Guided lesson

IV Graphs

2.102.9

3 guided lessons · 27 checks · 2. Electricity → Components in Series & Parallel Circuits

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IV graphs

Plot the current through a component against the voltage across it and the shape of the line tells you everything. Straight through the origin: the resistance never budges. Curved away: something inside is changing. This lesson learns to read both shapes and names the components that produce them.

What you know already

How a component responds to voltage is easiest to see as a graph. A straight line through the origin means steady, predictable behaviour; a curve means the component is changing underneath you. This note reads the IV graphs of wires, filament lamps and diodes, then shows the bench setup that measures them and the resistance dial that controls them.

Key points of understanding

An IV graph plots the current through a component against the voltage across it.
A straight line through the origin means the relationship is linear and the resistance stays constant.
A curved graph means the resistance changes as the current varies.
Wires and fixed resistors at steady temperature give linear, directly proportional graphs.
Filament lamps, diodes, LDRs and thermistors all produce non-linear graphs.

Linear & Non-linear elements, downloadable IGCSE & GCSE Physics revision notes — Linear IV graphs are straight lines through the origin, indicating a constant resistance. Non-linear IV graphs are curved, indicating a variable resistance

babed6564ed4-2-1-6-ohms-law-graph — The current is directly proportional to the potential difference (voltage) as the graph is a straight line through the origin

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1.IV graphs

2.10

Key points

An IV graph plots the current through a component against the voltage across it.
A straight line through the origin means the relationship is linear and the resistance stays constant.
A curved graph means the resistance changes as the current varies.
Wires and fixed resistors at steady temperature give linear, directly proportional graphs.
Filament lamps, diodes, LDRs and thermistors all produce non-linear graphs.

First pass

Straight-line graph: same resistance however hard you push. Curved graph: the resistance is drifting.

Analogy — A spring scale versus a stiffening hose: one reads the same every pull, the other fights back harder the more you open it.

Exam pointer — The word 'origin' matters — a straight line that misses the origin is not the proportional case.

Going deeper

Direct proportionality means doubling one quantity doubles the other, which is exactly what a straight line through the origin encodes.

Analogy — A taxi meter with a fixed rate: twice the distance, twice the fare, forever on the same straight line.

Exam pointer — At constant temperature is the exam qualifier for a fixed resistor's line — quote it when describing the linear case.

Checks

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1.An IV graph that is a straight line through the origin shows:
2.Which component gives a linear IV graph at constant temperature?
3.On a non-linear IV graph, the resistance:
4.For a wire at steady temperature, doubling the voltage across it:
5.Which statement about the resistance of a wire at constant temperature is true?
6.What shape does the IV graph of a fixed resistor take, and what does the shape tell you?
Written response · recall
Your answer
7.Name two components with non-linear IV graphs.
Written response · recall
Your answer
8.Explain what an IV graph running straight through the origin tells you about how the current behaves.
Written response · explain why
Your answer
9.A fixed resistor's IV graph passes through (2 V, 0.4 A) as a straight line through the origin. Predict the current at 5 V, then the resistance.
Written response · application
Your answer

2.IV graph for a filament bulb

2.10

Two stars of the non-linear world. The filament lamp heats itself as it works, and its own heat throttles it back. The diode is a one-way valve — generous forwards, immovable backwards. This lesson reads both graphs and the physics that shapes them.

Filament lamp IV graph, downloadable AS & A Level Physics revision notes — IV graph for a filament lamp

ad680122ff24-2-1-6-diode-i-v-graph — IV graph for a semiconductor diode

Key points

A filament lamp's graph bends over: the current grows more slowly than the voltage.
The rising current heats the filament, and hotter lattice atoms obstruct the electrons more.
So the lamp's resistance increases as the current grows.
A diode passes current in one direction only — the forward-bias direction.
In reverse bias a diode's resistance is extremely high and effectively no current flows.
On the diode's graph, forward bias shows a sharp rise on one side and reverse bias shows a flat zero region.

