Exploring and designing practice questions.

A student passes a large current through a thin metal wire to study its heating effect. Which is the most appropriate safety precaution?

• Allow the wire to cool before touching it, as it can become very hot
• Increase the current quickly so that the experiment finishes sooner
• Hold the wire firmly throughout to stop it from moving around
• Use the highest voltage available to get the clearest possible result

Answer: Allow the wire to cool before touching it, as it can become very hot

A current-carrying wire can reach a temperature that burns skin, so it must be allowed to cool before handling. The other options increase the hazard rather than reduce it.

A student investigates how the electrical resistance of a wire depends on its length. Which set of quantities should all be kept constant?

• The material, cross-sectional area and temperature of the wire
• The length, cross-sectional area and material of the wire
• The current, the length and the material of the wire
• The temperature, the length and the voltage across it

Answer: The material, cross-sectional area and temperature of the wire

Only length is deliberately changed, so material, area and temperature must be controlled. The wrong options all fix the length, which is the independent variable and must be allowed to vary.

A student plans to measure diffraction fringes using a laser. Which is the most appropriate risk-assessment step?

• Avoid looking into the beam or at its reflected rays
• Increase the laser power for a brighter, clearer pattern
• Keep the laser pointed toward the class during set-up
• Work in bright sunlight to make the fringes clearer

Answer: Avoid looking into the beam or at its reflected rays

A laser can damage the retina directly or by reflection, so the key precaution is to keep the beam away from eyes. Raising the power or aiming it at people increases the hazard.

Which research question is best suited to a quantitative physics investigation?

• Is copper wire a better choice for a circuit than nichrome wire?
• Why do some solid materials conduct electricity and others do not?
• How does the resistance of a nichrome wire vary with its length?
• What happens to a metal wire when an electric current passes through it?

Answer: How does the resistance of a nichrome wire vary with its length?

A usable question names a variable to change and a variable to measure, both quantitative. 'Better' and 'what happens' cannot be turned into an axis on a graph.

A trial run shows that the thermometer reading changes by only 0.5 °C across the whole planned range of the independent variable. Which is the best change to the design?

• Extend the range of the independent variable so that the response is larger
• Repeat each one of the planned readings five times over and take the mean value
• Round every recorded temperature to the nearest whole degree Celsius
• Keep the design unchanged but plot the results on a much larger scale

Answer: Extend the range of the independent variable so that the response is larger

A change of 0.5 °C read to ±0.5 °C is buried in its own uncertainty. Widening the range makes the effect large compared with the resolution; repeats and rescaling cannot rescue a signal that was never there.

A student has time for 30 readings in total. Which plan gives the best evidence for the shape of the relationship?

• Three values of the independent variable, with ten repeats at each value
• Two values of the independent variable, with fifteen repeats at each value
• Thirty different values of the independent variable, with no repeats at all
• Ten values of the independent variable, with three repeats at each value

Answer: Ten values of the independent variable, with three repeats at each value

Ten points define a curve while three repeats still give a mean and a spread. Two or three values cannot distinguish a straight line from a curve; thirty single readings give no measure of random uncertainty.