# Circuits helpWatch

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#1
I was doing question no. 51 (see image 1) and ended up choosing the incorrect answer. The second image provides an explanation for the correct choice. The section I don't understand from the explanation is" This parallel path does not affect the current flowing through or the voltage across R1 and R2 ...". I thought that current splits at the junction of parallel circuit, could someone please explain the answer.
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#2
Here are the images:
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4 days ago
#3
(Original post by Physics1872)
I was doing question no. 51 (see image 1) and ended up choosing the incorrect answer. The second image provides an explanation for the correct choice. The section I don't understand from the explanation is" This parallel path does not affect the current flowing through or the voltage across R1 and R2 ...". I thought that current splits at the junction of parallel circuit, could someone please explain the answer.
You have an ideal battery, so, regardless of the current that it is driving, the voltage across it does not change. It will therefore continue to supply the same current to the first branch, whilst supplying more for the, now active, branch.

The answer should really mention the voltage not changing first, as it's that that guarantees that the current through the original branch doesn't change.
Last edited by RogerOxon; 4 days ago
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#4
(Original post by RogerOxon)
You have an ideal battery, so, regardless of the current that it is driving, the voltage across it does not change. It will therefore continue to supply the same current to the first branch, whilst supplying more for the, now active, branch.

The answer should really mention the voltage not changing first, as it's that that guarantees that the current through the original branch doesn't change.
What assumptions are associated with ideal batteries? And I suppose that since P=IV, I and V must both be the same for power to remain the same so that's why the current is said to be the same perhaps.
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4 days ago
#5
(Original post by Physics1872)
What assumptions are associated with ideal batteries?
An ideal battery is one whose output voltage is constant, regardless of the current that is flowing. A practical battery is modelled by adding an internal resistance, which lowers the output voltage as the current increases.

With an ideal battery, adding any parallel path, including a short, makes no difference to the current flowing through an existing path, as the voltage across it remains the same.

With a practical battery, taking more current will lower the output voltage, and hence the current through existing paths.

(Original post by Physics1872)
And I suppose that since P=IV, I and V must both be the same for power to remain the same so that's why the current is said to be the same perhaps.
An ideal battery will deliver whatever power, or current, that is required to keep its stated output voltage. You put a 1 Ohm resistor across a 10V ideal battery, and it will deliver 10A. 0.001 Ohm, and it will deliver 10,000A.

The current through the existing path from one side of the battery to the other is conserved only because the voltage across it is.
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#6
(Original post by RogerOxon)
An ideal battery is one whose output voltage is constant, regardless of the current that is flowing. A practical battery is modelled by adding an internal resistance, which lowers the output voltage as the current increases.

With an ideal battery, adding any parallel path, including a short, makes no difference to the current flowing through an existing path, as the voltage across it remains the same.

With a practical battery, taking more current will lower the output voltage, and hence the current through existing paths.

An ideal battery will deliver whatever power, or current, that is required to keep its stated output voltage. You put a 1 Ohm resistor across a 10V ideal battery, and it will deliver 10A. 0.001 Ohm, and it will deliver 10,000A.

The current through the existing path from one side of the battery to the other is conserved only because the voltage across it is.
Thanks a lot for the help, much appreciated !!
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