How do potential dividers and potentiometers work?

Also, may you explain question 6 part d from the following paper?

http://papers.xtremepapers.com/CIE/Cambridge%20International%20A%20and%20AS%20Level/Physics%20%289702%29/9702_s10_qp_22.pdf

Thank you in advance.

Reply 1

uberteknik

Original post by asadmoosvi

Hello and welcome to TSR. :smile:

Physics study help forum works by you explaining where your difficulty arises in understanding / showing your work so far. Then we give you guidance and hints so you can work out the corrct answer for yourself rather than simply giving you the answer which requires no work on your behalf. :tongue:

Soooooo..........

What is it about potential dividers you do not understand that needs clarification?

Q 6(d):

If the resistance wire is linear, then the resistance between both halves at the mid point will be equal. However, theres a matter of that pesky voltmeter internal resistamce.

Can you see how that might alter the voltmeter reading?

Reply 2

Joinedup

Original post by asadmoosvi

in a fixed potential divider you have two fixed resistances (R₁ and R₂ in series with a supply voltage, V_supply across the pair.

Because they're in series you know some things.
1. the total resistance R_total = R₁ + R₂
2. the current through both is the same and given by I = V_supply / R_total

the voltage across a resistor is given by ohms law V=IR so the voltage across either resistor is given by V_x=R_x V_supply/R_total
---
e.g. if V_supply=10V
R₁=7 ohm
R₂=3 ohm
then
R _total = 7+3 = 10 ohm
I = 10/10 = 1 amps
V₁=R₁ * V_supply/R_total
=7*10/10
=7

you can repeat for R₂ and then confirm that V₁+V₂=V_supply as a sanity check

neat.

however it only works if the current in both resistors is the same, so is the current the same when that 5 ohm ammeter is poking about your circuit?

Reply 3

TaraStilton

How about potentiometers- how do they work?

Reply 4

uberteknik

Original post by TaraStilton

How about potentiometers- how do they work?

Hello and welcome to TSR. :smile:

Potentiometers (variable resistor) are nothing more than a variable potential divider.

The two ends of the potentiometer have a resistor connected between them with the resistive element exposed (called the track) so that a third sliding conductor (wiper) is able to move along the track.

The output voltage is picked off by the wiper whose position along the track determines the ratio of the divided resistance.

Hey presto: variable potential divider.

Thanks!

First equation: V1 = IR1
Second equation: V2 = IR2

How do you divide the first equation by the second to get V1/V2 = R1/R2? What does dividing the first equation by the second equation mean?

Reply 7

Stonebridge

Original post by TaraStilton

First equation: V1 = IR1
Second equation: V2 = IR2

How do you divide the first equation by the second to get V1/V2 = R1/R2? What does dividing the first equation by the second equation mean?

It means if, for example

y = 2x
and
z = 4x

You know z is 2 times bigger than y

In other words
y/z = 1/2

You can refer to this as dividing one equation by the other
y/z = 2x/4x

y/z = 1/2

This is what has been done in the example you have.