You are Here: Home >< Physics

# Error!!! watch

Announcements
1. My specifications say that I need to be able to :

- assess the uncertainty in a derived quantity by simple addition of actual,fractional or percentage uncertainties ( a rigorous statistical treatment is not required ).

Can anyone tell me how to do it. We have vacations till August and I have no teacher at hand.

I also have a question for this:

The power loss
P in a resistor is calculated using the formula P = V 2/R.( thats (v square) / R)

The uncertainty in the potential difference
V is 3% and the uncertainty in the resistance R is 2%.

What is the uncertainty in
P?

A-
4% B- 7% C- 8% D- 11%

Thanks !

2. dealing with the example will show the process nicely

if the there is a 3% max error in V then the max error in V^2 will be 9%

this will produce a max error in P of 9% also

an error of 2% in R could also result in a 2% error in P

in the extreme case the errors combined could produce an error in P of 11%, which is the answer to the question.

notice this is simply the "addition ... of percentage uncertainties".
3. (Original post by Drummy)
dealing with the example will show the process nicely

if the there is a 3% max error in V then the max error in V^2 will be 9%

this will produce a max error in P of 9% also

an error of 2% in R could also result in a 2% error in P

in the extreme case the errors combined could produce an error in P of 11%, which is the answer to the question.

notice this is simply the "addition ... of percentage uncertainties".

But the answer is 8% in the marking scheme.

So I am confused.
4. The error in V^2 is 2 X 3% = 6% not 9%

You multiply the error by the power.
5. Teachercool is correct (I am not worthy!)

0.97 x 0.97 = 0.9409 which is just 6% out, not 9%, so the answer is 8%
6. If the error for C is x%, then the error for Cn is nx%
7. thanks

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: July 16, 2007
Today on TSR

Choose Respect

### University open days

• Manchester Metropolitan University
Wed, 14 Nov '18
• University of Chester
Wed, 14 Nov '18
• Anglia Ruskin University
Ambitious, driven, developing your career & employability? Aspiring in your field, up-skilling after a career break? Then our Postgrad Open Evening is for you. Postgraduate
Wed, 14 Nov '18
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