Current Electricity

Charge, current, resistance, and the behaviour of circuit components.

Spec Points Covered
  • 3.5.1.1 Define 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)., 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)., and 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 difference to currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohms (Ω). using standard equations.
  • 3.5.1.1 Calculate charge flow, energyThe capacity to do work. Measured in joules (J). transfer, and resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). in simple circuits.
  • 3.5.1.2 Describe and explain I-V characteristics for ohmic conductors, filament lamps, and diodes.
  • 3.5.1.3 Explain how and why resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). changes with temperature for metals and thermistors.
  • 3.5.1.2 Use the resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m). equation and describe the required practical for measuring resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m)..
  • 3.5.1.3 Explain superconductivityThe property of certain materials to have zero resistivityA material property that quantifies how strongly it resists current. Measured in ohm-metres (Ω m). below a critical temperature (the transition temperature). and state applications of superconductors.
Notes
01 Charge & Current Electric current 3.5.1.1 → 02 Potential Difference & Voltage Potential difference 3.5.1.1 → 03 Resistance Resistance 3.5.1.1 → 04 Resistivity Resistivity 3.5.1.2 → 05 I–V Characteristics I–V characteristic 3.5.1.2 → 06 Resistance & Temperature NTC thermistor 3.5.1.3 →
Σ Key Equations Full Reference →
On Data Sheet
Not on Data Sheet
Resistance
$$R = \frac{V}{I}$$
  • Where:
    • $R$ = resistance (Ω)
    • $V$ = potential difference (V)
    • $I$ = current (A)
Definition of resistance. Applies to ALL components, not just ohmic ones.
Current
$$I = \frac{\Delta Q}{\Delta t}$$
  • Where:
    • $I$ = current (A)
    • $\Delta Q$ = charge flow (C)
    • $\Delta t$ = time interval (s)
Rearranges to Q = It. Not on the AQA data sheet — must memorise.
Resistivity
$$\rho = \frac{RA}{L}$$
  • Where:
    • $ρ$ = resistivity (Ω m)
    • $R$ = resistance (Ω)
    • $A$ = cross-sectional area (m²)
    • $L$ = length (m)
Material property. Not on the data sheet — must be memorised. Unit is Ω m, not Ω m⁻¹.
Potential difference
$$V = \frac{W}{Q}$$
  • Where:
    • $V$ = potential difference (V)
    • $W$ = work done / energy transferred (J)
    • $Q$ = charge (C)
1 V = 1 J C⁻¹. Not on the AQA data sheet — must memorise.
Electrical power
$$P = IV$$
  • Where:
    • $P$ = power (W)
    • $I$ = current (A)
    • $V$ = potential difference (V)
Primary power equation. Combine with R = V/I for the derived forms below.
Power (current form)
$$P = I^{2}R$$
  • Where:
    • $P$ = power (W)
    • $I$ = current (A)
    • $R$ = resistance (Ω)
Derived from P = IV and V = IR. Useful when V is unknown.
Power (voltage form)
$$P = \frac{V^{2}}{R}$$
  • Where:
    • $P$ = power (W)
    • $V$ = potential difference (V)
    • $R$ = resistance (Ω)
Derived from P = IV and I = V/R. Useful when I is unknown.
Q Retrieval Practice All 12 Questions →
Q1. Define electric current.
Electric current is the rate of flow of charge.
Q2. What does an electric current of one ampereThe SI unit of current. One ampere is a flow of one coulomb of charge per second. represent?
A rate of flow of charge of one coulombThe SI unit of charge. One coulomb is the charge transferred by a current of 1 A in 1 second. per second.
Q3. Define potential differenceThe energyThe capacity to do work. Measured in joules (J). transferred per unit charge between two points. Measured in volts (V)..
  • The work doneEnergy transferred when a force moves an object. In electrical circuits, W = QV (charge times potential difference). per unit charge.
  • V = W/Q.
Q4. What is resistance and how is it defined?
  • A measure of how much a component opposes the flow of current.
  • R = V/I.
  • Unit: ohm (Ω).
Q5. State Ohm's lawFor an ohmic conductor at constant temperature, current is directly proportional to potential difference..
The current through a metallic conductor is directly proportional to the p.d. across it, provided the physical conditions do not change.