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2.2.4Electricity

IV Graphs — Edexcel International GCSE Science Double Award 4SD0 - Physics Revision Notes & Topical Questions

10 min read 14 parts Lesson 8 of 18

📚 Edexcel International GCSE Science Double Award 4SD0 - Physics Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
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Part 1 of 7Obtaining an I–V graph

An I–V characteristic shows how the current in a component depends on the p.d. across it.

1
To measure it, connect the component in series with an ammeter and a variable power supply (or a variable resistor used as a potential divider).
2
Connect a voltmeter in parallel across the component.
3
Change the p.d. in steps, record V and I, and plot I on the y-axis against V on the x-axis.
4
To include both directions, reverse the supply connections and record negative values.

The circuit for the experiment uses a variable resistor as a potential divider so that the p.d. can be varied from zero.

Obtaining an I–V graph
Safety and accuracy points include keeping the current within the component's rating, especially for a diode,
  • which must always have a protective resistor in series, and waiting for the readings to settle for the lamp because the filament heats gradually.
Using the same meter ranges and checking for zero error improves accuracy.

Part 2 of 7Fixed resistor (ohmic conductor)

The graph is a straight line through the origin, and the line continues symmetrically for negative values. The current is directly proportional to the p.d., so the resistance R = V/I is constant (provided temperature is constant). The gradient of an I–V graph is 1/R, so a steeper line means lower resistance.

For reasons of symmetry, a graph of a metal resistor in reverse is a mirror image of the forward graph: reversing the p.d. reverses the current by the same amount. The same is true for a filament lamp. Only the diode breaks this symmetry because of its one-way behaviour.

Part 3 of 7Filament lamp

The graph starts as a straight line near the origin and then curves so that its gradient decreases as p.d. increases.

1
This means the resistance increases as the current rises.
2
The reason is that the current heats the filament, so the metal ions vibrate more and collide with the electrons more, increasing the resistance.
3
The graph is symmetrical for positive and negative p.d.
4
A lamp is therefore a non-ohmic conductor.

One way to remember the shapethe lamp's I–V curve flattens because as it heats up, it is harder to push more current through.

Filament lamp
In graph words, "the line bends towards the voltage axis".
The resistance at a point is the straight line from the origin to that point, and the slope of this line decreases as you move up the curve,
  • meaning R is increasing.

Part 4 of 7Diode

A diode allows current in one direction only.

1
In the forward direction the current is almost zero until the p.d. reaches about 0.6 V (for a silicon diode), then it rises very steeply.
2
In the reverse direction the current is practically zero for the usual p.d. values: the diode has a very high resistance.
3
The graph is therefore flat on the negative side, then a sharp upward curve on the positive side.
4
The diode is non-ohmic.

Part 5 of 7Reading values

To find resistance at a point on any I–V graph, read V and I at that point and calculate R = V/I; do not use the gradient of a curve at that point, because for a curved graph the gradient is not 1/R.

Reading values
For a straight graph through the origin the gradient method works.
Compare resistance at two points on a lamp graph: at a higher p.d. the ratio V/I is larger.

For an exam question that asks you to compare two I–V graphs for resistors on the same axes, the steeper line is the lower resistance. Check by choosing the same p.d. for both and reading the currents: the higher current means lower R.

Part 6 of 7Uses

Understanding I–V graphs helps in selecting components.

1
Diodes act as one-way valves in rectifier circuits, and as protection against reverse connection.
2
LEDs, which are diodes that emit light, light only when connected in the forward direction and need a series resistor to limit current.
3
Lamps and resistors are used for lighting and heating.
4
Thermistors and LDRs have resistance that changes with the environment and are covered in the components lesson.

Part 7 of 7Worked examples

  1. A resistor gives a current of 0.40 A at 2.0 V. What current does it carry at 6.0 V, if it is ohmic?

  2. A filament lamp has 1.0 V and 0.20 A at one point and 6.0 V and 0.60 A at another. Find the resistance at each point and comment.

  3. A diode has a forward p.d. of 0.70 V. A 6.0 V supply with a resistor in series gives a current of 0.020 A. Find the resistor.

Key terms

TermMeaning
I–V characteristicGraph of current against p.d. for a component.
Non-ohmicComponent whose resistance changes with p.d.
DiodeComponent allowing current in one direction.
FilamentThin wire in a lamp that gets hot and glows.
Forward biasConnection that lets a diode conduct.

Avoid these mistakes

1
Using the gradient of a curve at a point as the resistance (use V/I).
2
Mixing up axeson an I–V graph, the gradient is 1/R.
3
Saying a lamp has a constant resistance.
4
Thinking a diode conducts in both directions.
5
Forgetting that a diode needs a threshold forward p.d. before it conducts well.

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