# Physics - Electronics (AS)Watch

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#1
"A rheostat restricts current in a circuit and a potentiometer restricts voltage"

I know how the potentiometer works - it's like a variable resistor that allows a certain percentage of the total p.d. of the circuit proportional to it's position to an instrument. How does a rheostat work?
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15 years ago
#2
A rheostat works by varying the current of an appliance, like a lamp's brightness or something.
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#3
I know what it does, but how does it vary current?
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#4
How?
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15 years ago
#5
oh sorry ok:
It is effectively a variable resistor, it has a high resistance wired coil with a sliding contact. You can move the contact and hence change the current, i'm out of info
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#6
That's the problem, I can't see the difference between that and a potential divider. If you slide it along so that the circuit is extended then surely the voltage across it will increase and it will simply change the voltage across the other instruments.

I don't see how it can affect current by changing it's resistance when a very similar instrument can change voltage by changing its resistance.
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#7
Bah I hate electronics. Nothing ever makes any sense.
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15 years ago
#8
It's been a while since I did this, but here goes.
A rheostat is a variable resistance included as part of a circuit. As this resistance is varied, the current throught the circuit is also varied. See here for a more graphic illustration.
Now I'm not sure about this bit, but this is what I think. In a potentiometer, there is a circuit with a single resitor that is divided into two resistances (by a slider). Each of these two resistance has a p.d. that is the potential for two other output circuits. Now the sum of the p.d.s of the two resistances is constant and this makes up the "primary" voltage. As the slider moves, R1 (say) will vary and the p.d. over this will vary and so the output to circuit 1 will vary in potential. Output 2 will just be a dummy and won't be used (I think!)

HTH
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15 years ago
#9
(Original post by Fermat)
Each of these two resistance has a p.d. that is the potential for two other output circuits. Now the sum of the p.d.s of the two resistances is constant and this makes up the "primary" voltage. As the slider moves, R1 (say) will vary and the p.d. over this will vary and so the output to circuit 1 will vary in potential. Output 2 will just be a dummy and won't be used (I think!)

HTH
Its all about the ratio of the 2 that determines the output voltage. A bit like a potential divider say.
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#10
But in series current is constant, so by adding resistance in a rheostat aren't you just going to lower the voltage across the rest of the circuit?
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#11
What's an inductance? I've never heard that word before....
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#12
I don't know what a capacitor is either

It is something to do with an electromagnet, because I can't see any other significance of a being coiled around a core.
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15 years ago
#13
(Original post by piginapoke)
Time for some serious reading I think.

The purpose of the coil is to create a magnetic field, yes.
We aren't taught that at AS though.

Also, there are way more interesting things than electricity in Physics!

This is more electronics really...
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#14
Yep. But we're getting asked these questions...

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15 years ago
#15
(Original post by mik1a)
Yep. But we're getting asked these questions...

Only coz Edexcel sucks!

AQA leave it out of the syllabus completely!
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#16
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15 years ago
#17
(Original post by mik1a)
Never mind that, have you seen the trauma with the P3 Maths exam last year?

Never mind learning a few Physics words here and there!
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15 years ago
#18
(Original post by piginapoke)
I thought it was an optional module for A-level. Times have changed.
I don't think there is any option in Physics A-Level anymore, regardless of the syllabi.

On my syallabus, AQA A:

Module 1 - Particles, Radiation and Quantum Phenomena
Module 2 - Mechanics and molecular kinetic Theory
Module 3 - Electricity and Elastic Properties Of Solids
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