Electric Charge and Field

Question 1

What is an Electric Charge?

Answer 1

A fundamental property with two types of charges, positive and negative.

Question 2

How do the two different types of charges interact?

Answer 2

Same-sign charges repel, opposites attract

Question 3

Difference between Electrostatics and Electrodynamics?

Answer 3

Electrostatics is the study of charges at rest (w.r.t each other) and electrodynamics is the study of charges in motion (w.r.t each other).

Question 4

Describe the Conservation of Electric Charge.

Answer 4

The net amount of electric charge is constant in a closed system.

Question 5

Describe the Quantization of Electric Charge.

Answer 5

Since protons and electrons are natural unit electric charges, any observable amount of charge must have an integer multiple of those two.

Question 6

Describe Conductors and Insulators.

Answer 6

A conductor is an object that easily permits the flow of electrons with their loose electrons.

An insulator is an object that does not allow an easy flow of electrons due to not having loose electrons.

Question 7

Describe Charging by Conduction.

Answer 7

The direct transfer of charges by direct contact between objects.

Question 8

Describe Charging by Induction.

Answer 8

In a neutral object, without making contact, charges are displaced and shifted due to a charged object being brought near it.

Question 9

What is a point charge?

Answer 9

Charged objects with a distance between that is much greater than their sizes.

Question 10

Describe Coulomb’s Law.

Answer 10

The magnitude of the electric force between two charges is proportional to the product of their respective charges (in Coulombs) and inversely proportional to the squared distance between them.

Question 11

Where do the forces between two charges occur?

Answer 11

It occurs on a line connecting the two charges. This line of action is found by vectors making the shortest distance between the charges.

Question 12

Coulombs Law uses a constant “k”, what is that constant’s expression and its value?

Answer 12

1/(4πε_0) = 9 * 10 ^ 9

Question 13

Describe the Principle of Superposition.

Answer 13

The net force on a charge is the sum of the electric force vectors between every other charge in a system.

Question 14

Briefly describe an example of a scalar field and a vector field.

Answer 14

An example of a scalar field could be temperature and an example of a vector field could be an electric force field.

Question 15

What are the units for an electric field?

Answer 15

N/C
(Newtons/Coulombs)

Question 16

Describe the directions of electric fields around positive charges vs negative charges.

Answer 16

For positive charges, the force field exerts outwards while for negative charges the charges draw inward.

Question 17

For the three different dimensions of charge distributions, what is their respective expression for charge density?

Answer 17

For linear, λ is dQ/dL;
For surfaces, σ is dQ/dA;
For volumes, ρ is dQ/dV

Question 18

Explain uniform charge distribution.

Answer 18

The average density is the same as the local density of a system.

Question 19

Describe how the distance between field vector lines impacts field strength.

Answer 19

The closer lines appear to each other, the stronger the field is in that region. As they get further from each other, the field weakens in a region.

Question 20

How many field lines can exist at one point?

Answer 20

Only one field line may exist at one point, there should never be more than one. This is because this implies the lines intersect, which is impossible.

Question 21

Describe Electric Dipoles.

Answer 21

A pair of charges with equal magnitude charges, opposite signs and separated by a distance “d”.

Question 22

Dipole moment vector equation.

Answer 22

P = q * r-+ (C * m)

Question 23

When an electric dipole is placed in an electric field, how do the forces placed on it react?

Answer 23

The net force on the dipole is zero due to the forces on the positive and negative charges cancelling.

Question 24

Why do electric dipoles in an electric field experience torque? What is the formula?

Answer 24

While the net force is zero, the forces on the + and - ends of the dipole still experience a torque, thus a rotation. The formula is τ = p x E where p is the dipole moment vector.

Question 25

How do a dipole’s potential energy and the work done on it relate? What are their equations?

Answer 25

The potential energy of a dipole is equal to the negative of the work done onto it by an electric field, i.e. dU = -dW. So if the work done, W, equals p * E in Joules. Thus, U = -p * E.

Question 26

The equation for the flux differential element in an electric field.

Answer 26

dΦ = E * dA

Question 27

How to the net charge inside a surface affect the direction of the flux on the object?

Answer 27

For zero net charge, inward flux and outward flux cancel out.
For positive net charge, outward flux is experienced.
For negative net charge, inward flux is experienced.

Question 28

Describe Gauss’ Law.

Answer 28

The total electric flux experienced by any closed surface is equal to the total net charge divided by ε0, i.e. Φ = q (in)/ε0