B.5 Current and circuits

• Cells provide a source of emf.

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• Chemical cells and solar cells are energy sources in circuits. • Compare the advantages and disadvantages of different sources of electrical energy.

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• Explain the properties of electrical conductors and insulators in terms of the mobility of charge carriers. • Explain electrical resistance and its origin. • Use electrical resistance R=V/I.

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• Apply Ohm’s law: for an ohmic metal conductor at constant temperature, V ∝ I. • Distinguish ohmic and non-ohmic behaviour of electrical conductors, including the heating effect of resistors.

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• Electrical power: P=IV=I^2R=V^2/R.

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• Electric potential difference V is the work done W per unit charge q on moving a positive charge between two points along the path of the current: V=W/q. • Relate electrical energy transfer to power and time: E=Pt=IVt.

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• Cells convert chemical energy to electrical energy.

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• Circuit diagrams represent the arrangement of components in a circuit; use the required electrical circuit symbols provided in the Physics data booklet. • Conventional current follows the direction of positive charge flow. • Treat ammeters and voltmeters as ideal unless stated otherwise; a stated non-ideal meter has constant resistance.

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• Direct current I is a flow of charge carriers in one direction, with I=Δq/Δt. • Alternating-current circuit analysis is not required.

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• Distinguish direct current, whose direction does not reverse, from alternating current, whose direction reverses. • This distinction defines the syllabus boundary only; alternating-current circuit analysis is not required.

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• Use conservation of charge to explain why currents add at a junction in a parallel circuit. • Use conservation of energy to explain why potential differences add in a series circuit. • General multi-loop circuit analysis is not required.

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• Resistance depends on resistivity and dimensions: R=ρL/A.

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• Use series/parallel rules for current, voltage and equivalent resistance. • Series: same current; voltages and resistances add. • Parallel: same voltage; currents add; reciprocal resistance adds.

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• Cell emf with internal resistance: ε = I(R+r).

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• Resistors can have variable resistance. • Variable resistors are limited to thermistors, light-dependent resistors (LDRs) and potentiometers.

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