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4.9 Required Practical: Resistors in Series and Parallel

GCSE Physics · AQA · Chapter 4: Electric Circuits

A free GCSE Physics lesson for AQA (8463) from OBA School: required practical: measuring the total resistance of resistors in series and in parallel. 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

  • Measure the resistance of individual resistors and of combinations using an ammeter and a voltmeter
  • Test the rules for total resistance in series and in parallel against measured values
  • Identify variables and hazards, and process results including percentage differences
  • Explain why measured values differ from expected values and suggest improvements

Key words

  • Combination: Two or more resistors connected together, in series or in parallel.
  • Expected value: The value predicted by a rule, e.g. R1 + R2 for resistors in series.
  • Tolerance: How far a resistor’s real value may be from the value printed on it, e.g. ±5%.
  • Percentage difference: difference ÷ expected value × 100.
  • Valid: A valid experiment measures what it is meant to, with other variables controlled.

Aim and apparatus

Aim: to measure the total resistance of resistors connected in series and in parallel, and compare it with the resistances of the individual resistors.

Apparatus: battery or low-voltage supply, switch, ammeter, voltmeter, two or three fixed resistors (e.g. 10 Ω and 22 Ω), connecting leads with crocodile clips.

Method

1. Connect resistor R1 on its own, with the ammeter in series and the voltmeter across it. Close the switch, record V and I, then open the switch.

2. Repeat with resistor R2 on its own.

3. Connect R1 and R2 in series. Put the voltmeter across both resistors. Record V and I.

4. Connect R1 and R2 in parallel. Put the voltmeter across the pair. Record V and I.

5. Repeat every reading and find means. Calculate each resistance with R = V ÷ I.

6. Extension: repeat with three resistors, or add identical resistors in parallel one at a time.

  • independent; In this practical the arrangement: one resistor, two in series, two in parallel (or the number of resistors)
  • dependent; In this practical total resistance of the arrangement (calculated from V and I)
  • control; In this practical the same resistors each time; the same supply p.d.; temperature of the resistors (switch off between readings)
  • hazard; In this practical resistors can get hot – use a low p.d., switch off between readings and do not touch hot resistors

Results and analysis

  • R1 alone (10 Ω label); p.d. V (V) 3.00; current I (A) 0.296; R = V ÷ I (Ω) 10.1
  • R2 alone (22 Ω label); p.d. V (V) 3.00; current I (A) 0.135; R = V ÷ I (Ω) 22.2
  • R1 and R2 in series; p.d. V (V) 3.00; current I (A) 0.093; R = V ÷ I (Ω) 32.3
  • R1 and R2 in parallel; p.d. V (V) 3.00; current I (A) 0.430; R = V ÷ I (Ω) 7.0
  • Series: expected R1 + R2 = 10.1 + 22.2 = 32.3 Ω. Measured 32.3 Ω – the rule is supported.
  • Parallel: measured 7.0 Ω, which is less than the smaller resistor (10.1 Ω) – the rule is supported.

Worked example: percentage difference

The labels say 10 Ω and 22 Ω, so the expected series resistance is 32 Ω. The measured value is 32.3 Ω.
percentage difference = (32.3 − 32) ÷ 32 × 100

Answer: 0.9% – well within a ±5% resistor tolerance

Evaluation: errors and improvements

  • Resistor tolerance: the real resistance can differ from the printed value (e.g. ±5%), so use your measured single-resistor values to work out the expected totals.
  • Contact resistance at loose clips adds a little extra resistance – tighten all connections.
  • Heating raises resistance: keep the p.d. low and switch off between readings.
  • Resolution: a small current such as 0.093 A needs an ammeter that reads to 0.001 A (or use a milliammeter).

Exam tip: A multimeter on its resistance (Ω) setting can be used to check each value, but in the exam describe the ammeter–voltmeter method and R = V ÷ I.

Common mistakes

  • Putting the voltmeter across only one resistor when measuring the total resistance of a combination.
  • Expecting the parallel total to be between the two resistances – it is less than the smallest.
  • Comparing with the printed values only and forgetting resistor tolerance.
  • Giving resistance in amps or volts – always state the unit, Ω.

Revision questions

Try these, then check your answers below each one.

Question 1. A student measures two resistors as 33 Ω and 47 Ω. What total resistance should she expect when they are connected in series?

Answer: 80 Ω

Working: 33 + 47 = 80 Ω.

Question 2. The same two resistors (33 Ω and 47 Ω) are connected in parallel. Which value is the most likely measured total resistance? (Choose from: about 19 Ω, about 80 Ω, about 40 Ω, about 47 Ω.)

Answer: about 19 Ω

Working: Only 19 Ω is less than 33 Ω. (The exact value is about 19.4 Ω.)

Question 3. True or false? Adding a third resistor in parallel with two others increases the total current from the supply.

Answer: True

Working: True. The new branch adds its own current while the other branch currents stay the same, so the total current rises and the total resistance falls.

Related lessons

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This lesson is section 4.9 (AGCPH0409) 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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