25.1 - Variables & Controls

What a variables question looks like

Variables appear in two main ways on practical papers:

• Planning questions - Where you state the key variables to keep constant and specify values for the independent variable.
• Experiment questions - Explaining why a quantity was kept constant or identifying what must stay the same for a valid comparison.

The three questions behind every variable

1. What is being changed? - The independent variable.
2. What is being measured? - The dependent variable.
3. What else could change the measurement, and so must be held constant? - The constant variables. These must always be named specifically.

A typical question

state the key variables that you would keep constant

The three kinds of variable

Take an investigation into how the resistance of a wire depends on its length: a student connects a length of resistance wire into a circuit, measures the current $I$ and potential difference $V$, calculates resistance from $R=\frac{V}{I}$, and repeats for different lengths.

The independent variable

• The independent variable is the one quantity you deliberately change - here, the length of wire.
• There's only ever one independent variable. If two quantities change at once, you can't tell which caused the change in the result.
• In planning questions, specify at least five values spread over a sensible range (e.g. "lengths of 20, 40, 60, 80, and 100 cm").

The dependent variable

• The dependent variable is the quantity you measure or determine - here, the resistance $R$.
• It doesn't have to be read directly from an instrument; it can be calculated from other measurements (such as current and potential difference).

Constant variables

• Constant variables are all other quantities that could affect the outcome, kept the same so only the independent variable affects the result.
• For the wire experiment, constant variables include:
  • The same wire throughout (same material and diameter).
  • The same power supply setting.
  • Keeping the wire at a constant temperature (not letting it heat up between readings).
• Never write "keep everything else the same" - You must name the specific quantity.
• Never list the dependent variable as a constant variable.

Comparing two experiments

Common exam questions

Question 1

A student investigates how the extension of a spring depends on the load hung from it. Name the independent and the dependent variable.

The independent variable is the load, in newtons. The dependent variable is the extension of the spring, in centimetres.

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Question 2

In that experiment, state two variables that should be kept constant.

The same spring throughout, so its stiffness does not change, and the same starting length measured from the same point on the spring each time.

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Question 3

A student writes "keep all other conditions the same" as her answer for the constant variables. Why does this score nothing?

Because it does not name a single variable. The mark is for identifying a specific quantity that is held still, such as "the same volume of water", so a general phrase identifies nothing to credit.

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Question 4

Why must only one variable be changed at a time?

Because if two things change between readings, you cannot tell which of them caused the measured quantity to change, so no conclusion can be drawn from the results.

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Question 5

A student investigating the rate of cooling of water says she will keep the temperature constant. What is wrong with this?

The temperature is the dependent variable, the thing she is measuring as it changes. What she should keep constant is the starting temperature, along with the volume of water and the container.

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Question 6

A plan says "use different masses". Why is this a weak way to state the independent variable?

It gives no values and no range. A plan that scores states specific values, such as 100, 200, 300, 400, and 500 g, because five sets of readings spread over a sensible range is what the mark scheme looks for.

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Question 7

A student measures the current and the potential difference in order to find the resistance of a wire. Which of these three is the dependent variable?

The resistance. The current and the potential difference are the readings taken in order to calculate it, but the resistance is the quantity being investigated as the length is changed.