Sections 11-14

Question 1

What is another way of writing the potential of a number of point charges? Why is this?

Answer 1

Sum from i=1 to n of 1/2 * qi * Vi

This is because the potential at q1 due to q1 is q2/(4pi* e0* r) and vice versa, so rearrange potential of 2 and substitute this in.

Question 2

What is the general equation for energy density?

Answer 2

energy density = energy/volume = u

Question 3

What is the equation for energy density including epsilon 0?

Answer 3

u = 1/2 * ϵ0 * E^2

Question 4

What is a dielectric?

Answer 4

Insulating material in which there are no free charges.

Question 5

What is the equation for relative permittivity?

Answer 5

ϵr = C/C0 where C0 is the capacitance when separated by a vacuum and C is when separated by the dielectric

Question 6

What is the equation for permittivity of a material?

Answer 6

ϵ = ϵr*ϵ0

Question 7

How is a surface charge induced in a dielectric?

Answer 7

If you apply an electric field across and atom the electron charge distribution will move in the opposite direction of the field, making dipole. Dielectric has no charge so the charges appear on the surface.

Question 8

What is the dielectric breakdown?

Answer 8

When dielectrics become conductors as the electrons become unbound.

Question 9

How does a plasma ball work?

Answer 9

Spherical capacitor filled with noble gas. Has radius 0.1m and V=60kV.

Question 10

What is the equation for the current density?

Answer 10

J = I/A = nqv, where n is the number density of free charges and v is the mean velocity of the charges.

Question 11

How do you find the mean speed of electrons in a wire?

Answer 11

From start point to point P, electron travels v*t, so volume of region passing P is v*t*A, where A is the cross sectional area. Number of free charges in volume is n*v*t*A where n is the number density. Charge passing P in t is dQ = q*n*v*t*A. Then I = dQ/dt, so find the derivative and rearrange for v.

Question 12

What is resistivity?

Answer 12

Determines how strong a current can flow for a given imposed electric field. Conductor - low resistivity, Insulator - high resistivity.

Question 13

What is the reciprocal of resistivity?

Answer 13

Electrical conductivity

Question 14

What is the equation for electrical resistance for any conductor?

What is the equation for electrical resistivity?

Answer 14

ρL/A

ρ = E/J, where J is the current flowing per unit cross-sectional area (I/A)

Question 15

State ohms law.

Answer 15

R = V/I

Question 16

What is the total resistance of resistors in series/parallel?

Answer 16

Series - R = R1 + R2 + … (same current)

Parallel - 1/R = 1/R1 + 1/R2 + … (same voltage)

Question 17

What are the 3 main equations for power?

Answer 17

P = V*I

P = R*I^2

P = V^2 / R

Question 18

What is e.m.f?

Answer 18

The emf of a battery is the voltage across the terminals if there are no losses in the battery.

Question 19

What us the equation for voltage including internal resistance?

Answer 19

V = ϵ - rI where ϵ is the e.m.f and r is the internal resistance.

Question 20

How do you use maxwell loops?

Answer 20

Choose a direction of the loop. Find the voltages across each resistor and the total e.m.f in the loop and then solve. Also use Kirchoff’s Laws about junctions.

Question 21

What is the time constant and what is its equation?

Answer 21

The time it takes a capacitor to be charged. The equation is τ = RC