Copyright © University of Cambridge. All rights reserved.

## 'Electric Kettle' printed from http://nrich.maths.org/

The diagram shows a simple circuit: the cell provides energy, *V* volts, which causes a current, *I* amps, to flow around the circuit. There is also a resistance, *R* ohms.

This circuit provides a simple model for what happens in an electric kettle: a resistance converts electrical energy into heat energy by impeding the flow of electrons around the circuit.

The table shows data collected from a circuit like this.

**Resistance (ohms)** |
**Temperature (degrees Celsius)** |

5 |
44.9 |

6 |
50 |

7 |
55.1 |

8 |
59.9 |

9 |
65 |

10 |
70.1 |

- Draw a graph of this data, with the resistance on the horizontal axis, putting a straight line through the points.
- Find the gradient of the line.
- Find the equation of the line.

Once you have found the equation, discuss these questions:

- What resistance would you need to heat water to 100 C°?
- What would the temperature be if the resistance was zero?
- Do you think that in practice, any circuit can have zero resistance?