You are Here: Home >< Physics

# Circuit Question! watch

1. Just wanted to add one more thing. When we assume that none of the current passes through the wire of resistor B, then we also assume that resistor B doesn't exists in the circuit. So is that a fact of an ideal situation or what?
2. I guess you're supposed to assume when S2 is closed it has zero resistance.

what's having a zero resistance in parallel going to do to the voltage across resistor B and what's the current in B resulting from that voltage going to be?
3. (Original post by Joinedup)
I guess you're supposed to assume when S2 is closed it has zero resistance.

what's having a zero resistance in parallel going to do to the voltage across resistor B and what's the current in B resulting from that voltage going to be?
If something has no resistance than maybe it has no potential difference across it and no p.d represents that no current is flowing through it. If that was the case then all the current would've been flowing through wire of resistor B, and it should have had been considered even when switch S2 was closed. But when i don't consider it in the circuit when switch S2 is closed, then I get the answers stated in the mark scheme. I think that I'd have confused you too- would you like me to post the whole question OR you understand what I'm trying to say?

Edit: As the questions call these labelled things 'elements' - the examiner report says that when switch S2 will be closed the 'element' B won't be operating. That looks pretty strange to me.
4. Yeh they're called circuit 'elements' Dude you're being thrown off by silly words! This is what I always hated about A-level/IB physics, their idea of difficulty is wording a question weirdly rather than raw problem solving.

Currents take the path of least resistance, so naturally most of the charge will flow through S2 and completely ignore the element. This is sort of how short circuiting works.

If you look at the equation I = V/R, as R -> 0 then I -> infinite. So mathematically, the current through S2 is much larger than the current through the element.

It may also help to think of this at the most fundamental level. If a system can avoid doing work against something (which would be the case if current flowed through the resistor, hence why you had a potential across it anyway) then it will do it, i.e. all systems try to minimise its energy.
5. (Original post by trm90)
Yeh they're called circuit 'elements' Dude you're being thrown off by silly words! This is what I always hated about A-level/IB physics, their idea of difficulty is wording a question weirdly rather than raw problem solving.

Currents take the path of least resistance, so naturally most of the charge will flow through S2 and completely ignore the element. This is sort of how short circuiting works.

If you look at the equation I = V/R, as R -&gt; 0 then I -&gt; infinite. So mathematically, the current through S2 is much larger than the current through the element.

It may also help to think of this at the most fundamental level. If a system can avoid doing work against something (which would be the case if current flowed through the resistor, hence why you had a potential across it anyway) then it will do it, i.e. all systems try to minimise its energy.
Oh, cool! So should I always remember and apply this whenever any element is connected in parallel with a wire with zero resistance? And btw if in future any question asks the disadvantage of a circuit like this, then may I write that it will cause short circuit?
6. (Original post by Zishi)
Oh, cool! So should I always remember and apply this whenever any element is connected in parallel with a wire with zero resistance? And btw if in future any question asks the disadvantage of a circuit like this, then may I write that it will cause short circuit?
No I wouldn't do that - the informal use of 'short circuit' is whan someone's talking about an unexpected, accidental electrical fault...

if it's not an accident, not unexpected (i.e. part of the way a circuit has been designed to work) and isn't a fault it's not really a disadvantage - just a technical description of how a bit of circcuit works.
7. (Original post by Joinedup)
No I wouldn't do that - the informal use of 'short circuit' is whan someone's talking about an unexpected, accidental electrical fault...

if it's not an accident, not unexpected (i.e. part of the way a circuit has been designed to work) and isn't a fault it's not really a disadvantage - just a technical description of how a bit of circcuit works.
Oh, as trm90 said that this is sort of how short circuiting works that's why I was asking. So when's a circuit shorted btw?
8. (Original post by Zishi)
Oh, as trm90 said that this is sort of how short circuiting works that's why I was asking. So when's a circuit shorted btw?

http://en.wikipedia.org/wiki/Short_circuit

like it says it's only normal to call something a short circuit if it's abnormal.
9. (Original post by Joinedup)
http://en.wikipedia.org/wiki/Short_circuit

like it says it's only normal to call something a short circuit if it's abnormal.
Oh, I got it. Thanks.
10. I shouldn't have used 'normal' and 'abnormal' in the same sentence like that.

an example of a short circuit would be a fault caused by a rat chewing the insulation off a flex till the different conductors touched each other.
11. (Original post by Joinedup)
I shouldn't have used 'normal' and 'abnormal' in the same sentence like that.

an example of a short circuit would be a fault caused by a rat chewing the insulation off a flex till the different conductors touched each other.
So is it obvious that a short circuit will result in heating up of an element?
12. (Original post by Zishi)
So is it obvious that a short circuit will result in heating up of an element?
in the rat gnawing example it'd probably just blow a fuse (or trip a circuit breaker) or start a fire.

I'd avoid using the phrase 'short circuit' in exams unless you're definately talking about an abnormal event - and not closing a switch. I think we were discouraged from using 'short circuit' altogether when I did A level physics as it's entered the general non-technical language to mean any sort of electrical fault.
13. (Original post by Joinedup)
in the rat gnawing example it'd probably just blow a fuse (or trip a circuit breaker) or start a fire.

I'd avoid using the phrase 'short circuit' in exams unless you're definately talking about an abnormal event - and not closing a switch. I think we were discouraged from using 'short circuit' altogether when I did A level physics as it's entered the general non-technical language to mean any sort of electrical fault.
Hmm, I'll be careful with that word, then.

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: April 5, 2011
Today on TSR

### Are you living on a tight budget at uni?

From budgets to cutbacks...

### University open days

1. Loughborough University
Fri, 21 Sep '18
2. University of Cambridge
Fri, 21 Sep '18
3. Richmond, The American International University in London