Ohm's Law

This lesson covers Ohm's Law and the current–voltage characteristics of different components, as required by the AQA GCSE Physics specification (4.2.1.2 and 4.2.1.3). Understanding how current varies with potential difference is essential for predicting the behaviour of circuits.

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What Is Ohm's Law?

Ohm's Law states that:

The current through an ohmic conductor is directly proportional to the potential difference across it, provided the temperature remains constant.

This means that if you double the potential difference, the current will also double — as long as the temperature does not change. The relationship can be expressed as:

V = I x R

Where R (resistance) remains constant for an ohmic conductor at constant temperature.

Ohmic Conductors

An ohmic conductor is a component that obeys Ohm's Law. This means it has a constant resistance — the ratio V/I stays the same no matter what values of V and I are used.

Examples of ohmic conductors:

• A resistor (at constant temperature)
• A length of wire (at constant temperature)

Exam Tip: Ohm's Law only applies to ohmic conductors at constant temperature. Many components (filament lamps, diodes, thermistors, LDRs) are NOT ohmic conductors. Be precise: do not say "all components obey Ohm's Law."

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I–V Characteristics

An I–V characteristic (or I–V graph) is a graph showing how the current through a component varies as the potential difference across it changes. These graphs reveal whether a component is an ohmic conductor or not.

1. Resistor (at constant temperature) — Ohmic Conductor

The I–V graph for a resistor at constant temperature is a straight line through the origin.

Feature	Observation
Shape	Straight line through the origin
What it shows	Current is directly proportional to p.d.
Resistance	Constant (same gradient everywhere)
Obeys Ohm's Law?	Yes

The gradient of the line equals 1/R (since I = V/R). A steeper line means a lower resistance.

The graph is the same shape in both the positive and negative directions (the line extends into the third quadrant), because reversing the p.d. simply reverses the current direction.

2. Filament Lamp — Non-Ohmic

The I–V graph for a filament lamp is a curve that starts steep and then flattens.

Feature	Observation
Shape	S-shaped curve through the origin
What it shows	Current is NOT directly proportional to p.d.
Resistance	Increases as current increases
Obeys Ohm's Law?	No

Explanation: As current increases, the temperature of the filament increases. The metal ions in the filament vibrate more, causing more frequent collisions with the electrons (charge carriers). This means the resistance increases as the lamp gets hotter.

Exam Tip: A common 6-mark question asks you to explain why the filament lamp does not obey Ohm's Law. The key points are: (1) as current increases, temperature increases; (2) ions vibrate more at higher temperatures; (3) electrons collide more frequently with ions; (4) so resistance increases; (5) therefore current is not proportional to p.d.

3. Diode

The I–V graph for a diode shows that current flows in one direction only (the forward direction).

Feature	Observation
Shape	No current in reverse direction; exponential increase in forward direction
What it shows	Current only flows in one direction
Resistance	Very high in reverse; very low in forward direction (above threshold)
Obeys Ohm's Law?	No

Key points about diodes:

• A diode has a very high resistance in the reverse direction — effectively no current flows.
• In the forward direction, current starts to flow once the p.d. exceeds approximately 0.6–0.7 V (the threshold voltage for silicon diodes).
• Above the threshold, the resistance drops very quickly and current increases rapidly.
• LEDs (light-emitting diodes) behave in the same way but also emit light when current flows in the forward direction.

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Resistors in Practice

Fixed Resistors

A fixed resistor has a constant resistance value. The resistance does not change (assuming temperature stays constant). Fixed resistors are used to control the current in a circuit.

Variable Resistors

A variable resistor (also called a rheostat or potentiometer) allows the resistance to be changed by moving a slider or turning a dial. They are used to:

• Control the brightness of a lamp
• Control the volume of a speaker
• Control the speed of a motor
• Vary the current in a circuit during an investigation

graph LR
    A[Battery] --> B[Variable Resistor]
    B --> C[Lamp]
    C --> A

When the resistance of the variable resistor is increased, the current in the circuit decreases (from V = IR, if V is constant and R increases, I must decrease). This makes the lamp dimmer.

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Thermistors

A thermistor is a resistor whose resistance changes with temperature.

For the most common type (NTC thermistor — Negative Temperature Coefficient):

• As temperature increases, resistance decreases.
• As temperature decreases, resistance increases.

Temperature	Resistance	Current (for fixed p.d.)
High	Low	High
Low	High	Low

Applications of thermistors:

• Thermostats in central heating systems
• Temperature sensors in car engines
• Fire alarms and smoke detectors
• Baby incubators in hospitals

graph TD
    A[Temperature Increases] --> B[Resistance of Thermistor Decreases]
    B --> C[Current in Circuit Increases]
    C --> D[Device Responds - e.g. fan switches on]

Exam Tip: When describing how a thermistor is used in a sensing circuit, always state the sequence clearly: temperature changes -> resistance changes -> current changes -> output device responds. Examiners want to see each step of the logic chain.

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Light Dependent Resistors (LDRs)

An LDR (light dependent resistor) is a resistor whose resistance changes with light intensity.

• As light intensity increases, resistance decreases.
• As light intensity decreases, resistance increases.

Light Intensity	Resistance	Current (for fixed p.d.)
High (bright)	Low	High
Low (dark)	High	Low