Potential Divider

Overview

In this lesson, you build the basic idea of a potential divider: two resistors share the supply voltage, and the share across each resistor depends on the resistance. Once that idea is secure, the next lesson on variable potential dividers becomes much easier.

What You Need to Know

• A potential divider is a series circuit used to produce an output potential difference that is only part of the total supply voltage.
• In a series potential divider, the larger resistor gets the larger share of the total p.d.
• For two resistors, the ratio of the p.d.s across them matches the ratio of their resistances.
• You should be able to read a potential-divider circuit diagram and identify where the output is taken from.
• The idea only works if you keep the relationship between total p.d., resistor values, and output p.d. clear.

How to Work Through It

1. Start by sketching a simple two-resistor series circuit and labelling the supply p.d.
2. Compare what happens when the two resistors are equal and when one is larger than the other.
3. Practise short questions where you decide which resistor gets the larger p.d. and where the output should be measured.
4. Finish with ratio questions that connect resistor values to output voltage.

Check Your Understanding

• If the two resistors are equal, what fraction of the supply p.d. appears across each one?
• If one resistor is larger than the other, which one gets the larger potential difference?
• Can you explain why the output p.d. changes when one resistor value changes?

Common Mistakes

• Mixing up current and potential difference. In a series circuit, the current is the same, but the p.d. can be different across each resistor.
• Assuming the output is always half the supply. That is only true when the two resistors are equal.
• Forgetting that the output is measured across part of the circuit, not across the whole supply.

Next Steps

• Use the practice questions to get comfortable with resistor ratios and output voltage.
• Carry the same idea into the next lesson, where the resistance changes and the output becomes variable.