4.4 Component Characteristics

A free GCSE Physics lesson for AQA (8463) from OBA School: i–V graphs of a filament lamp and a diode; thermistors, LDRs and simple sensing circuits. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Sketch and explain the I–V graphs of a fixed resistor, a filament lamp and a diode
• Explain why the resistance of a filament lamp increases as its temperature increases
• Calculate the resistance at a point on a curved I–V graph using R = V ÷ I
• Describe how the resistance of a thermistor and of an LDR change, and give uses of each
• Explain how a simple sensing circuit responds to changes in temperature or light

Key words

• I–V characteristic: A graph of current (y) against potential difference (x) for a component.
• Non-ohmic: A component whose resistance changes as the current changes, so its I–V graph is not a straight line.
• Filament lamp: A lamp with a thin metal wire that glows when it gets very hot.
• Diode: Lets current through in the forward direction only; very high resistance in the reverse direction.
• LED: Light-emitting diode: a diode that gives out light when a current flows forwards through it.
• Thermistor: A resistor whose resistance decreases as its temperature increases.
• LDR: Light-dependent resistor: its resistance decreases as the light intensity increases.

Ohmic and non-ohmic components

For a fixed resistor at constant temperature the I–V graph is a straight line through the origin (see 4.3): its resistance is constant. Many components are non-ohmic – their resistance changes with the current, so their I–V graphs curve.

Key fact: You can always find the resistance at a point using R = V ÷ I, reading V and I off the graph at that point. On a curve, the resistance is different at different points.

The filament lamp

• As the current increases, the filament gets hotter. The metal ions vibrate more, so electrons collide with them more often: the resistance increases.
• So the line curves away from the current axis and gets flatter: each extra volt adds less current.
• Reversing the p.d. reverses the current but gives the same shape: the resistance does not depend on the direction.

Worked example: resistance at two points

From the graph: at 1.0 V, I = 0.16 A, so R = 1.0 ÷ 0.16 = 6.25 Ω.
At 3.0 V, I = 0.30 A, so R = 3.0 ÷ 0.30 = 10 Ω.

Answer: the resistance rises from about 6 Ω to 10 Ω as the filament heats up

A cold filament has a low resistance, so a large current flows at the moment it is switched on. This is why filament bulbs usually fail at switch-on.

The diode and the LED

• Forward direction: almost no current until the p.d. reaches about 0.6 V, then the current rises steeply. The resistance falls as the p.d. increases.
• Reverse direction: the current is almost zero, so the resistance is very high.
• A light-emitting diode (LED) works the same way but gives out light when a forward current flows. LEDs are used as indicator lights and in lighting because they waste little energy.

Worked example: resistance of a diode

A diode carries 12 mA when the p.d. across it is 0.66 V.
I = 12 ÷ 1000 = 0.012 A; R = V ÷ I = 0.66 ÷ 0.012

Answer: R = 55 Ω

Thermistors and LDRs

• thermistor; Resistance decreases when… its temperature increases; Typical uses thermostats in heating systems, fire alarms, fridge and oven temperature sensors
• LDR; Resistance decreases when… the light intensity increases; Typical uses automatic street lights and night lights, light meters, burglar alarms

Exam tip: Learn the direction: hotter or brighter → lower resistance → bigger current.

Sensing circuits

A sensor and a fixed resistor connected in series with a battery make a simple sensing circuit. A voltmeter across the fixed resistor gives an output p.d. that changes with the conditions.

Worked example: what happens when the room warms up

The thermistor gets hotter, so its resistance decreases.
The total resistance of the circuit decreases, so the current increases.
The p.d. across the fixed resistor (V = I R) increases.

Answer: the voltmeter reading rises; this output can switch on a fan or an alarm

Swapping the thermistor for an LDR makes a light sensor: in brighter light the output p.d. across the fixed resistor rises; in the dark it falls.

Common mistakes

• Saying the lamp’s resistance decreases as it gets hotter. Metal filaments have a higher resistance when hot.
• Using the gradient of a curved I–V graph as the resistance. Always use R = V ÷ I at the point.
• Drawing a diode graph with current in the reverse direction. Reverse current is (almost) zero.
• Mixing up thermistors and LDRs: thermistors respond to temperature, LDRs to light.
• Forgetting to convert mA to A when calculating a diode’s resistance.

Revision questions

Try these, then check your answers below each one.

Question 1. Which describes the I–V graph of a filament lamp? (Choose from: A curve through the origin that gets less steep as the p.d. increases, with the same shape for negative p.d., A straight line through the origin, Zero current until about 0.6 V, then rising steeply, and zero current in reverse, A horizontal line.)

Answer: A curve through the origin that gets less steep as the p.d. increases, with the same shape for negative p.d.

Working: As the filament heats up its resistance rises, so the line gets flatter. Reversing the p.d. gives the same shape. The straight line is a fixed resistor; the one-way curve is a diode.

Question 2. A filament lamp has a p.d. of 4.5 V across it and a current of 0.30 A through it. Calculate its resistance at this point.

Answer: 15 Ω

Working: R = V ÷ I = 4.5 ÷ 0.30 = 15 Ω.

Question 3. A diode carries a current of 25 mA when the p.d. across it is 0.75 V. Calculate its resistance.

Answer: 30 Ω

Working: I = 0.025 A. R = 0.75 ÷ 0.025 = 30 Ω.

Related lessons

• Previous lesson: 4.3 Potential Difference and Resistance
• Next lesson: 4.5 Series Circuits
• In this chapter: 4.1 Electrical Charges and Fields
• In this chapter: 4.2 Current and Charge
• In this chapter: 4.6 Parallel Circuits

Continue in the full course

This lesson is section 4.4 (AGCPH0404) of our free GCSE Physics course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Physics course.