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Consider this circuit.

What would I get if I measure over R1? It should be less than 12v, right? But then what volatage is LAMP1 running at? Is it still 12v, or is it the voltage I measured across R1? If it's 12v, then why didn't the voltmeter show 12v since it's connected to the supply in the same way as the lamp (IE over R1)?

I'm confused because the wiki article on parallel and series circuits states that:

If the light bulbs are connected in parallel, the currents through the light bulbs combine to form the current in the battery, while the voltage drop is 6.0 V across each bulb and they all glow.

(The wiki article uses a 6V supply instead of a 12V for their example)

So what is the voltage drop across LAMP1, and if it is different to the measurement across R1, then why?

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  • \$\begingroup\$ "What would I get if I measure over R1? It should be less than 12v, right?" - Why do you think this? \$\endgroup\$ – RedGrittyBrick Dec 6 '13 at 10:25
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V1, R1, and LAMP1 are connected in parallel in your circuit. That means that the voltage across R1 is equal to V1 or 12 volts. Likewise, the voltage across LAMP1 is also equal to V1 or 12 volts. Thus the current through R1 is 12 volts/100 ohms or 120 milliamperes. The current through LAMP1 is 12 volts/100 ohms or also 120 milliamperes. Note that the current through R1 is the same as the current through LAMP1 because their resistance is the same, not because they are connected in parallel. In a practical circuit, the interconnecting wires would have some small resistance (small compared to 100 ohms) so that the voltage across R1 would be slightly less than V1. The voltage across LAMP1 would also be slightly less than R1 due to the voltage drop in the wires between R1 and LAMP1.

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Evidently, you don't understand the following: parallel connected circuit elements have identical voltages

This is an elementary, fundamental result of ideal circuit theory. According to the schematic you've provided, there is just one voltage present, the voltage across the parallel connected circuit elements.

Now, if the physical wires connecting the circuit elements have significant resistance, then you will measure a different voltage across each circuit element.

It will also be the case that your schematic does not accurately reflect the actual state of affairs.

In other words, it will no longer be the case that the circuit elements are connected in parallel as the schematic suggests; the wires should be modelled as resistors with their resistance shown on the schematic.

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In this question, you'll measure 12 volts across R1, and 12 volts across LAMP1. R1 will have 12/100 = 0.12 amp's through it, and LAMP1 will have the same 0.12 amps through it. The total current from V1 will be 0.12 + 0.12 = 0.24 amps.

I'm not sure why you think there would be less than 12 volts across R1. Referring to the wiki article, R1 and LAMP1 are connected in parallel.

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