You are Here: Home >< Physics

# Percentage capacitor discharge Watch

1. Hiya all,

how would I go about doing the following question sorry?

A 50microF capacitor is charged by connecting it to a 6V battery then discharged through a 100kOhm resistor.

Estimate the resistance of the resistor that you would use in place of the 100kOhm resistor if the discharge is to be 99% completed within about 5s

I've already calculated charge stored and the time constant.

Thank you! ^__^
2. (Original post by NothingCrushesUs)
Hiya all,

how would I go about doing the following question sorry?

A 50microF capacitor is charged by connecting it to a 6V battery then discharged through a 100kOhm resistor.

Estimate the resistance of the resistor that you would use in place of the 100kOhm resistor if the discharge is to be 99% completed within about 5s

I've already calculated charge stored and the time constant.

Thank you! ^__^
Is this part of a larger question?
I ask because there is no need to know the voltage (6V) or the value of the 100kOhm resistor as you replace it with a different one for this question.
You don't need to know the charge on it when fully charged.
All you need to know is that there is now only 1% of the charge remaining after 5 seconds.

Then use

then
Q/Qo = 0.01 for 99% discharge
Take logs of both sides to find R for t=5s
3. (Original post by Stonebridge)
Is this part of a larger question?
I ask because there is no need to know the voltage (6V) or the value of the 100kOhm resistor as you replace it with a different one for this question.
You don't need to know the charge on it when fully charged.
All you need to know is that there is now only 1% of the charge remaining after 5 seconds.

Then use

Q = Qo e^-t/CR
then
Q/Qo = 0.01 for 99% discharge
Take logs of both sides to find R for t=5s
It is indeed part of a larger question; thanks for the help!
4. (Original post by Stonebridge)
Is this part of a larger question?
I ask because there is no need to know the voltage (6V) or the value of the 100kOhm resistor as you replace it with a different one for this question.
You don't need to know the charge on it when fully charged.
All you need to know is that there is now only 1% of the charge remaining after 5 seconds.

Then use

then
Q/Qo = 0.01 for 99% discharge
Take logs of both sides to find R for t=5s
But the problem is the question asks you to "estimate" so i was wondering if there was a way to find out the value without using the formulas. The answer in the back is 20kΩ which is 1/5 of the original resistance but if you were to use the formula you get 22kΩ.

Thoughts?
5. (Original post by -Maz-)
But the problem is the question asks you to "estimate" so i was wondering if there was a way to find out the value without using the formulas. The answer in the back is 20kΩ which is 1/5 of the original resistance but if you were to use the formula you get 22kΩ.

Thoughts?
How did you find such an old question?

Anyways, the chief examiners are very articulate, especially in physics. Remember that the decay rate of charge equation is a probability distribution
. This means that, ultimately, capacitor decay is a random process (to an extent) that varies with a certain degree of uncertainty with the decay equation. That's why they use 'estimate.'

The answer in the back is given as 20KΩ because one of the values used in the calculation/ prior to the calculation (the voltage?) will have been written to 1 significant figure. Any calculated value can only be as accurate as the least accurate input value. so although the 2 sig fig answer is the correct mathematics/calculation, It's not accurate. I.E 2 sig fig 22KΩ = 20kΩ (1 sig fig).

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: May 26, 2013
Today on TSR

### Oxford grad sues for £1m

Claims damages because he didn't get a first

### 'No evidence of shots fired' at Oxford Circus

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

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

• Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

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

• 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

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.