# Talk

## Hell's teeth Y10 Physics- circuit diagrams! Bear with me!

(7 Posts)
Tansie Tue 25-Mar-14 18:12:51

Bloody hell! I recall struggling with this myself 35 years ago (!) and here I am, trying to help DS1! He's doing higher tier fast-paced triple science- and shouldn't be. We now know that. His school only offer triple at 'fast pace' and we were assured DS was up to it, but he isn't, and we're stuck with it!

So here we are, me trying to come to terms with what happens when you stick resistance into a circuit in series with one in parallel. Or something.

He, no we have to find the following- wish I could do circuit diagrams on MN!: We have a circuit diagram presenting both 'in series' and 'in parallel' components. We're asked ? the voltage of a certain battery, the ammeter immediately beyond it is also '?'; beyond that the circuit splits! The first split has another ? ammeter, then a bulb of unknown (but not required resistance- all the 4 bulbs have the same resistance in this question)- we're asked ? the voltage across this bulb. This split then rejoins the whole circuit back to the cell. The second split starts with an ammeter measuring 1A- beyond this we have 3 identical bulbs (each having the same unknown resistance as the bulb on the first split); We're asked for the voltage across each of these bulbs though we're told the first has 3V across it. This second split then also rejoins the original circuit back to the cell.

Jeez.

My ishooz and suggestions:

- If all the bulbs present the same resistance, surely as we're given the voltage across one of them i.e. 3V, even though 3 are in series and one is in parallel with them, they all have a voltage of 3V across them? So it's a 3V battery?

- If 1 amp (as we're told) services a series of 3 bulbs and if 'bigger resistance = lower current' the ratio is "one amp to the 3 resistors, therefore 3 amps to the one resistor" (the one in parallel to these three) I can work out that the first split's ammeter should read 3A.

AM I right?

And congratulations if you've stuck with me!!

indignatio Tue 25-Mar-14 19:25:20

DH wants a go ....

If I have understood the circuit you start with a battery which goes to an ammeter. Then it splits into two parallel branches. The first branch has an ammeter and one bulb. The second parallel branch has an ammeter then three bulbs in series?

If so then this is the answer

You are told that the first bulb on the branch with three bulbs in series has 3volts across it. Since all the bulbs are in series, they all have the same current through them. Since they all have the same resistance (and the same current through them) then each one has the same voltage across it I.e. 3 volts across each one.

Therefore the battery is 9 volts (3 bulbs in series each with 3volts across it)

The single bulb in the other branch therefore has 9 volts across it and so carries 3 times as much current as the ones in the other branch (which only have 3volts across each one). The ammeter on the branch with 3 bulbs says 1 amp so the current in the branch with a single bulb is 3amps

The current from the battery is 4 amps (3 in one branch and 1amp in the other) so that is the reading on the ammeter nearest the battery

hench Tue 25-Mar-14 19:27:33

_||________
| |
| |
|_X1_X2_X3_@1_|
| |
| |
|____X4__@2___|

Does it look something like this?

With 3V across one of the 3 X's (bulbs) in a line (eg X1)
and @1 the ammeter reading 1 Amp?

If so, then you are right about the amps and almost right about the volts.

X1, X2 and X3 will have the same voltage across them - 3V each, but the battery voltage must be 3x3V=9V as the voltage across things in series adds together to give the total. This then means the voltage across X4 is also 9V (not 3). This also explains why X4 gets a current of 3Amps (although your reasoning was quite sound) as it has a bigger voltage across it to push more current through.

hench Tue 25-Mar-14 19:29:52

diagram failed there, as the spaces got compressed (& I missed an ammeter). But I agree with indignatios dh

RegainingUnconsciousness Tue 25-Mar-14 19:34:33

my favourite resource might help: www.my-gcsescience.com

Tue 25-Mar-14 19:42:29

I don't know at all, and not even any point me reading all above: but there must be masses of stuff on line about it, and Khan Academy must have some surely?

Nocomet Tue 25-Mar-14 19:54:21

DH reads circuit diagrams for fun, but if I pass this to him, he will blind you with science, wander into a level and leave you more puzzled.

BBC bite sized, CGP revision books or khan will confuse you far less.

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