x Turn on thread page Beta
 You are Here: Home >< Physics

# Capacitance - urgent help please XD watch

1. 2.0mF capacitor used as a backup power source. PD of 5.0V when fully charged.
Required to supply constant 1.0 microAmps and can be used until PD falls by 10%.
How long can it be used for?
2. V = Vnaught e^-t/rc

Use vnaught as 10% of v

rc we can calculate

use t = -(Ln(v/vnaught))RC
3. (Original post by Kyx)
V = Vnaught e^-t/rc

Use vnaught as 10% of v

rc we can calculate

use t = -(Ln(v/vnaught))RC
Thank you.
I thought of that but how can we work out RC if we don't have the resistance?
4. P = iv

p = v^2 / r

(Original post by myabzz)
thank you.
I thought of that but how can we work out rc if we don't have the resistance?
5. (Original post by Kyx)
P = iv

p = v^2 / r
Thank you, I think it is right
6. (Original post by myabzz)
Thank you, I think it is right
np

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: April 2, 2016
Today on TSR

### Boyfriend slept with someone else

...we were on a break

### Mum says she'll curse me if I don't go to uni

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE