MATERIALS • a circuit board • two cells • a piece of nichrome wire or a strand of steel wool • connecting wires • a switch • a small bowl of water
METHOD Step 1 Set up the circuit shown in Figure 1 alongside. Step 2 Before you close the switch, feel the nichrome wire.
Step 3 Close the switch and feel the nichrome wire again.
Step 4 Place the nichrome wire in the water and feel the water after a few minutes.
Figure 1 Heating a nichrome wire with current
RESULTS Write two or three sentences to describe your observations.
QUESTIONS 1 Write a conclusion to explain your results. 2 Predict how you think the information that you have from this investigation can be applied. (Consider the advantages and the disadvantages of the heating effect.)
Fuses and short circuits
Fuses prevent circuits from overheating. An overheated circuit can be dangerous and lead to fires. Circuits can overheat if a short circuit occurs.
Fuses A fuse is a type of safety device. It is used to protect the circuit (and people) from the heating effect of an electric current. A fuse or circuit breaker reduces the danger of electricity that can overheat conducting wires and lead to a fire.
A fuse consists of a very thin piece of wire with a low melting point. The current flows through the wire and if the current becomes too big, the wire will get hot and melt. This breaks the circuit and the current will stop flowing. If a fuse “blows” the electrical fault must first be fixed and then the fuse can be replaced.
Fuses are found in the electrical systems of cars. Some older houses have fuses to protect the wiring in the house. Newer houses tend to have trip switches instead of fuses. A trip switch uses the magnetic effect of current (see page 114) to break a circuit when the current becomes too large. This helps prevent a fire. A trip switch can be reset once the fault has been fixed.
Figure 2 Fuses prevent overheating in circuits
nichrome wire
New words
short circuit – when an electric current flows through a path with low resistance in a circuit and not through the rest of the circuit
fuse – a device used in electric circuits to prevent overheating
Unit 3 Effects of an electric current 113
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