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Circuit question!!


weneedhelp2002

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There are three parts to the following question about electrical circuits... First, when there is one light bulb in a circuit, parallel or series, and you unscrew the lightbulb, there is no more current but the base of the unscrewed lightbulb (the part that's connected to the circuit, not the actual bulb) still has voltage. Why? Secondly, there are two light bulbs in a series circuit and one is unscrewed (again there is no current in the circuit), but there is still voltage in the unscrewed lightbulb base, while there is no voltage in the lightbulb that is screwed in, no matter what order the two lights are in the circuit. Why does this happen? Lastly, if you have two light bulbs in a series circuit and remove them both, there is no voltage in any of the bases. Why is there no voltage when you remove both clight bulbs, but there is voltage when you only remove one?

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In the first case the voltage that you across the empty socket is the same voltage that made the light bulb light up when it was in place. It might be slightly higher, because now that no current is flowing there are no voltage drops in the wires. Voltage does not require a complete circuit - there's a voltage between the ends of a battery when it's just sitting on the shelf. That is the voltage that causes current to flow in the load when you attach one. The same can be said of the two slots of an AC outlet that has nothing plugged in, except that's an sinusoidal AC voltage rather than a fixed DC voltage.

 

In the second case, with the series bulbs, when you remove the first bulb you stop current from flowing in the system. With no current flowing, there can be no voltage across the second light bulb, because it is basically a resistor and resistors only develop voltage when a current flows. So the prongs of the socket that it's plugged into must have zero voltage across them as well.

 

In the last case, let's draw a picture. We'll assume the external power supply applies voltage across the outer ends of the drawing.

 

+V ---------------------A1 A2-----------------------B1 B2---------------------- -V

 

A and B are the bulbs. In this situation you would measure voltage between points A1 and B2. However, you won't measure voltage between A1 and A2 or between B1 and B2, because neither of those point pairs connect to both ends of the voltage source. In other words, when you remove both bulbs from the circuit you completely disconnect the wire A1-B2 from the circuit. That wire contains one of the prongs of both sockets, so neither socket will show a voltage.

 

Does that help?


Let me say that a slightly different way. With only one bulb removed, both prongs of the empty socket still see a conducting path to the two sides of the supply. There will be no voltage drop across the second bult (the one that's still in the circuit) because no current is flowing. With both bulbs removed, each socket has one terminal that is connected to the supply, but the other one is not. So neither socket will show a voltage, though an appropriate measurement using terminals from both sockets will.

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