3.5.1.2
I–V Characteristics
Current Electricity — AQA A-Level Physics
Key Definition
I–V characteristic — A graph showing how the currentThe rate of flow of charge. Measured in amperes (A). through a component varies with the potential differenceThe energy transferred per unit charge between two points. Measured in volts (V). across it.
- Data collected using: variable resistor or potential dividerA circuit that uses two or more resistors in series to produce a fraction of the source voltageThe 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). Informal term for potential difference. across one of the resistors., ammeterAn instrument that measures current. Connected in series with the component. Has very low resistance so it doesn't affect the circuit. (series), voltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. (parallel).
- Reverse p.d. achieved by swapping battery connections.
- A potential dividerA circuit that uses two or more resistors in series to produce a fraction of the source voltageThe 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). Informal term for potential difference. across one of the resistors. circuit allows p.d. to be varied from 0 V upwards.
Circuit for measuring I–V characteristics: powerThe rate of energy transfer. Measured in watts (W). supply, variable resistor or potential dividerA circuit that uses two or more resistors in series to produce a fraction of the source voltageThe 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). Informal term for potential difference. across one of the resistors., ammeterAn instrument that measures current. Connected in series with the component. Has very low resistance so it doesn't affect the circuit. in series, voltmeterAn instrument that measures potential difference. Connected in parallel across the component. Has very high resistance. in parallel across test component.
Ohmic Conductor
- I–V graph is a straight line through the origin.
- CurrentThe rate of flow of charge. Measured in amperes (A). is directly proportional to p.d.Potential difference. The energy transferred per unit charge between two points in a circuit. Measured in volts (V).
- ResistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential differenceThe energy transferred per unit charge between two points. Measured in volts (V). to currentThe rate of flow of charge. Measured in amperes (A).. Measured in ohms (Ω). is constant.
- $ResistanceThe opposition to currentThe rate of flow of charge. Measured in amperes (A). flow. The ratio of potential difference to current. Measured in ohms (Ω). = 1 / gradient of the I–V graph.$
I–V graph for an ohmic conductorA conductor that obeys Ohm's law: current is directly proportional to potential difference at constant temperature.: straight line through the origin with positive gradient, symmetric in both positive and negative quadrants.
Filament Lamp
- I–V graph is a curve with decreasing gradient.
- ResistanceThe opposition to current flow. The ratio of potential differenceThe energy transferred per unit charge between two points. Measured in volts (V). to current. Measured in ohms (Ω). increases as currentThe rate of flow of charge. Measured in amperes (A). increases.
- More current causes the filament temperature to rise.
- Hotter atoms/ions vibrate more and collide more with conduction electronsDelocalised electrons in a metal that are free to move through the lattice structure and carry electric current..
- More collisions mean greater opposition to charge flow: higher resistanceThe opposition to current flow. The ratio of potential differenceThe energy transferred per unit charge between two points. Measured in volts (V). to current. Measured in ohms (Ω)..
- ResistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). at any $point = V/I$ at that point on the graph.
I–V graph for a filament lampA lamp containing a thin wire that heats up when current flows, increasing its resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω).. Its I-V characteristic is non-linear.: curve through the origin with gradient that decreases at higher V, symmetric S-shape across both quadrants.
Diode
- A diodeA component that allows current to flow in one direction only (forward bias). Has a threshold voltage of approximately 0.6 V for silicon. only allows current in one direction (the forward direction).
- The p.d. must exceed a threshold voltageThe minimum forward voltage needed for a diode or LED to begin conducting. (~0.6 V for silicon) before current flows.
- In reverse bias, current is approximately zero.
- Very high resistanceThe opposition to current flow. The ratio of potential difference to current. Measured in ohms (Ω). in reverse; very low resistance above the threshold in forward bias.
I–V graph for a diodeA component that allows current to flow in one direction only (forward bias). Has a threshold voltage of approximately 0.6 V for silicon.: zero current in reverse bias (negative V region), negligible current below ~0.6 V in forward bias, then steep exponential rise in current.
Common Mistake
MEDIUM
Describing the filament lampA lamp containing a thin wire that heats up when current flows, increasing its resistance. Its I-V characteristic is non-linear. graph as 'exponential'. The correct description is 'the gradient decreases' or 'the curve flattens', showing increasing resistance.
Examiner Tips and Tricks
- When asked to 'explain' an I–V characteristic, link shape to a physical mechanism.
- For a filament lampA lamp containing a thin wire that heats up when current flows, increasing its resistance. Its I-V characteristic is non-linear.: more current → higher temperature → more lattice vibrations → more collisions with electrons → higher resistance → less current per voltThe SI unit of potential difference and EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V).. One volt is one joule per coulomb..