Act14_sol - Period 14 Activity Sheet Solutions: Electrical...

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Period 14 Activity Sheet Solutions: Electrical Safety and Transmission Activity 14.1: How Do Fuses and Circuit Breakers Prevent Fires? a) Fuse Demonstration with a Short Circuit Connect a piece of solder and a four bulb tray in series to a step-down transformer using connecting wires. Solder is thin wire with a low melting point that represents a fuse in this activity. The transformer lowers the voltage to a safe level. Note the brightness of the bulbs. Then create a short circuit by attaching a connecting wire between points A and B. 1) How does the path of the current change after you attach the wire between point A and point B? Describe what happens to the circuit when you add the connecting wire. Before attaching the wire, the current flows from the outlet through the transformer, the connecting wires, the solder, the light bulbs, and back through the transformer to the outlet. A wire connected between points A and B creates a pathway for current to flow around the bulbs and creates a short circuit. Joule heating in the solder causes the solder to melt, opening the circuit. Since current cannot flow through an open circuit, the light bulbs go out. 2) Which has greater resistance – the bulb tray or the wire connecting A and B? Explain how you know. The bulb tray has more resistance than the connecting wire. Current flows more easily through devices with lower resistance. When the wire is attached, most of the current bypasses the bulb tray and flows through the wire. Therefore, the wire must have less resistance than the bulb tray. 3) What happens to the current flowing through a circuit if the circuit resistance is decreased? How can this cause an electrical fire? Lowering the resistance allows more current to flow through the circuit. As current flows, some of its electrical energy is converted into thermal energy, which heats the wires. If the current is large enough, wires can be heated to the combustion temperature of the materials they touch and cause a fire. b) Fuse Demonstration with an Overloaded Circuit Your instructor will demonstrate an overloaded circuit with a fuse. Describe what happens as more bulbs in the circuit are lit. The fuse (solder wire) melts, breaking the circuit. The bulbs go out. c) How Can Fuses Prevent Fires? 1) How is connecting more light bulbs in a circuit similar to shorting a wire across a bulb? The light bulbs are connected in parallel, so adding more parallel pathways (more bulbs) reduces the resistance of this circuit. Reduced resistance allows more current to flow, which heats the fuse until it melts. This produces the same effect as reducing the circuit resistance by adding a pathway of less resistance around the bulbs in Activity 14.1.a. 2)
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This note was uploaded on 06/11/2011 for the course PHYSICS 103 taught by Professor Staff during the Spring '10 term at Ohio State.

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Act14_sol - Period 14 Activity Sheet Solutions: Electrical...

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