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4.8 Required Practical: Component I–V Characteristics

GCSE Physics · AQA · Chapter 4: Electric Circuits

A free GCSE Physics lesson for AQA (8463) from OBA School: required practical: investigating the I–V characteristics of a fixed resistor, a filament lamp and a diode. 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

  • Assemble a circuit to measure the current through and the p.d. across a component
  • Describe a method that obtains both positive and negative values of p.d.
  • Plot I–V graphs and calculate the resistance at points on them
  • Identify variables, hazards, errors and improvements for this practical

Key words

  • I–V characteristic: A graph of current against p.d. for a component, for both directions of current.
  • Variable resistor: Used to change the p.d. across the component in steps (a variable power supply can be used instead).
  • Protective resistor: A fixed resistor in series with a diode to stop the current becoming too large.
  • Zero error: A meter that does not read zero when it should; a systematic error.
  • Repeatable: Results are repeatable if the same person gets similar results using the same method and equipment.

Aim and apparatus

Aim: to find how the current through a fixed resistor, a filament lamp and a diode depends on the p.d. across it.

Apparatus: battery or low-voltage supply, switch, variable resistor, ammeter (or milliammeter for the diode), voltmeter, fixed resistor, filament lamp in a holder, diode with a protective resistor, leads.

Method

1. Connect the fixed resistor in the circuit. Set the variable resistor to its largest resistance.

2. Close the switch. Record the p.d. and the current. Open the switch.

3. Change the variable resistor to increase the p.d. in steps (e.g. 0.5 V) and record V and I each time, up to the component’s safe maximum.

4. Reverse the connections to the component (or to the supply) and repeat to get negative values of p.d. and current.

5. Repeat each set of readings and calculate mean values.

6. Replace the resistor with the filament lamp, then the diode, and repeat steps 1–5.

  • independent; In this practical potential difference across the component
  • dependent; In this practical current through the component
  • control; In this practical the component being tested; for the fixed resistor, its temperature (switch off between readings)
  • hazard; In this practical a filament lamp or resistor can get hot – do not touch it, let it cool, use low p.d.s; a diode can burn out – use a protective resistor and do not exceed its maximum current

Results and analysis

Plot current on the y-axis against p.d. on the x-axis. Negative readings go in the bottom-left quarter. Draw a smooth curve or straight line of best fit.

  • current (A); 0 0; 1.0 0.11; 2.0 0.17; 3.0 0.21; 4.0 0.25; 5.0 0.28; 6.0 0.30

Worked example: resistance from a diode reading

At 0.68 V the milliammeter reads 17 mA.
R = V ÷ I = 0.68 ÷ 0.017

Answer: R = 40 Ω

Exam tip: Find the resistance at a point with R = V ÷ I using the readings at that point – not from the gradient of the curve.

Evaluation: errors and improvements

  • Zero errors: check that both meters read zero before switching on; subtract any zero error from every reading.
  • Heating: the fixed resistor warms up if the current is left on, so its resistance rises. Switch off between readings.
  • Filament lamp: wait a few seconds for the reading to settle at each step, because the filament takes time to reach a steady temperature.
  • Diode: the current rises steeply over a small range of p.d., so take more readings in that region to show the shape accurately.
  • Repeats: repeating readings and taking a mean reduces the effect of random errors and shows whether results are repeatable.

Common mistakes

  • Connecting the voltmeter across the variable resistor or the supply instead of across the component.
  • Forgetting to reverse the component to get negative values.
  • Using the gradient to find the resistance of a lamp or diode.
  • Testing a diode without a protective resistor – it can overheat and fail.
  • Mixing up the axes. Unless a question says otherwise, plot current on the y-axis and p.d. on the x-axis – and read each axis label carefully in the exam.

Revision questions

Try these, then check your answers below each one.

Question 1. How do you get negative values of p.d. and current when finding a component’s I–V characteristic? (Choose from: Reverse the connections to the component, Use a larger variable resistor, Turn the ammeter round but nothing else, Switch off the circuit.)

Answer: Reverse the connections to the component

Working: Swapping the connections to the component (or the supply) reverses the p.d. and the current.

Question 2. During the practical, a filament lamp has 3.0 V across it and 0.25 A through it. Calculate its resistance.

Answer: 12 Ω

Working: R = 3.0 ÷ 0.25 = 12 Ω.

Question 3. A diode has 0.70 V across it and a current of 35 mA. Calculate its resistance.

Answer: 20 Ω

Working: I = 0.035 A. R = 0.70 ÷ 0.035 = 20 Ω.

Related lessons

Continue in the full course

This lesson is section 4.8 (AGCPH0408) 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.

 

 

 

 

 

 

 

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