PME
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I calculated the potential difference across a resistor in a circuit correctly (2.2V). The e.m.f is 9V and internal resistance is negligible.
There was a second resistor but it's presence didn't matter.

A capacitor was then placed "parallel" across the resistor I calculated a 2.2 voltage for. The question asked "State the value of the maximum potential difference across the capacitor in this circuit."
Normally with a capacitor in series the answer would be 9V as the resistors do not make a difference.

The answer, however, was 2.2 volts. Why does this happen when a capacitor is placed across a resistor?
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Stonebridge
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(Original post by PME)
I calculated the potential difference across a resistor in a circuit correctly (2.2V). The e.m.f is 9V and internal resistance is negligible.
There was a second resistor but it's presence didn't matter.

A capacitor was then placed "parallel" across the resistor I calculated a 2.2 voltage for. The question asked "State the value of the maximum potential difference across the capacitor in this circuit."
Normally with a capacitor in series the answer would be 9V as the resistors do not make a difference.

The answer, however, was 2.2 volts. Why does this happen when a capacitor is placed across a resistor?
If the capacitor and resistor are in parallel and the resistor has a pd of 2.2V across it then the capacitor will (must) have the same pd. That's what being in parallel means.
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Sparatus
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The voltage across two points is the amount of energy gained (if it's a battery or other emf source) or lost (if it's a resistor or something that's charging) per unit charge as it flows across those two points. Consider a unit of charge (a bunch of electrons) with flowing towards the resistor/capacity combo in your circuit. If it has not gone through any other components yet after leaving the battery (or emf source) then it has 9J of energy. (If it went through a 5V component before it got here then it would have 9-5 = 4J, etc).

This means that each electron has 9 * (charge of electron / charge of entire bunch ) Joules of energy. In other words, the energy of the bunch can be assumed to be evenly distributed among its electrons.

When the bunch reaches the junction splitting the path towards the resistor and the capacitor, it's split up with one portion going towards the resistor and another going towards the capacitor.

If you think about it, the split is given by the ratio of currents that flow in the two routes. The higher the current in a route, the larger the portions of the incoming bunches that needs to go through it per second. However, the energy that each electron has is still the same. So, at any time, if you pick a bunch of electrons in one of the two routes that's the same size as the bunch we considered earlier (Amounting to 1 unit of charge), then the total energy of this new bunch would be exactly the same as the old one. In other words the energy per unit charge (which is the definition of voltage) would be the same.

Hence the voltage across parallel routes are equal. So since the only component in each of the two routes in your circuit are the capacitor and resistor, they each suck up the same amount of energy per unit charge because that's what's available to them. (If you have other components in your circuit, you might see this better if you imagine they weren't there).
'
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