You are Here: Home >< Physics

# Physics, Azzopardi book page 291 question 3 watch

2. Sorry you've not had any responses about this. Are you sure you've posted in the right place? Here's a link to our subject forum which should help get you more responses if you post there.

Just quoting in Fox Corner so she can move the thread if needed
Spoiler:
Show
(Original post by Fox Corner)
x
3. Could you please post the question?
4. (Original post by tempo91)
Could you please post the question?
Two 15 micro-Farad capacitors are connected in parallel, and the pair are connected in series with a 45 micro-Farad capacitor and a battery. If the p.d. across the parallel pair is 30 V, Calculate:
a) the charge on each capacitor
b) the e.m.f. of the battery
c) the total capacitance of the circuit
d) the total energy stored in the capacitors.
5. Firstly it would help to draw a circuit a diagram to understand the scenario.

For now, let the two 15 micro-Farad capacitors and 30 micro-Farad capacitor be , and respectively.

As for part a, you can calculate the charge on the capacitors in parallel because the question tells you the potential difference and capacitance across the parallel components using . To calculate the charge on 45 micro-Farad capacitor, you can treat the parallel components as if they were a single component equivalent to the ones in parallel. So since charge and capacitance can be added in parallel, so you'd have a 30 micro-Farad (15 + 15) and a 45 micro-Farad capacitor bearing in mind the charge on the 30 micro-Farad capacitor is now the sum of charge across the 15 micro-Farad capacitors.

You know in series, the charge across all capacitors are the same so

As for part b, the e.m.f of the battery is the sum of all potential difference across the components. You know that 30V goes across the capacitors in parallel and you can use to work out the p.d. across the 45 micro-Farad capacitor. Add everything up and you'll get your answer.

As for part C, do the same method from part a and treat the circuit as if it was a series circuit.

As for part d, use an energy equation where your C is the total capacitance and V is your e.m.f. A long method is to calculate the energy on each capacitor and then add them up.
6. (Original post by tempo91)
Firstly it would help to draw a circuit a diagram to understand the scenario.

For now, let the two 15 micro-Farad capacitors and 30 micro-Farad capacitor be , and respectively.

As for part a, you can calculate the charge on the capacitors in parallel because the question tells you the potential difference and capacitance across the parallel components using . To calculate the charge on 45 micro-Farad capacitor, you can treat the parallel components as if they were a single component equivalent to the ones in parallel. So since charge and capacitance can be added in parallel, so you'd have a 30 micro-Farad (15 + 15) and a 45 micro-Farad capacitor bearing in mind the charge on the 30 micro-Farad capacitor is now the sum of charge across the 15 micro-Farad capacitors.

You know in series, the charge across all capacitors are the same so

As for part b, the e.m.f of the battery is the sum of all potential difference across the components. You know that 30V goes across the capacitors in parallel and you can use to work out the p.d. across the 45 micro-Farad capacitor. Add everything up and you'll get your answer.

As for part C, do the same method from part a and treat the circuit as if it was a series circuit.

As for part d, use an energy equation where your C is the total capacitance and V is your e.m.f. A long method is to calculate the energy on each capacitor and then add them up.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: November 6, 2016
The home of Results and Clearing

### 1,068

people online now

### 1,567,000

students helped last year
Today on TSR

### University open days

1. Sheffield Hallam University
Tue, 21 Aug '18
2. Bournemouth University
Wed, 22 Aug '18
3. University of Buckingham