AQA GCSE Physics (8463) — Electricity
Imagine trying to run down a hallway packed with people moving side to side; the more they move, the harder it is for you to get through. In a metal wire, current is the flow of free (delocalised) electrons moving through a lattice of fixed positive metal ions.
As these electrons flow, they collide with the metal ions, transferring energy from their kinetic energy store to the ions' thermal energy store. This energy transfer causes the metal ions to vibrate more vigorously with greater frequency and amplitude.
Because the ions are vibrating more, they become "larger targets," which increases the frequency of collisions with the flowing electrons. These more frequent collisions further impede the flow of charge, resulting in an increase in electrical resistance and a measurable rise in temperature.
Some components maintain a constant resistance regardless of the current flowing through them, provided their temperature remains constant. These are known as ohmic conductors (such as fixed resistors and metal wires).
For these components, the current is directly proportional to the potential difference across them. Because the relationship is directly proportional, if the potential difference doubles, the current also doubles. This means the ratio of potential difference to current remains constant, strictly following Ohm's Law ().
An I-V characteristic is a graph showing how the current () flowing through a component changes as the potential difference () across it is varied. The shape of the graph tells you whether a component is a linear element or a non-linear element.
On a standard I-V graph where current is on the y-axis and potential difference is on the x-axis, the gradient represents . A steeper line indicates a lower resistance, while a shallower line indicates a higher resistance.
Fixed Resistors (Ohmic) The I-V characteristic for an ohmic conductor is a straight line passing exactly through the origin . Because the line is straight, the gradient is constant, proving that the resistance does not change as the current changes. The relationship is symmetrical, meaning if you reverse the power supply connections, the graph extends as a straight line into the negative (third) quadrant.
Filament Lamps (Non-Ohmic) A filament lamp is a non-linear element, meaning its resistance is not constant. Its I-V characteristic forms an S-shaped curve that passes through the origin.
As the potential difference increases, the thin metal filament gets very hot, causing the metal ions to vibrate more and increasing the frequency of collisions with electrons. This increases the resistance, which makes the graph curve and become shallower at higher voltages because the current increases at a slower rate.
Diodes (Non-Ohmic) A diode is a non-linear element designed to allow current to flow in one direction only, known as forward bias. In the reverse direction (reverse bias), the diode has an extremely high resistance, meaning the current remains at zero and the graph sits completely flat on the x-axis.
Even in the forward direction, current remains at zero until a specific threshold voltage is reached (typically to ). Once this threshold is exceeded, the resistance drops sharply, and the current increases rapidly, creating a steep upward curve.
To gather data for an I-V graph, you must set up a circuit with a power supply, an ammeter connected in series with the component, and a voltmeter connected in parallel across it. You also need a variable resistor to change the potential difference across the component and the current through it, allowing you to take multiple readings.
Crucially, the temperature of the component must be kept constant to ensure a fair test. You should turn the switch off between readings to prevent the component from heating up and changing its resistance.
You can use the formula to find the resistance at any point on an I-V graph.
Question: A fixed resistor has an I-V graph passing through the origin. At one point, the potential difference is and the current is . Calculate the resistance.
Step 1: Write down what you know from the graph.
Step 2: Rearrange the equation for resistance.
Step 3: Substitute the values and calculate.
Students often state that resistance increases because electrons collide with 'atoms' — AQA mark schemes specifically require you to say 'metal ions'.
In 6-mark questions explaining resistance in a filament lamp, examiners expect a logical sequence: current/voltage increases → temperature increases → ions vibrate more → collision frequency increases → resistance increases.
When sketching the I-V graph for a diode, ensure the line stays exactly flat on the x-axis for all negative voltages; do not let your pen 'flick' up or down.
Remember that on a standard I-V graph (where current is on the y-axis), the gradient represents 1/R, so a steeper line actually means a lower resistance.
Always mention 'at a constant temperature' when defining an ohmic conductor or describing direct proportionality in a fixed resistor.
Free (delocalised) electrons
Electrons that are not bound to a specific atom and can move freely through the lattice to carry an electric current.
Lattice
The regular, repeating, three-dimensional arrangement of positive metal ions within a solid conductor.
Metal ions
Positively charged particles in a metallic lattice that vibrate more vigorously when heated, causing resistance.
Resistance
A measure of the opposition to the flow of electric current, caused by collisions between free electrons and vibrating metal ions.
Ohmic conductors
Conductors that obey Ohm's Law, where current is directly proportional to potential difference at a constant temperature, resulting in a constant resistance.
Directly proportional
A linear relationship where the ratio of two variables remains constant, represented graphically by a straight line passing through the origin.
I-V characteristic
A graph showing how the current flowing through a component changes as the potential difference across it is varied.
Linear element
A circuit component, like a fixed resistor, that produces a straight-line I-V graph because its resistance remains constant.
Non-linear element
A circuit component, like a filament lamp or diode, whose resistance changes with current or voltage, producing a curved I-V graph.
Filament lamp
A light bulb with a thin metal wire that gets hot when current flows, causing its resistance to increase and producing an S-shaped I-V curve.
Diode
A component that only allows current to flow in one direction and has a very high resistance in the reverse direction.
Forward bias
The orientation of a diode that allows electric current to flow.
Reverse bias
The orientation of a diode that blocks current flow due to its extremely high resistance.
Threshold voltage
The minimum potential difference (typically 0.6 V to 0.7 V) required for a diode to begin conducting significantly in the forward direction.
Variable resistor
A circuit component used to alter the potential difference and current in a circuit, enabling multiple readings to be taken.
Put your knowledge into practice — try past paper questions for Physics
Free (delocalised) electrons
Electrons that are not bound to a specific atom and can move freely through the lattice to carry an electric current.
Lattice
The regular, repeating, three-dimensional arrangement of positive metal ions within a solid conductor.
Metal ions
Positively charged particles in a metallic lattice that vibrate more vigorously when heated, causing resistance.
Resistance
A measure of the opposition to the flow of electric current, caused by collisions between free electrons and vibrating metal ions.
Ohmic conductors
Conductors that obey Ohm's Law, where current is directly proportional to potential difference at a constant temperature, resulting in a constant resistance.
Directly proportional
A linear relationship where the ratio of two variables remains constant, represented graphically by a straight line passing through the origin.
I-V characteristic
A graph showing how the current flowing through a component changes as the potential difference across it is varied.
Linear element
A circuit component, like a fixed resistor, that produces a straight-line I-V graph because its resistance remains constant.
Non-linear element
A circuit component, like a filament lamp or diode, whose resistance changes with current or voltage, producing a curved I-V graph.
Filament lamp
A light bulb with a thin metal wire that gets hot when current flows, causing its resistance to increase and producing an S-shaped I-V curve.
Diode
A component that only allows current to flow in one direction and has a very high resistance in the reverse direction.
Forward bias
The orientation of a diode that allows electric current to flow.
Reverse bias
The orientation of a diode that blocks current flow due to its extremely high resistance.
Threshold voltage
The minimum potential difference (typically 0.6 V to 0.7 V) required for a diode to begin conducting significantly in the forward direction.
Variable resistor
A circuit component used to alter the potential difference and current in a circuit, enabling multiple readings to be taken.
