# physics - electricity questionWatch

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#1

1. If I have a parallel circuit, and there are say three branches, if one of the branches breaks say a filament lamp burns out, would the current increase in the other two branches? I know p.d. and resistance would be unaffected for the other two right?

2. If I had a simple parallel circuit, where say 1.4 A went to one branch, and 1.2 A to another branch, in my textbook, it just states that 'each parallel branch can be thought of as a separate circuit. If changes are made to one branch, the other branches are not affected'. They then added another branch and 1.6 A goes to this branch. Where did the number 1.6 A come from? Did they just pick it randomly // how could i calculate it if a question came up in an exam?

Sorry, there is a lot and I hope it makes sense :/
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4 weeks ago
#2
(Original post by Klara_b155)

1. If I have a parallel circuit, and there are say three branches, if one of the branches breaks say a filament lamp burns out, would the current increase in the other two branches? I know p.d. and resistance would be unaffected for the other two right?

2. If I had a simple parallel circuit, where say 1.4 A went to one branch, and 1.2 A to another branch, in my textbook, it just states that 'each parallel branch can be thought of as a separate circuit. If changes are made to one branch, the other branches are not affected'. They then added another branch and 1.6 A goes to this branch. Where did the number 1.6 A come from? Did they just pick it randomly // how could i calculate it if a question came up in an exam?

Sorry, there is a lot and I hope it makes sense :/
The lightbulbs in a house are connected in parallel across a constant voltage source (with switches) - when a bulb fails or is switched off it doesn't affect the current in the other lightbulbs and it wouldn't be convenient if it did... you want to be able to control the lightbulbs independently of what the other lightbulbs are doing.
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#3
Ah okay, thank you so much for replying! Do you know what the reason for that is please?
(Original post by Joinedup)
The lightbulbs in a house are connected in parallel across a constant voltage source (with switches) - when a bulb fails or is switched off it doesn't affect the current in the other lightbulbs and it wouldn't be convenient if it did... you want to be able to control the lightbulbs independently of what the other lightbulbs are doing.
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4 weeks ago
#4
(Original post by Klara_b155)
Ah okay, thank you so much for replying! Do you know what the reason for that is please?
the PD is the same across each parallel path (like you said in the OP)
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#5
Okay I am about to seem really stupid, and I am sorry for bothering, but if you have the same p.d. along each branch, and the Rt (total resistance) of the circuit would change as one branch now is not applicable, so the current (total) should change, and then the distribution of it would be differerent? I know I am wrong but I am really puzzled :/ sorry
(Original post by Joinedup)
the PD is the same across each parallel path (like you said in the OP)
0
4 weeks ago
#6
Oh yeah the total current would vary... it's the sum of all the branch currents

e.g. 1 if you had parallel branch currents of 1.4A and 1.2A the total would be 2.6A
if a third parallel branch was added with a current of 1.6A the total would be 4.2A

e.g. 2 if you knew the total current and all but one of the branch currents you could work out the missing branch current by subtracting the known branch currents from the known total
(Original post by Klara_b155)
Okay I am about to seem really stupid, and I am sorry for bothering, but if you have the same p.d. along each branch, and the Rt (total resistance) of the circuit would change as one branch now is not applicable, so the current (total) should change, and then the distribution of it would be differerent? I know I am wrong but I am really puzzled :/ sorry
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#7
but surely if the total current changes, the amount distributed to each branch would change. So if initally there were three branches of equal resistance, then the current would be split three ways. Then if one branch was short-circuited for instance if a bulb broke, then the current would be split two ways, so they should get more current, no?
(Original post by Joinedup)
Oh yeah the total current would vary... it's the sum of all the branch currents

e.g. 1 if you had parallel branch currents of 1.4A and 1.2A the total would be 2.6A
if a third parallel branch was added with a current of 1.6A the total would be 4.2A

e.g. 2 if you knew the total current and all but one of the branch currents you could work out the missing branch current by subtracting the known branch currents from the known total
0
4 weeks ago
#8
(Original post by Klara_b155)
but surely if the total current changes, the amount distributed to each branch would change. So if initally there were three branches of equal resistance, then the current would be split three ways. Then if one branch was short-circuited for instance if a bulb broke, then the current would be split two ways, so they should get more current, no?
you've got cause and effect reversed... with a constant voltage source changing the current in one of the branches causes the total current to change.

The total current is the result of what's happening in the parallel branches
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#9
oh i see. Thank you so much !
(Original post by Joinedup)
you've got cause and effect reversed... with a constant voltage source changing the current in one of the branches causes the total current to change.

The total current is the result of what's happening in the parallel branches
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