K323 / 2027
Physical quantities and measurement overview

Lesson 6 of 8

Planning an experiment

Make a guess. You are not marked.

You test how a pendulum's length affects its period. What must you keep the same?

  1. The period, so that the results can be compared fairly.
  2. The length, so that the period can be measured more accurately each time.
  3. The amplitude and bob mass, so that length is the one variable that changes.
Show the answer

The amplitude and bob mass, so that length is the one variable that changes.

In a fair test you change the independent variable, length, measure the dependent variable, period, and keep everything else that could affect it the same.

A planning question asks you to describe an experiment you will not carry out. The marks go to a clear structure: what you change, what you measure, what you keep the same, and how the results answer the question.

The six parts of a plan

  1. Variables. The independent variable is what you change. The dependent variable is what you measure. Controlled variables are what you keep the same, and you say how.
  2. Apparatus. List it and draw a labelled diagram of the set-up.
  3. Procedure. Write numbered steps. Say how you measure each quantity and with which instrument.
  4. Range. Use at least 5 values of the independent variable over a wide range. Repeat each reading and take an average.
  5. Table, graph and conclusion. Give the table headings, say what to plot against what, and say what the graph shape or gradient would show.
  6. Risks and precautions. Name a real hazard in this experiment and the precaution that keeps the risk small.

Worked plan

How does the resistance of a wire depend on its length?

Task. You have a long piece of resistance wire, a metre rule, a cell, a switch, an ammeter, a voltmeter, connecting leads and two crocodile clips. Plan an experiment to find how the resistance R of the wire depends on its length l.

Variables. The independent variable is the length l of wire between the clips. The dependent variable is the resistance R, calculated from R = V / I. Keep the material and the diameter of the wire constant by using the same piece of wire for every length. Keep the temperature of the wire steady by using a small current and opening the switch between readings.

Apparatus and set-up. Tape the wire straight along the metre rule. Connect the cell, switch and ammeter in series with the wire, using one crocodile clip fixed at the 0.0 cm mark and one sliding clip. Connect the voltmeter across the wire between the two clips.

Procedure.

  1. Place the sliding clip so that l = 20.0 cm. Read l from the metre rule with your eye directly above the clip.
  2. Close the switch. Record the voltmeter reading V and the ammeter reading I. Open the switch straight away.
  3. Calculate R = V / I.
  4. Repeat for l = 30.0, 40.0, 50.0, 60.0 and 70.0 cm.
  5. Repeat the whole set of readings and find the mean R for each length.

Table. l / cm, V / V, I / A, R / Ω, and a column for the mean R / Ω.

Graph and conclusion. Plot R against l. If the points lie on a straight line through the origin, R is directly proportional to l. The gradient is the resistance of each centimetre of the wire. The gradient example uses results from this plan.

Risks and precautions. A short length of wire can draw a large current and become hot enough to burn. Keep the current small, open the switch between readings, and do not touch the wire while the switch is closed.

Accuracy. Check that both meters read zero before you start. Make sure the clips grip the bare wire firmly, and measure l between the points where the clips touch the wire.

Use the same six headings for any plan: how the period of a pendulum depends on its length, how the temperature of water changes as it cools, or how the current in a circuit depends on the voltage. Only the details change.

Check your understanding

A student plans to investigate how the resistance of a wire depends on its length. Which quantity must she keep constant?

  1. The current shown by the ammeter
  2. The length of the wire
  3. The diameter of the wire
Show the answer

The diameter of the wire

A thicker wire has a smaller resistance, so the diameter must stay the same. Using one piece of wire for every length keeps both the diameter and the material constant.

Practise an exam question

Write your answer first, then tick the mark points it makes.

Exam-style question

Plan an experiment to find out how the period of a simple pendulum depends on its length. Your plan should name the variables, describe the procedure and say how you would use your results. [5]

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