Current, Potential Difference and Resistance Notes

Electric current is the rate of flow of charge. In a metal it is a flow of electrons. It is measured in amperes (A); charge is in coulombs (C) and time in seconds (s).

Rearranging gives $I = \dfrac{Q}{t}$ and $t = \dfrac{Q}{I}$. Show the equation, the substitution with units and the final unit.

Potential difference (p.d., often called voltage) is the energy transferred per unit charge, measured in volts (V). A p.d. of 1 V means 1 J is transferred for every coulomb.

Rearranging gives $W = VQ$ and $Q = \dfrac{W}{V}$. A voltmeter is connected in parallel across the component.

Resistance is measured in ohms (Ω). For a given p.d., higher resistance means lower current. For a given current, higher resistance means higher p.d.

1 Ω means 1 V produces 1 A. An ohmic conductor, such as a resistor at constant temperature, obeys Ohm's law: p.d. is directly proportional to current.

Hotter metal ions vibrate more about their fixed positions. Free electrons collide with the ions more often, so each collision reduces their momentum and the flow of charge slows.

This is the Higher tier explanation: more collisions means more opposition to current, so higher resistance. Calculations assume constant temperature unless told otherwise.

Work through these steps:

1. List the known values with units.
2. Identify the unknown.
3. Choose the equation.
4. Rearrange so the unknown is the subject.
5. Substitute, with units.
6. Calculate and state the answer with units.

Examiners award marks for method, not just the final answer.