MYTUTOR SUBJECT ANSWERS

590 views

A capacitor discharge circuit of time constant 45ms includes a capacitor and resistor. The capacitor has a capacitance of 18µF What is the resistance of the resistor?

This question is in one of the CIE Physics Pre-U sample papers, and if you know the definition of the time constant for a capacitor circuit (=RC), its very easy, however even if you didnt it canbe derived fairly easily.

Consider a circuit containing just a capacitor C and resistor R, with the capacitor initially storing some charge Q0 with a voltage V0 across it. Call the curent flowing i, the voltage across the capacitor as time progresses Vc and the changing charge Q. By Kirchoff's law,

Vc + iR=0

Then we know that dQ/dt = i and C=Q/Vc, so we can say:

Q/C + RdQ/dt=0 and so RCdQ/dt + Q=0 

This has solution Q=Q0exp(-t/RC) and comparing this with the standard form of a time decaying property, X=X0exp(-t/k), where k is the time constant, gives k=RC, and then the problem is trivial.

Sam C. A Level Physics tutor, GCSE Physics tutor, A Level Maths tutor...

1 year ago

Answered by Sam, an A Level Physics tutor with MyTutor


Still stuck? Get one-to-one help from a personally interviewed subject specialist

103 SUBJECT SPECIALISTS

£24 /hr

Dan B.

Degree: MPhys Physics (Masters) - Exeter University

Subjects offered:Physics, Maths

Physics
Maths

“Hi I'm Dan and I'm in the first year of a four-year MPhys Physics degree at the University of Exeter. I am very patient and will tailor the sessions' content and pace to suit your individual needs...”

PremiumTimothy N. A Level Design & Technology tutor, GCSE Design & Technolog...
£36 /hr

Timothy N.

Degree: Architecture and Environmental Engineering (Masters) - Nottingham University

Subjects offered:Physics, Maths+ 2 more

Physics
Maths
Design & Technology
-Personal Statements-

“Hi there, I have a passion for helping students achieve, and believe that with my years of experience tutoring, we will be able to surpass the grades you want!”

£24 /hr

Ayusha A.

Degree: BEng electrical and electronics engineering (Bachelors) - Newcastle University

Subjects offered:Physics, Maths+ 1 more

Physics
Maths
Further Mathematics

“About me: I am a final year Electrical and Electronic Engineering student at Newcastle University. I took Mathematics, Further Mathematics, Chemistry and Physics as my A-level subjects. I did peer mentoring in university and also have...”

About the author

Sam C.

Currently unavailable: for new students

Degree: MEng Engineering Science (Masters) - Oxford, New College University

Subjects offered:Physics, Maths+ 2 more

Physics
Maths
Further Mathematics
.PAT.

“ABOUT ME:I am studying Engineering Science at New College, Oxford. I am also a key member of the prestigious University Boat Club, coxing the Oxford Blue boat in the 2016 Boat Race. I am passionate about academic success and about Ma...”

You may also like...

Posts by Sam

A capacitor discharge circuit of time constant 45ms includes a capacitor and resistor. The capacitor has a capacitance of 18µF What is the resistance of the resistor?

How do I evaluate something like 3070^2-3069^2?

Other A Level Physics questions

In still air an aircraft flies at 200 m/s . The aircraft is heading due north in still air when it flies into a steady wind of 50 m/s blowing from the west. Calculate the magnitude and direction of the resultant velocity?

What is the main evidence for the Big Bang theory?

Define the term "Gravitational Potential" and write down a formula which defines it.

Convection, conduction and radiation in space

View A Level Physics tutors

We use cookies to improve your site experience. By continuing to use this website, we'll assume that you're OK with this. Dismiss

mtw:mercury1:status:ok