Little help with Physics coursework

'lo,

I missed first year of this schools GCSE courses, due to moving house/plane/school. (Even country. ).

So I'm currently working substantially hard to try and catch up with an entire year of coursework.

This bit of Physics coursework is straight forward but has got me stumped on something. It's about how the length of wire changes the resistance. Can anyone tell me why the variable resistor is in there? Was it changed for each different length of the wire? If it was then obviously that makes it an unfair test??? If not, then what's the point of it being in there?

I don't know the above because I obviously didn't do the experiment like everybody else.

Here is diagram of the circuit

Hope you can help.

Thanks,

 

No idea why that's in there.

Changing the length of the wire will change the resistance (longer == more resistance).

As long as the v/resistor is kept the same as you vary the length of the wire, the results won't be 'unfair' in any way...

I'd ignore it

 

Yup, exactly what I thought. The results show an increase in voltage as the wire length is increased though? :/

 

I just thought - the variable resistor might be in there to demonstrate that resistors in series add up?

Voltage shouldn't increase as length increases though...

*mutters something about welsh electronics *

 

Damn, this is confusing.

The voltage increases as the wire length increases, and you're saying it shouldn't? Which means they did it with the variable resistor, which makes the test unfair?

 

I noticed that was slyy added in afterwards. Wales rocks!

The sheep will get you... oh yes, they will.

 

surely as length increases the resistance increases (R=pl/A IIRC) so as V=IR, V will increase ??

Also the variable resistor might be to limit the current flowing. As i remember in college we did this with loads of wires and had to tweak the resistor to stop it burning as long as you dont toucj it as you increase the length it wont amke a difference

 

The variable resistor is there to probably show you that in a series circuit the two resistors will add up to the same resistance. As the wire increases in length the voltage goes up:

V=IR the current will be constant but the resistance will increase making the number larger each time

And yes the variable resistor must stay in the same position each time

 

BjD: Yep, that sounds right.

The voltage is going up as the wire length is increased, and the current is going down, and the resistance is going up.

So, you're saying the variable resistor is kept the same, definately?

 

Sorry, voltage over the *resistor* increases as the resistance goes up

was thinking throughout the whole circuit

V_r = IR, I = constant and R increases making V_r increase!

god, and to think I have an A @ Alevel electronics

 

Lol, just realised i did the same thing over here Rich

 

I was thinking "huh? how can you get more volts than you started with "

 

yep, as i recall we used thin wire so obviouslt needed to limit the current, we just tweaked it and left it.

ph33r teh sheep!

 

BjD, just as an aside, are you in Lufbra, and do you know a guy called Matt Kellet?

 

By looking at others coursework online, it seems that the variable resistor is there to keep the current the same on each test (at 2A).

On my results, the current is different for each test, and decreases as the length of the wire increases. So I don't know where he got these results, but they must be wrong?

I'm going to see him tomorrow to try and resolve it.

Thanks for the help all. 8)

 

yeh that sounds about right, i remember doing this project, we didnt use a resistor though, just a variable power supply instead

 

yeah im in uni in lufbra that name doesnt ring any bells tho

 

They're still right. As the wire resistance increases it takes a bigger share of the full supply voltage provided the other resistance is kept the same or reduced to give the same total circuit resistance. The share is different for the two cases, but still bigger.

Ohm Rule for Yorkshire

 

Actually, Where the current is measured in your circuit, is before any of the x lenght of wire. using the Variable resistor to create the same current going into the measured Voltage section (x) And as the others said, V=IR, and if you keep I constant with the pot, where it is measured (before the Voltmeter), and you increase R, V HAS to increase, up to the supply V. You cannot go past the supply V in any case, and when the x is longer than that, it is unable to keep the current I constant. The voltage drop across the pot decreases as the length increases, but only till the resistance is 0 at the pot. At that point, the problem falls apart.

The circuit is really a way to make a constant current supply out of a constant voltage supply.

 

Went to see him today, he said it's all right.

Apparently the voltage goes up as length is increased and the current goes down. The variable resistor is left alone.

So all is ok.

Thanks,