# Homework Help: Voltage-divider and d'Arsonval voltmeter problem

1. May 4, 2016

• Member advised to use the homework template for posts in the homework sections of PF.
The voltage-divider circuit is designed so that the no-load output voltage is 8/10ths of the input voltage. A d'Arsonval voltmeter having a sensitivity of 200 ohms/V and a full-scale rating of 150 V is used to check the operation of the circuit.
a) What will the voltmeter read if it is placed across the 126 V source?
b) What will the voltmeter read if it is placed across the 60 kohms resistor?
c) What will the voltmeter read if it is placed across the 15 kohms resistor?
d) Will the voltmeter readings obtained in parts (b) and (c) add to the reading recorded in part (a)? Explain why or why not.

Output voltage is 100.8 V. About d'Arsonval voltmeter: using sensitivity and full-scale rating we can find resistance of the voltmeter? Than it is 30 kohms. I think in part (a) answer is 126 V, is it correct? Have some problems with parts (b)-(d). Thanks for sharing your thoughts!

#### Attached Files:

• ###### physics.PNG
File size:
16.6 KB
Views:
101
Last edited: May 4, 2016
2. May 4, 2016

3. May 4, 2016

### Staff: Mentor

Your answer for (a) is correct. Please show us your work on the rest of the parts of the problem. What is the equation for a Voltage Divider?

4. May 5, 2016

Voltage divider equation: Uout = Uin * R2 / (R1 + R2). And for my problem Uout = 126 V * 60 kohms / (60 kohms + 15 kohms) = 100.8 V.

5. May 6, 2016

### CWatters

Try drawing the equivalent circuit for the set up in b)

6. May 6, 2016

Here it is.

File size:
5.4 KB
Views:
94
7. May 7, 2016

### CWatters

OK now replace the real world volt meter with an ideal meter and a resistor

8. May 7, 2016

Can I use this equations: U2 = I * R;
I = Uin/(R1 + R2)?
I = 126 V / (15 kohms + 60 kohms) = 1.68 mA; U2 = 1.68 mA * 60 kohms = 100.8 V. Is that correct?

9. May 8, 2016

### CWatters

Thats the voltage that an ideal meter would display not the voltage this meter would display. See my post #7.