supply pd, set by the ratio of the resistances.

Front: SPOT THE ERROR
Say what is wrong with this claim: Current gets used up on the way round.
Back: Current is not used up: charge is conserved, so the current returning to the battery equals the current leaving it. What is transferred round the circuit is energy per coulomb, the pd.

Front: RECALL
State the unit of resistivity.
Back: The ohm metre (Ω m), not the ohm per metre.

Front: DEFINITION
Define: Superconductor.
Back: A material whose resistivity falls to exactly zero when cooled to or below its critical temperature.

Front: DEFINITION
Define: Ohm's law.
Back: The current through an ohmic conductor is proportional to the potential difference across it, provided physical conditions such as temperature stay constant.

Front: RECALL
State the drift speed equation.
Back: I = nAvq, where n is the number of charge carriers per cubic metre, A the cross-sectional area, v the drift speed and q the charge on each carrier.

Front: RECALL
State the typical electron drift speed in a copper wire.
Back: Around 10^−4 m s^−1, a fraction of a millimetre per second.

Front: DEFINITION
Define: Terminal potential difference.
Back: The potential difference across a source's terminals: V = ε − Ir, equal to the emf when no current flows.

Front: EQUATION · NOT ON THE DATA SHEET
Potential divider output:
V_out = ?\(V_{out} = \, ?\)V_out = ?
Back: V_out = V_in × R_2R_1 + R_2\(V_{out} = V_{in} \times \frac{R_{2}}{R_{1} + R_{2}}\)V_out = V_in × R_2/(R_1 + R_2)

Front: RECALL
State the effect of doubling a wire's diameter on its resistance.
Back: The resistance falls by a factor of four, because area scales with diameter squared.

Front: DEFINITION
Define: Potential difference.
Back: The work done per unit charge moving between two points: V = W/Q, in volts.

Front: RECALL
State the short-circuit current of a cell.
Back: With the terminals shorted, R = 0 and the current takes its maximum value I = ε/r.

Front: SPOT THE ERROR
Say what is wrong with this claim: The battery decides the voltage across each component.
Back: The battery fixes only the total pd across the series chain. The same current flows through each resistor, so each takes a share of the pd proportional to its resistance.

Front: DEFINITION
Define: Electric current.
Back: The rate of flow of charge: I = ΔQ/Δt.

Front: RECALL
Explain why an LDR's resistance falls in brighter light.
Back: Absorbed photons free extra conduction electrons, so the number density n rises and the same pd drives a larger current.

Front: RECALL
State the combined resistance of resistors in parallel.
Back: Always less than the smallest individual branch resistance.

Front: DEFINITION
Define: Light-dependent resistor (LDR).
Back: A component whose resistance falls as the light intensity falling on it rises.

Front: RECALL
State the applications of superconductors.
Back: Very strong electromagnets, as in MRI scanners and maglev, and power cables with no resistive loss.

Front: DEFINITION
Define: Threshold voltage of a diode.
Back: The forward potential difference, about 0.6 V, above which a diode conducts with low resistance.

Front: DEFINITION
Define: Kirchhoff's first law.
Back: The total current into a junction equals the total current out: conservation of charge.

Front: RECALL
Explain why a lamp lights instantly despite slow electron drift.
Back: The electric field is established round the circuit at nearly the speed of light, so electrons everywhere start drifting almost at once.

Front: RECALL
Explain how to read resistance from an I-V characteristic.
Back: R is the ratio V/I at the chosen point, never the gradient of a curved characteristic.