Physics problem-rheostat

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viorelflower

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I had a problem in physics but I never learned anything about it in highschool.We passed this lesson with no idea what a rheostat does or is.
Can someone explain me?I hope that you learned this during your training as an electrician because I never learned that.
Well if I still have the problem I will show you the details:
E=15v,r=5 ohms,R(AB)=40 ohms,Resistor of Ampmeter is 0.
Calculate:
a)ampmeter indication when the cursor C is in the position B
b)rheostat voltage between points A and M when the cursor C is on the position M
c)voltage of every battery supply(E,r) when cursor C is on the position M
d)indication of the ampmeter when one of the batteries is polarized inverse.
I have no idea how to solve this.Can someone explain to me by solving this problem?

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I assume this is homework. What calculations have you made?
This is not homework.This is a problem that I had for my IB and I didn't know how to solve it.Do you know how to solve it?

I know most of dc laws but I have no idea what a rheostat is and if I try to solve this problem I can have a lot of errors.

A rheostat in my case are 2 resistors?I have no idea if this is a solution to my problem.

 
Here’s a hint. A rheostat is a variable resistor.
But won't that make it like a derivative in math?I mean R=V/I is constant but how do you know that the value of the resistor doesn't change from time to time?

 
Well if R=40 ohms won't that change it to another value?

Do I use voltage divider and current divider formulas?I have no idea what to use right now.

 
Yep, just got in and having a quick look before footy.

A rheostat is a linear variable resistor, so if it's half way along the linear variable resistor of 40 Ohms.....

Never got taught this stuff on sparky course, this is basic school physics.

 
I'm not trying to put anyone down, just vary their level of undertsanding  :^O  - I've kind of firgotten most of this stuff myself these days. Something to do with lack of use.

 
Thanks

never heard of it

is it an alternative to something?........GCSEs, A levels?
A level but harder than that.

In physics IB i choosed electricity(DC) and mechanics(which was easier than electricity).Do you think that I should choose something else if I couldn't solve this problem?

 
you have a voltage, you have resistances, what's the ampage V=IR ergo I = V/R., etc etc.

a/ C is at full length of the rheostat so 40 Ohms, add 5 ohms for each battery that makes 50 Ohms total.  Battery voltage with batteries in series is given as 15V  each, in series you add the voltages so 30/50 = 0.6 Amps

b/ you have 30V across a resistance of 20 Ohms ie half the linear resistor - work it out. They are asking for the potential difference between the 2 points in volts.

c/ Not sure what they are getting at here - it's 35 years since I left school, but I would say 15V at oth lower battery terminals,  30V at the higher battery becuase that would be 2 batteries in series - could be wrong about that.

d/ if you reverse polarity of one battery, they will cancel each other out rather than add voltages together so you would see no amps flowing through the circuit.

Any errors in the above will soon be corrected by another forum member..... :^O

 

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