25.9 - Planning an Investigation

What a planning question looks like

• Question 4 on every Cambridge practical paper is a plan, worth 7 marks out of 40 - more than any other question.
• It sets an experiment you haven't done, then gives the same five bullets every series:

A student investigates the rate of cooling of hot water in a container which has a lid. Plan an experiment which will enable him to compare the effect of lids of different thicknesses on the rate of cooling.

You should:

• state any additional apparatus required
• explain briefly how you would do the investigation
• state the key variables that you would keep constant
• draw a table, or tables, with column headings, to show how you would display your readings (you are not required to enter any readings in the table)
• explain how you would use your readings to reach a conclusion. [7]

• Behind the five bullets, the mark scheme has seven marking points, labelled MP1 to MP7.
• The examiner isn't looking for a single "correct" method; they're ticking off whether each of the seven points is present. Treat the question as a checklist to run down.

Two things not to spend time on

1. Safety - No marks are awarded for safety, and papers often state you don't need to include safety precautions.
2. Readings in the table - The experiment hasn't been carried out, so leave the table empty.

The seven ingredients of a plan

Every plan is marked against the same seven points. Turning each one into a sentence gives a full-mark answer.

Written out in full:

I would need a thermometer and a stop-watch. I would pour hot water into the container, put the thinnest lid on, record the starting temperature, and record the temperature again after five minutes, using the stop-watch to time it. I would then repeat this with lids of different thicknesses, using at least five different thicknesses. I would keep the volume of water and the starting temperature of the water the same each time. I would record the results in a table with columns for the thickness of the lid in cm, the temperature in °C, and the time in s. I would then compare the temperature drop for each lid, and plot a graph of temperature drop against lid thickness, to see whether a thicker lid gives a smaller drop.

Three ways to earn MP7

1. At least five sets of readings - Use at least five different values across a sensible range for the independent variable.
2. A second control variable - Name another specific quantity kept constant.
3. Repeating and averaging - State that each measurement is repeated to calculate a mean.
4. Including more than one gives a backup if the examiner rejects one.

Matching the table to the method (MP5)

• The table must match every quantity measured in your method.
• If your method describes measuring current, potential difference, and length, the table needs a column for all three, each with its quantity and unit. Missing a column loses MP5.

Circuit planning questions

• If the question asks you to "complete the circuit diagram", MP1 is a drawing mark rather than an apparatus mark (such as adding a voltmeter in parallel across a component with the correct circuit symbol).
• MP2 through MP7 remain the same.

Common exam questions

Question 1

The planning question is worth 7 marks. What are the seven things a plan should contain?

The additional apparatus; the method, saying what you measure; a statement that you repeat with a new value of the independent variable; a named control variable; a table with headed columns; how you would use the readings to reach a conclusion; and one additional point, which can be a second control variable, at least five sets of readings, or repeats with an average.

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Question 2

A student writes half a page on the method and nothing else. Roughly how many of the seven marks can they get?

One or two. The marks are spread one per point, so a plan that covers all seven briefly scores far better than one that covers a single point in great detail.

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Question 3

Why does "keep everything else the same" score nothing for the control variable?

Because it does not name a single quantity. The mark is for identifying a specific thing held constant, such as the same volume of water or the same starting temperature.

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Question 4

A plan describes doing the experiment once. Which marking point is lost?

MP3, the one for repeating with a new value of the independent variable. Without changing the thing under test there is no investigation, so a sentence saying you repeat for different values is essential.

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Question 5

Should you include a safety precaution in a Cambridge physics plan?

No. The paper does not award a mark for safety here, and it occasionally says on the page that you are not required to write about safety precautions. Spend the line on a marking point instead.

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Question 6

Should you fill in the table with example readings?

No. The paper says you are not required to enter any readings, because the experiment has not been done. The mark is for the structure - the correct columns, each headed with a quantity and a unit.

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Question 7

Name three different ways to earn the seventh marking point.

Stating a second control variable; saying you would take at least five sets of readings; or saying you would repeat each measurement and take an average.

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Question 8

How should a plan say the readings will be used to reach a conclusion?

Either by comparing the readings to see whether changing the independent variable changes the dependent variable, or by plotting a graph, naming which quantity goes on which axis.

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Question 9

A plan's method says the student will measure the current, the potential difference, and the length of the wire, but the table has only two columns. What is wrong?

The table has to match the method. Every quantity the method says you will measure needs its own column, headed with the quantity and its unit, so this table needs three.