Why do wires get hot when a current passes through them?

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Presentation transcript:

Why do wires get hot when a current passes through them? 6.3 Fuses Why do wires get hot when a current passes through them?

6.3 Fuses 20 April 2017 Fuses and circuit breakers cut off the electrical circuit when the current gets too high. Circuit breakers have advantages over fuses. The correct size fuse is important. Double insulated devices have a shell of insulating material and so do not need to be earthed.

Fuses Symbol Fuses contain a thin piece of wire which melts when the current is higher than the value shown on it. This prevents fires. Fuse size = Power in Watts 230V E.g. a toaster has a power of 920W. Fuses are available in 3A, 5A and 13A sizes. Which one should be used? Fuse size = 920W / 230V = 4A. Use 5A the next highest value.

Testing a fuse 1 A Fuse wire Set up this circuit with 3cm of fuse wire on a heat proof mat. Switch on the power supply and move the variable resistor. What current does the fuse melt at? Connect a lead from point 1 to the red terminal and switch on. What happens?

The Earth Wire Electrical objects with a metal case must be earthed – the case connected to the earth with a wire. This provides a path for electricity to flow if there is a fault “blowing” the fuse. Safe Faulty Fuse Fault Earth Live Neutral Fuse Metal Case Live Neutral Earth Metal Case

Circuit Breakers Circuit breakers are electromagnetic switches which cut off the power when the current gets too high. Advantages Faster than fuses Can be reset

Key points A fuse contains a thin piece of wire which melts when too much current passes through it, cutting off the power. A circuit breaker is an electromagnetic switch that opens and cuts off the power if too much current passes through it.