First pass

Lamp: the graph flattens as it warms, because heat adds resistance. Diode: current one way, blocked the other.

Analogy — The lamp is a runner tiring as they go; the diode is a turnstile that spins one way and locks the other.

Exam pointer — Name the mechanism — hot filament, vibrating atoms — not just the curve, when asked to explain the lamp's graph.

Going deeper

The lamp's slowdown is negative feedback: more current heats the filament, and the heat raises the resistance that restrains the current.

Analogy — A self-regulating door that gets heavier the harder you push it — progress continues, but always a little less easily.

Exam pointer — For non-linear graphs, resistance at a point is voltage divided by current at that point — the straight-line gradient shortcut does not apply.

Checks

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1.As the voltage across a filament lamp increases, the current:
2.The filament's resistance grows with current because:
3.In reverse bias, a diode:
4.A diode conducts:
5.Compared with a fixed resistor's graph, a filament lamp's graph:
6.Describe the shape of a filament lamp's IV graph and state what happens to its resistance.
Written response · recall
Your answer
7.What is forward bias for a diode?
Written response · recall
Your answer
8.Explain why a diode passes no current in reverse bias.
Written response · explain why
Your answer
9.A diode is placed in a circuit backwards. State what the ammeter reads and what the IV graph shows on that side.
Written response · application
Your answer

3.Investigating current and voltage

2.102.9

Graphs do not draw themselves. Bench practice: adjust the current with a variable resistor, read the voltage off a voltmeter, and record the pairs. Step the current up, plot what you collected, and the component's IV curve appears. This lesson walks the setup and the resistance dial behind it.

Investigation of Current and Voltage, downloadable IGCSE & GCSE Physics revision notes — These circuits enable the investigation of current and voltage for a filament lamp or diode to be investigated

Fixed & Variable Resistor, downloadable IGCSE & GCSE Physics revision notes — Fixed and variable resistor circuit symbols

Key points

The investigation needs an ammeter, a voltmeter, a variable resistor, a power source and wires.
The variable resistor adjusts the current, which is the independent variable.
The voltmeter reads the voltage across the component — the dependent variable.
Recording current and voltage pairs while stepping the current up yields the component's IV graph.
A higher resistance in a circuit means a smaller current; a lower resistance means a larger one.
Fixed resistors keep one resistance; variable resistors change theirs by altering the wire length in circuit.

First pass

Set the current with the variable resistor, read the voltage, step, repeat — the pairs plot the graph.

Analogy — Tuning a radio dial (variable resistor), noting the volume (voltage) at each setting, then plotting the response.

Exam pointer — Independent variable = what you set (current); dependent variable = what you measure (voltage). Get the labels right for the marks.

Going deeper

The variable resistor is the experiment's steering wheel: lengthening its wire raises resistance and eases the current down for the next reading.

Analogy — A dimmer slider on a lamp — slide it and the brightness steps smoothly through the range you want to record.

Exam pointer — Quote the resistance-current see-saw (higher resistance, smaller current) for the qualitative-effect part of the question.

Checks

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1.In the IV investigation, the independent variable is:
2.The instrument that varies the current in the investigation is the:
3.Increasing a circuit's resistance while the supply stays fixed makes the current:
4.A variable resistor changes its resistance by:
5.A longer variable-resistor wire gives:
6.List the equipment needed to investigate how current varies with voltage.
Written response · recall
Your answer
7.State which quantity is the independent variable in the IV investigation and which instrument changes it.
Written response · recall
Your answer
8.Explain why the voltmeter's readings form the dependent variable of the investigation.
Written response · explain why
Your answer
9.An investigator steps the current up five times, recording the voltage each time. What can be plotted from the pairs, and which shape signals a constant resistance?
Written response · application
Your answer

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Back to the revision note All revision notes

1.IV graphs

2.10

2.IV graph for a filament bulb

2.10

3.Investigating current and voltage

2.102.9