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# HELP! Experiment:investigate the formula for parallel combinations of resistors watch

1. Hi all!
the aim of the experiment is the title. Here is the diagram for the circuit.
Procedure:
1) Lay the loop of wire along the meter ruler as two parallel straight sections. Record the total length of wire L in meters. Set up the circuit as the diagram shown
2) Attach a croc. clip to the juncton of twisted ends. The second clip such that distance 'x' is 10cm. Record the pd and current.
Move the second clip around the loop recording values for x, V, I

Calculate the resistance R of the wire loop for each value of "x" using I and V.

Calculate the theoretical resistance by using the following equation:-
(attached as png file)
Attachment 730078
p - resistivity
x - distance
L- total length of the wire
A- area of cross section

Then we were supposed to plot the theoretical values and measured values onto a graph and we were supposed to get a gradient of 1.

My Doubt
Why was that loop of wire used and distance varied? I know that somehow they provided a parallel circuit.

Why does the resistance rise and then fall as the clip is moved around the loop?

While calculating %difference, would it be between the difference in gradients or the difference in the résistances Rx and R'?
2. I suppose it's quite convenient... you can adjust both resistances by moving one croc clip. Probably you'll make a greater number of measurements in a limited amount of time.

Doesn't seem to have any disadvantages over a method that has you adjusting two croc clips and measuring two lengths
3. (Original post by Joinedup)
I suppose it's quite convenient... you can adjust both resistances by moving one croc clip. Probably you'll make a greater number of measurements in a limited amount of time.

Doesn't seem to have any disadvantages over a method that has you adjusting two croc clips and measuring two lengths
Thanks a lot! So they act as variable resistor? to vary resistance?
I still dont understand why do i need to move the croc clip?
4. (Original post by Iluv2edgedsword)
Thanks a lot! So they act as variable resistor? to vary resistance?
I still dont understand why do i need to move the croc clip?
yeah you get 2 variable resistors in parallel.
resistance of a single piece of that resistance wire will be proportional to it's length.
you're measuring the distance between croc clips of the resistance wire - so you get two pieces of resistance wire in parallell, one has length x and the other has length L-x
5. (Original post by Joinedup)
yeah you get 2 variable resistors in parallel.
resistance of a single piece of that resistance wire will be proportional to it's length.
you're measuring the distance between croc clips of the resistance wire - so you get two pieces of resistance wire in parallell, one has length x and the other has length L-x
oohh...so that means resistance measured(V/I) will give me the resistance of both the wires? I really appreciate your help!
6. I still dont understand why resistance rises and falls when the croc clip is moved around loop....
7. If you draw out a circuit diagram of two resistors in parallel (which is effetively the resistances of the loop with the croc clips), and split say 10 ohms between the resistors, calculate the effetive resistance of the resistors in parrallel. So make them both 5 ohms then maybe 2 ohms and 8 ohms and so on.

The long distance between the croc clips has higher resistance than the shorter distance between the croc clips. So as x decreases resistance decreases.
If you draw out a circuit diagram of two resistors in parallel (which is effetively the resistances of the loop with the croc clips), and split say 10 ohms between the resistors, calculate the effetive resistance of the resistors in parrallel. So make them both 5 ohms then maybe 2 ohms and 8 ohms and so on.
I made them both 5ohms but the effective resistance wasn't 10ohms but 2.5 ohms though since the formula's different for calculating resistance in parallel
R1*R2
R1+R2

The long distance between the croc clips has higher resistance than the shorter distance between the croc clips. So as x decreases resistance decreases.
But toward the end when X, distance was the highest, the resistance was lowest as well..

Here is the pic of my results:-
Thanks a lot for your help!
9. Here's my results and after peer checking they were deemed to be accurate results...
Attached Images

10. (Original post by Iluv2edgedsword)
Here's my results and after peer checking they were deemed to be accurate results...
V=IR
For first set of results with x=0.1

R= 0.11/0.047 = 2.34

When x = 0.6
R= 0.24/0.0429 = 5.59

Hence resistance increases as distance x increases. Your results prove this
11. (Original post by G.Y)
V=IR
For first set of results with x=0.1

R= 0.11/0.047 = 2.34

When x = 0.6
R= 0.24/0.0429 = 5.59

Hence resistance increases as distance x increases. Your results prove this
What about resistance from 0.7 m to 1.1m? Thanks for your input!😊
12. (Original post by Iluv2edgedsword)
What about resistance from 0.7 m to 1.1m? Thanks for your input!😊
I guess from those results you've got a loop circumference of about 1.2m.

if you look at the diagram you posted in the OP you'll see that there is a clockwise current path and an anti clockwise current path... as one gets longer the other gets shorter.

now the formula for parallel resistances says
1/Rtotal = 1/R1 + 1/R2

(resistance should be proportional to length for a uniform wire)

and you can start making predictions about what resistance you should be measuring from that...

Things to think about... symmetry, what happens when both the lengths are equal?

would you expect the overall resistance to be the same or different whether the croc clip was 5cm to the left of the halfway point or 5cm to the right?

limits - what happens when you put the croc clip all the way up to one end so that one of the lengths is zero? (better do this as a *thought experiment only* as ammeters and power supplies tend not to like it)
13. (Original post by Joinedup)
I guess from those results you've got a loop circumference of about 1.2m.

(Original post by Joinedup)
and you can start making predictions about what resistance you should be measuring from that...

Things to think about... symmetry, what happens when both the lengths are equal?

would you expect the overall resistance to be the same or different whether the croc clip was 5cm to the left of the halfway point or 5cm to the right?

limits - what happens when you put the croc clip all the way up to one end so that one of the lengths is zero? (better do this as a *thought experiment only* as ammeters and power supplies tend not to like it)
Yeah now I understand, i think... as the length of clockwise path rise, the length of anticlockwise path decrease...they're antagonistic to each other so in both cases, resistances would be low since the current chooses the easy short path rather than long one.

So when both the pathways are equal, resistance reaches max as that point both pathways have equal max length right? Thanks a lot for the explanation!!!

While calculating %difference, would it be between the difference in gradients or the difference in the résistances Rx and R'?
14. (Original post by Iluv2edgedsword)

Yeah now I understand, i think... as the length of clockwise path rise, the length of anticlockwise path decrease...they're antagonistic to each other so in both cases, resistances would be low since the current chooses the easy short path rather than long one.

So when both the pathways are equal, resistance reaches max as that point both pathways have equal max length right? Thanks a lot for the explanation!!!

While calculating %difference, would it be between the difference in gradients or the difference in the résistances Rx and R'?
I don't like to think about charge carriers 'choosing' tbh...

Some current always goes down the high resistance path in a parallel circuit... the proportion follows fairly obviously from Ohm's law.

e.g. if you've got parallel resistors of 1 Ohm and 10 Ohm with 5 Volts across them

I=V/R gives you
5 Amps through the 1 Ohm and 0.5 Amps through the 10 Ohm
the total current would be 5.5A

effective resistance of those parallel resistors from R=V/I
5/5.5 = 0.91 Ohms

which is also what you'd get from the parallel resistor formula
1/R = 1/R1 + 1/R2

1/R = 1 + 1/10
1/R = 1.1
R=0.91
so it's all consistent

---
normal thing to do would be to compare your experimental results with the predictions made using the appropriate theory and comment on the outcome... but go with whatever you've been asked for.
15. Yeah... true... there should be some current flowing through the high resistant portion... Thanks for the clarification using the formulas!

Normally, I compare the percentage difference with percentage uncertainty where the former should be lesser than the latter. But here it's totally different. Anyways, I got it though.

Thanks a lot for all your help!

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