MYTUTOR SUBJECT ANSWERS

290 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...

7 months ago

Answered by Sam, an A Level Physics tutor with MyTutor


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

56 SUBJECT SPECIALISTS

£20 /hr

Baran K.

Degree: Mathematics (Bachelors) - Cambridge University

Subjects offered: Physics, Maths+ 3 more

Physics
Maths
Further Mathematics
.STEP.
-Personal Statements-

“About Me I am a friendly, outgoing first-year Maths student at the University of Cambridge. I am passionate about anything vaguely maths-related and love to help others improve their understanding of maths and physics! In my free tim...”

MyTutor guarantee

£20 /hr

Laura K.

Degree: Chemical Engineering (Masters) - Bath University

Subjects offered: Physics, Science+ 3 more

Physics
Science
Maths
Further Mathematics
Chemistry

“Hey there! I am Laura and I am currently studying Chemical Engineering at University of Bath. I am really passionate about Science and I am willing to share my knowledge through tutoring you. My passion and expertise in science can be...”

£20 /hr

Tom V.

Degree: Mechanical Engineering (Masters) - Leeds University

Subjects offered: Physics, Maths

Physics
Maths

“About MeHello! I'm Tom, I'm in my first year studying mechanical engineering at the University of Leeds. I've always loved sicence, being particularly drawn to physics and maths from a young age. I loved the methodical, logical appro...”

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

Two railway trucks of masses m and 3m move towards each other in opposite directions with speeds 2v and v respectively. These trucks collide and stick together. What is the speed of the trucks after the collision?

What is meant by an excited atom?

Convection, conduction and radiation in space

What's the highest height a ball thrown straight up will reach?

View A Level Physics tutors

Cookies:

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

mtw:mercury1:status:ok