# Potential Difference and Resistance

## Potential Difference and Resistance Revision

**Potential Difference and Resistance**

As well as current, we can also measure the **potential difference (pd)** of a circuit and the **resistance** of a circuit.

**Definitions**

**Potential difference** is the **driving force** that causes electric charges to flow around a circuit. The units of potential difference are **Volts** (\text{V})and it can be measured using a **voltmeter**. A voltmeter must be connected in parallel with the component for which you want to know the potential difference.

Resistance **slows down** the flow of electrical charges in a circuit. The units of resistance are **Ohms **(\Omega).

Current, potential difference and resistance are related by the following equation:

\color{aa57ff}{V=IR}

- V= the potential difference in volts \text{(V)}
- I = the current in amps \text{(A)}
- R = the resistance in Ohms (\Omega).

**Required Practical**

**Investigating Factors Affecting Resistance of Electrical Circuits**

There are many different ways of altering the **current** and **potential difference** of a circuit. In this experiment, you can investigate the effects of changing the **length of a wire**.

**Doing the experiment**

- Set up a circuit as shown in the circuit diagram.
- Using the
**ruler**, measure the distance between the two crocodile clips. This is the**“wire length”**. - Close the switch and record the current on the
**ammeter**and the potential difference on the**voltmeter**. - Move the claw clips so that the wire length is bigger or smaller. Repeat the measurements of length, current and potential difference.
- Repeat the experiment for a number of different wire lengths.

You can now use your results to test the relationship between resistance and the wire length.

- Rearrange \text{V}=\text{IR} to get \text{R}=\dfrac{\text{V}}{\text{I}} and use this equation to calculate the resistance of the wire for each wire length.
**Plot**the**resistance versus the wire length**.- Draw a
**line of best fit**. This should be a straight line that**passes through the origin**of the graph. - The line of best fit shows that the resistance is
**directly proportional**to the wire length.

You can also alter this experiment to investigate the effect of adding resistors in **series and parallel**.

**Example: Calculating Current from Potential Difference and Resistance**

The potential difference across a 20 \Omega resistor is 8\text{ V}. What is the current across the resistor?

**[2 marks]**

so

\begin{aligned}I&=\dfrac{\text{V}}{\text{R}}\\&=\dfrac{\textcolor{2730e9}{8}}{\textcolor{7cb447}{20}}=\bold{0.4\text{ A}}\end{aligned}

## Potential Difference and Resistance Example Questions

**Question 1:** Define resistance.

**[1 mark]**

A property that** slows down the flow of electrical charge** (or current).

**Question 2:** Describe how you might measure the potential difference across a lamp.

**[2 marks]**

Connect a **voltmeter** to the lamp** in parallel**.

**Question 3:** The current across a 4\Omega resistor is 3\text{ A}. What is the potential difference across the resistor?

**[2 marks]**

**Question 4:** Describe an experiment you could do to investigate the effect of the length of a wire on resistance. Use a circuit diagram to help your description.

**[6 marks]**

Set up a **circuit** as shown in the circuit diagram. Using the ruler, measure the **distance between the two crocodile clips**. This is the “wire length”. Close the switch and **record the current** on the ammeter and **the potential difference** on the voltmeter. Move the crocodile clips so that the wire length is bigger or smaller. **Repeat the measurements of length, current and potential difference** and then repeat the experiment for a **number of different wire lengths**.