14.4Chapter 14: Electrical Circuits, Resistance and Power

Planning investigations: resistance of a wire — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 6 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision

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Lesson summary

Physics 14.4 Planning investigations: resistance of a wire — Length and Cross-Sectional Area — Alhussan International Academy — Physics (Stages 7–9) notes: Decide when to use primary data and describe how to plan an investigation.Observations — The filaments of lamps are very thi…

Testable questions
Not easily testable
Independent variable
Possible variables
Control variables
Secondary sources

Part 1 of 6Choosing a question to test

Observations

The filaments of lamps are very thin, curled wires, which leads to questions about length, thickness and material.

Testable questions

Choose ideas that can be tested by collecting data, such as how the resistance of a wire depends on its length.

Not easily testable

A question such as why some bulbs look brighter than others is interesting but hard to test with data.

Part 2 of 6Variables

Independent variable

The length of the wire.
- (or its thickness, cross-sectional area)

Dependent variable

The resistance, calculated from voltage and current.

Possible variables

Length, thickness, material and voltage of the battery.

Control variables

They are kept the same so the data show the link between the chosen variable and the resistance.
When changing length, keep the thickness, material, voltage and temperature the same.
When changing thickness, keep the length, material, voltage and temperature the same.

Part 3 of 6Preliminary work

Purpose

Preliminary work decides how to do the experiment safely and how to get a good range of results.

Secondary sources

Use them to find the most suitable wire.

Trying voltages

Try out different voltages for different lengths.

Choice

The wire gets very hot at high voltages, so a lower voltage such as 1.5 V is chosen.

Part 4 of 6Choosing equipment

Ammeter

A digital ammeter measures the current in the wire to the nearest 0.1 A.

Changing length

Clips connect to different positions on the wire.

Ruler

It measures the length of wire between the clips to the nearest 1 mm.

Circuit

A variable power supply is connected in series with the wire and the ammeter.

Part 5 of 6Assessing hazards

Hot wire

The wire can get very hot.

Control measures

Put the wire on a heatproof mat and do not leave the battery connected for long.

Effect on results

A hot wire also gives a higher resistance, so switch off between readings.

Part 6 of 6Obtaining and analysing evidence

Measure the current for different lengths of wire.

Repeat each measurement three times to make results more reliable.

Discard anomalous results before calculating averages.

Calculate resistance R = V ÷ I for each length.

Plot resistance against length to find a pattern.

Length

For a uniform wire the points lie on a straight line through the origin, so resistance is proportional to length.

Cross-sectional area

Resistance is inversely proportional to the area; a wire of twice the area has half the resistance.

Key terms

Term	Meaning
Primary data	Data you collect yourself in an experiment.
Preliminary work	Trial experiments to choose methods, values and safe conditions.
Cross-sectional area	The area of a cut across a wire, showing its thickness.
Anomalous result	A result that does not fit the pattern.
Control variable	A variable kept the same in a fair test.