3.5.1.1
Resistance
Current Electricity — AQA A-Level Physics
Key Definition
Resistance — A measure of how much a component opposes the flow of currentThe rate of flow of charge. Measured in amperes (A). through it.
$$R = \frac{V}{I}$$
- Where:
- $R$ = resistance (Ω)
- $V$ = potential difference (V)
- $I$ = current (A)
- ResistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). between two points. Measured in volts (V). to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω). (R) is measured in ohms (Ω).
- 1 Ω = 1 V A⁻¹.
- To find resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω).: measure currentThe rate of flow of charge. Measured in amperes (A). through and p.d.Potential difference. The energy transferred per unit charge between two points in a circuit. Measured in volts (V). across the component.
- $R = V/I$ is the definition of resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω).. It applies to all components, not just ohmic ones.
Key Definition
Ohm's law — The currentThe rate of flow of charge. Measured in amperes (A). through a metallic conductor is directly proportional to the potential differenceThe energy transferred per unit charge between two points. Measured in volts (V). across it, provided physical conditions (e.g. temperature) do not change.
- Ohm's lawFor an ohmic conductor at constant temperature, current is directly proportional to potential difference. is a special case. Not all components obey it.
- Components that obey Ohm's lawFor an ohmic conductor at constant temperature, current is directly proportional to potential difference. are called ohmic conductorsComponents where current is directly proportional to p.d. at constant temperature (e.g. a metal wire)..
- Ohmic conductors have constant resistanceThe opposition to current flow. The ratio of potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V). to current. Measured in ohms (Ω)..
- 'Physical conditions' means temperature, light intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared., strainThe fractional change in length of a material under stress. It is dimensionless (no units)., etc.
Common Mistake
MEDIUM
Stating that $R = V/I$ is Ohm's lawFor an ohmic conductor at constant temperature, current is directly proportional to potential difference.. It is not. $R = V/I$ is the definition of resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω).. Ohm's law states that I is proportional to V at constant temperature.
Examiner Tips and Tricks
For full marks on Ohm's law, you must include 'provided physical conditions do not change' or 'at constant temperature'.