Register to reply

Dielectric and Energy

by 123yt
Tags: dielectric, energy
Share this thread:
123yt
#1
Apr24-11, 08:53 PM
P: 32
1. The problem statement, all variables and given/known data
Consider a horizontal square plate capacitor of area 1*1 m2, capacitance in vacuum 2 uF, which contains a dielectric material with dielectric constant K=5. The dielectric slides frictionlessly and is attached via a massless string and a massless pulley to a block of mass 2.5 kg. The block pulls the dielectric from the capacitor as it falls. Compute the speed of the block at the instant the dielectric leaves the capacitor assuming it starts at rest and that the voltage across the capacitor after the dielectric is removed is measured to be 500 V.


2. Relevant equations
U = 1/2 CV^2 = q^2 / 2C


3. The attempt at a solution
Energy without dielectric = 1/2 * 2 * 10^-6 * 500^2 = 0.25
Energy with dielectric = 0.25 / k = 0.05

Energy change = 0.25 - 0.05 = 0.2

0.2 = 1/2 mv^2
v^2 = 0.4 / 2.5
v = 0.4 m/s

Actual answer is 4.4091 m/s .
Phys.Org News Partner Science news on Phys.org
Experts defend operational earthquake forecasting, counter critiques
EU urged to convert TV frequencies to mobile broadband
Sierra Nevada freshwater runoff could drop 26 percent by 2100
ehild
#2
Apr25-11, 12:12 AM
HW Helper
Thanks
P: 10,668
Quote Quote by 123yt View Post
Energy change = 0.25 - 0.05 = 0.2
Do not forget the change of the potential energy of the box.

ehild


Register to reply

Related Discussions
Energy density in a dielectric Introductory Physics Homework 0
5. Energy of a Capacitor in the Presence of a Dielectric Introductory Physics Homework 0
Energy of a Capacitor in the Presence of a Dielectric Introductory Physics Homework 1
Energy to make a dielectric explode? Advanced Physics Homework 5
Electric energy of a dielectric sphere Introductory Physics Homework 2