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OCR - P3: Electric circuits

P3.2 What determines the current in an electric circuit?
Background to the topic What you should be able to do:

An electric current is the rate of flow of charge; in an electric circuit the metal conductors (the components and wires) contain many charges that are free to move.

When a circuit is made, the battery causes these free charges to move, and these charges are not used up but flow in a continuous loop.

In a given circuit, the larger the potential difference across the power supply the bigger the current.

Components (for example, resistors, lamps, motors) resist the flow of charge through them; the resistance of connecting wires is usually so small that it can be ignored.

The larger the resistance in a given circuit, the smaller the current will be.

Representational models of electric circuits use physical analogies to help think about how an electric circuit works, and to predict what happens when a variable is changed .

1. Recall that current is a rate of flow of charge, that for a charge to flow, a source of potential difference and a closed circuit are needed and that a current has the same value at any point in a single closed loop.

2. Recall and use the relationship between quantity of charge, current and time:

charge (C) = current (A) × time (s)

3. Recall that current (I) depends on both resistance (R) and potential difference (V) and the units in which these quantities are measured

4. a) Recall and apply the relationship between I, R, and V, to calculate the currents, potential differences and resistances in d.c. series circuits:

potential difference (V) = current (A) × resistance (Ω)

b) Describe an experiment to investigate the resistance of a wire and be able to draw the circuit diagram of the circuit used.

5. Recall that for some components the value of R remains constant (fixed resistors) but that in others it can change as the current changes (e.g. heating elements, lamp filaments)

6. a) Use graphs to explore whether circuit elements are linear or non-linear and relate the curves produced to their function and properties.

b) Describe experiments to investigate the I-V characteristics of circuit elements.

To include: lamps, diodes, LDRs and thermistors.

Be able to draw circuit diagrams for the circuits used.

7. Represent circuits with the conventions of positive and negative terminals, and the symbols that represent common circuit elements, filament lamps, diodes, LDRs and thermistors, switches and fixed and variable resistors.