# Electric circuitWatch

Announcements
#1
Can anyone explain to me why the answer is D. I understand the potentiometer principle but could not analyze this question
0
4 years ago
#2
(Original post by TammyChong)
Can anyone explain to me why the answer is D. I understand the potentiometer principle but could not analyze this question
I found it easier to put some numbers in to simplify the reasoning.

1. Since the galvanometer doesn't deflect, the voltage XN = the voltage of the galvanometer's cell. Suppose these two voltages are 8 V

2. The voltage along the potentiometer drops linearly from max at Y to 0 at X.

3. Suppose the battery at the top provides 15 V and that the drop across the variable resistor is 5 V. Then the drop across the pot. is 10 V.

4. The distance YN must give us a drop of 2 V so YN must be 2/10 = 1/5 of the length of the pot (by point 2 above).

5. We now increase the variable resistance so we get a drop of 6 V across it.

6. ... can you do the rest?
0
4 years ago
#3
(Original post by TammyChong)
Can anyone explain to me why the answer is D. I understand the potentiometer principle but could not analyze this question
This is a variation on the 'Wheatsone Bridge' principle. i.e. nulling the p.d. across the galvanometer.

a) The resistance of the wire in series with the variable resistor forms a potential divider. Ignoring the galvanometer reading for the moment, what happens the p.d. across the wire between XY if the variable resistor increases?

i.e. use the potential divider equation to do this.

b) Now, by sliding the resistance-wire contact, if the p.d. developed across the resistance-wire is the same as the p.d. across the terminals of the galvanometer battery, no current will flow through the galvanometer which will then read zero.

i.e. the contact point with the wire itself forms a potential divider between points XY. By sliding the contact, the point where the p.d. of the single cell battery and the p.d. across the XY potential divider is the same, will produce no p.d. across the galvanometer and hence it will read zero.
0
X

new posts
Back
to top
Latest
My Feed

### Oops, nobody has postedin the last few hours.

Why not re-start the conversation?

see more

### See more of what you like onThe Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

### University open days

• The University of Law
Solicitor Series: Assessing Trainee Skills – LPC, GDL and MA Law - Guildford campus Postgraduate
Wed, 29 Jan '20
• Nottingham Trent University
Wed, 29 Jan '20
• University of Groningen
Fri, 31 Jan '20

### Poll

Join the discussion

#### What's stopping you doing a masters?

It's too expensive (53)
24.09%
My career doesn't need one (30)
13.64%
I'm sick of studying (45)
20.45%
I can't find a course I want to do (5)
2.27%
I don't know enough about them (15)
6.82%
Nothing, I'm going to do it! (72)
32.73%