Electricity

Question 1

What happens in interactions among elementary particles?

Answer 1

• equal quantities of positive and negative charge are produced or destroyed
• the net charge of the universe is unchanged
• i.e. electric charge is conserved

Question 2

What is the SI unit of electric charge?

Answer 2

the Coulomb

Question 3

Coulomb’s Law

Answer 3

F = (kq1q2)/r^2

F = magnitude of the electric force exerted by charge q1 on another charge q2 when the charges are separated by a distance r

Question 4

Coulomb’s Constant

Answer 4

k = 8.99 x 10^9 Nm^2/C^2 = 1/4pie0

Question 5

Two small spheres attract one another electrostatically. Which of the following statements is true?

a) at least one sphere must be charged
b) neither sphere needs to be charged
c) both charges must be charged and the charges must have the same sign
d) both spheres must be charged and the charges must have opposite signs

Answer 5

a) a single charge object can cause the other object to become charged, so that there is an electrostatic force between the two objects

Question 6

Electric Field

Equation

Answer 6

E = (k*q1)/r^2
Electric field, E, at a point P which is a distance from a point charge q1
Also, F=Eq -> E = F/q

Question 7

Electric Field Definition

Answer 7

E is the force on a charge divided by the magnitude of that charge, i.e. the force per unit charge E = F/q

Question 8

Finding Total Electric Field

Answer 8

Can be found by measuring the force on a small charge and dividing by q
E = lim(q->0) F / q
You have to take the limit of small q so that the test charge does not disturb the electric field of q with its own electric field

Question 9

Electric Potential Energy

Definition

Answer 9

the change in potential energy of a charge in an electric field when it is displaced a certain distance

Question 10

Electric Potential Energy

Equation

Answer 10

dU = -qEdl

Question 11

Potential Difference

Definition

Answer 11

Change in potential energy per unit charge

Question 12

Potential Difference

Equation

Answer 12

dV = dU/q = -Edl

Question 13

Change in Potential Difference

Equation

Answer 13

ΔV = Vb-Va = -∫Edl (between b and a)

Question 14

Potential at a distance R from a point charge q

Answer 14

ΔV = kq/R

using V = 0 at r = ∞ in the integral

Question 15

Electrostatic Energy of Two Point Charges, q1 and q2

Answer 15

U = (kq1q2)/r12

Question 16

Electrostatic Energy of Three Point Charges, q1, q2, q3

Answer 16

U = (kq1q2)/r12 + (kq1q3)/r13 + (kq2q3)/r23

Question 17

Electrostatic Energy - Continuous Charges

Answer 17

U = (Q,0)∫ V(q) dq

Eneryg , U, required to add a charge element dq in the presence of a potential V(q) generated by a charge distribution q

Question 18

Capacitance

Equation

Answer 18

C = Q / V = d / εA

Question 19

Capacitance

Definition

Answer 19

a measure of the capacity to store a charge, Q, for a given potential difference V

Question 20

Self Capacitance of a Spherical Conductor

Answer 20

C = 4πε0R

Question 21

Unit of Capacitance

Answer 21

1 Farad = 1 Coulomb / 1 Volt

Question 22

Permitivity of Free Space

Answer 22

ε0 = 8.85 x 10^(-12) F/m

Question 23

Surface Charge Density

Answer 23

σ = Q / A

Question 24

Electric Field Between Two Parallel Plates

Answer 24

E = V/d = σ/ε

Question 25

Electric Energy Stored in a Capacitor

Equation

Answer 25

U = (1/2)*Q²/C = (1/2)QV

= (1/2)CV²

Question 26

What is electrostatic field energy?

Answer 26

The energy required to charge a capacitor can also be thought of as the energy required to establish the electric field, the electrostatic field energy

Question 27

Electrostatic Field Energy

Equation

Answer 27

U = (1/2)εE²(Ad)

Question 28

Electric Field Energy Density

Definition

Answer 28

energy stored in the electric field per unit volume

Question 29

Electric Field Energy Density

Equation

Answer 29

u = (1/2)εE²

Question 30

Capacitance of a Cylindrical Capacitor

Answer 30

C = 2πεL / ln(R1/R2)