# Homework Help: Potentiometer in circuit

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1. Jun 2, 2015

### PhysicStud01

1. The problem statement, all variables and given/known data

2. Relevant equations
No equations required

3. The attempt at a solution
The correct choice is A.
I know that a potentiometer provides some of its resistance of its total resistance and the other part is not contributed.

Current flows from the + of the battery and re-enters the -ve terminal.

What I can't understand is how the current goes in the diagrams shown and how to identify which part of the resistance should be considered and which not.

I've searched in books, on the internet, ... but could not find a proper explanation. If possible please, try to explain how each circuit works, not just the correct answer.

Thanks a lot

2. Jun 2, 2015

### theodoros.mihos

Identical is not exactly the same. If you see difference on a couple may exist more difference on another. But your choice is correct.

3. Jun 2, 2015

### PhysicStud01

Sorry, but i did not quite understand what you said

4. Jun 2, 2015

### DEvens

In X you have a wire across the side of the variable resistor between d and the right side. Make yourself a diagram that shows what that looks like. What is the effective resistance of a wire across a resistor?

5. Jun 2, 2015

### theodoros.mihos

Current not continue to x-shape after d because can find a path with no distance. So W and X works the same.

6. Jun 2, 2015

### PhysicStud01

Actually, it's not a reason for the correct answer that i want. I can't understand the flow of current and the part of the potentiometer that the resistance is being contributed. can you please explain, in terms of current, resistance - in a physics way - what's really happening in each case. in this way, i think i'll be able to identify the correct choice directly

7. Jun 2, 2015

### theodoros.mihos

Point d and the black ball have the same potential(voltage) because wire connection. So resistance right from d will not have current flow because voltage not exist. (is zero)

8. Jun 2, 2015

### PhysicStud01

but, isn't d a distance? so there will be different potentials. the distance d indicates the resistance?

+ edit: what about when the contact is from the battery itself OR what about as in the last circuit?

9. Jun 2, 2015

### theodoros.mihos

Usually, d is the part of potentiometer distance of whole length D. When potentiometer is in position d that mean the active resistance is $d/D\cdot{R}_{max}$, or the oposite $(D-d)/D\cdot{R}_{max}$.

10. Jun 2, 2015

### PhysicStud01

when is it the first one (i can't properly understand it) and when is it the opposite>

i know about the potential divider equation, but can't identify which part to use.

can we proceed as this please:
we try to compare each diagram with another. we see the visual difference in the diagram and how this difference affect / or not the result. can you please try to explain it in this way for all of the 4 circuit. this would really help with my understand of the physics of potentiometer, i think.

is there a circuit where the potentiometer does not contribute no matter where the slide is??
how can i understand the last one?
how does the additional wire in the 2nd diagram not affect the circuit and is similar to the first one?

11. Jun 2, 2015

### theodoros.mihos

Let d divide potentiometer resistance $R_0$ to two parts: $R_1,\,R_2$ with $R_1+R_2 = R_0$. Let the other resistance to be $R$.
Then. the active resistance for (A) and (X) is $R+R_1$. Can you see the active resistance on the others?

12. Jun 2, 2015

### PhysicStud01

yeah, but isn't this a justification of the answer.

i want to know what really happens?
sometimes the potiometer is connected directly to the battery, sometimes it's the slider.??
it does not have to be a small explanation. It's not a problem for me to have a full detailed description of what's happening, in every details if possible.

THanks

13. Jun 3, 2015

### PhysicStud01

any please, I really need a proper explanation for the poteniometers

14. Jun 3, 2015

### Staff: Mentor

You need to understand only that the slider on a potentiometer is a sliding connection to anywhere in the middle* of a length of resistance wire. (By "middle" I mean a variable location anywhere you choose somewhere between one end of the resistance wire and its other end.)

Let's see if we can highlight the similarities and differences in the circuits you are shown.

In arrangement W, use a red pen and trace out the path that a couple of electrons would take if they were to traverse the circuit from one end of the battery to its other end.

Now, do the same with arrangement X. (Take care with this one.)

15. Jun 3, 2015

### PhysicStud01

OK, I see.
W, X and Y are clear.

but fo r the last one, i'm still in doubt. i'll consider current instread of electrons.
flow: + battery, 'd', sliding contact, ameeter, - battery (does it neglect the resistor here??)

16. Jun 3, 2015

### theodoros.mihos

Because wire connection, d and black point are on the same voltage so...

17. Jun 3, 2015

### Staff: Mentor

There is a zero resistance path ("a short circuit") around the resistor in the final arrangement.

You'd choose the easy path, wouldn't you? Well, so does the current!

18. Jun 3, 2015

### PhysicStud01

THanks. I think it's a bit clear now.

But one lsat thing, how do i know which side of the contact should i take (i have assumed its the left). i believe a poteniometer constant a wire of zero resistnace - that's why one part of it does not contribute as the current flows through the 0 ressitance wire. but this wire could be either on the right or left? then the whole circuit would change, right?

19. Jun 3, 2015

### Staff: Mentor

A potentiometer consists of a wire having significant resistance. It is rare that we can regard its resistance as negligible; and never as zero ohms.

A good reason why you might ignore half of the potentiometer is when that end of its resistance wire element is not connected anywhere!