BTEC Applied Science Unit 3 - Investigating the Relationship Between Potential Difference and Current in a Lamp

Circuit symbols for electrical applications

In the BTEC Applied Science Unit 3 exam, learners are often asked to write a plan for a practical investigation. One common example is investigating how the current through a lamp changes as the potential difference (voltage) is increased.

This blog post explains exactly how to structure your answer, what examiners are looking for, and how to apply it to the lamp investigation shown in the figures.

Step 1: Start with a Clear Aim

The aim should clearly state what you are investigating and what you are measuring.

Example aim:

To investigate how the current through a filament lamp changes when the potential difference across the lamp is increased.

✔ Keep it short

✔ Use correct scientific terms (current, potential difference)

Step 2: Identify Variables

Even if the question doesn’t explicitly ask for variables, including them shows strong scientific understanding.

Example:

• Independent variable: Potential difference across the lamp (V)

• Dependent variable: Current through the lamp (A)

• Control variables:

• Same lamp used throughout

• Same circuit components

• Same ambient temperature

• Same measuring equipment

Step 3: Selection and Justification of Equipment

You must name the equipment AND explain why it is used.

Circuit diagram that could be used for this investigation

Example:

• A variable D.C. power supply (0–16 V) or a variable resistor is used to vary the potential difference across the lamp.

• An ammeter is used in series to measure the current flowing through the lamp.

• A lamp and lamp holder ensure the lamp is securely connected.

• A switch allows the circuit to be safely opened and closed.

• Connecting leads are used to complete the circuit.

Examiners reward answers that show you understand how the equipment works, not just what it is called.

Step 4: Write a Step-by-Step Method

Your method should be logical, detailed, and repeatable.

Example:

1. Set up the circuit as shown, with the ammeter connected in series with the lamp and the variable D.C. power supply.

2. Set the power supply to a low potential difference, such as 2 V.

3. Close the switch and record the current shown on the ammeter.

4. Increase the potential difference in regular steps (for example, 2 V intervals).

5. Record the current at each potential difference.

6. Repeat each measurement at least three times.

7. Calculate the mean current for each voltage value.

✔ Use numbered steps

✔ Include repeats

✔ Use units where appropriate

Step 5: Quantities, Range, and Measurements

This section shows examiners that you understand data quality.

Example:

• The quantities measured are potential difference (V) and current (A).

• The potential difference will be varied from 0 V to 16 V.

• Measurements will be taken at regular intervals.

• Each reading will be repeated three times to improve reliability.

Step 6: Health and Safety

Always include at least one realistic hazard.

Example:

• The lamp may become hot, so it should not be touched during the investigation.

• The circuit should be switched off before adjusting connections.

• Electrical equipment should be checked for damaged wires.

Step 7: Brief Data Processing and Analysis

You don’t need full calculations — just show you know what will be done with the data.

Example:

The mean current for each voltage will be calculated. A graph of current against potential difference will be plotted to show the relationship between them. The graph is expected to be non-linear because the resistance of the filament increases as it heats up.

Common Mistakes to Avoid

❌ Writing vague methods (“measure the current”)

❌ Forgetting repeats

❌ Listing equipment without justification

❌ No mention of safety

❌ No reference to data analysis

Teacher Tip for Students

If you imagine someone else trying to repeat your experiment using only your plan, would they be able to do it successfully?

If the answer is yes, you’re on track for high marks.

Key Takeaway

• To score well on a “write a plan” question:

• Be structured

• Be specific

• Justify your choices

• Link your plan to the science behind the practical

If you need more structured support to achieve your BTEC Applied Science Unit 3 exam goals, get in touch. With 1:1 tuition, we can help you get the Distinction grade you are aiming for.