3.5.1.1
Charge & Current
Current Electricity — AQA A-Level Physics
Key Definition
Electric current — The rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C)..
$$I = \frac{\Delta Q}{\Delta t}$$
- 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). (I) is measured in amperes (A).
- ChargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). (Q) is measured in coulombs (C).
- 1 A = 1 C s⁻¹ (one coulombThe SI unit of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. One coulomb is the chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). transferred by a current of 1 A in 1 second. of charge per second).
- Charge is quantisedExisting only in discrete values. Charge is quantised because it can only exist as whole-number multiples of the elementary charge e.: it comes in multiples of e = 1.60 × 10⁻¹⁹ C.
- Number of electrons: $n = Q / e$.
Key Definition
Conventional current — Flows from positive to negative. Electron flow is in the opposite direction.
- CurrentThe rate of flow of charge. Measured in amperes (A). is measured using an ammeterAn instrument that measures currentThe rate of flow of charge. Measured in amperes (A).. Connected in series with the component. Has very low resistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohms (Ω). so it doesn't affect the circuit..
- An ammeterAn instrument that measures current. Connected in series with the component. Has very low resistance so it doesn't affect the circuit. must be connected in seriesComponents connected end-to-end in a single path, so current flows through each in turn. in the circuit.
- Charge is conserved: current entering a junction equals current leaving it.
Examiner Tips and Tricks
- Always state 'rate of flow of charge' in definitions, not just 'flow of charge'.
- The word 'rate' earns the mark.
Common Mistake
MEDIUM
Confusing current direction with electron flow. Conventional currentThe direction of current flow defined as from positive to negative. Opposite to the direction of electron flow. goes from + to −. Electrons drift from − to +.