Electrostatics

Question 1

Electrostatic Force

Answer 1

The force existing between stationary charges

Question 2

How do electric charges react to one another?

Answer 2

Like charges repel, while unlike charges attract.

Question 3

What happens when you rub a glass rod with a piece of silk?

Answer 3

The electrons flow from the glass rod to the silk cloth due to friction, leading to static cling.

Question 4

Coulomb

Answer 4

SI unit of charge

Question 5

Fundamental Unit of Charge

Answer 5

e = 1.60 X 10^-19 C (the charge of an electron when negative and proton when positive)

Question 6

Coulomb’s Law

Answer 6

F = k q1 q2/ r^2

Question 7

Coulomb’s Constant

Answer 7

k = Coulomb’s Constant = Electrostatic Constant = 1/ (4πε0) = 8.99 X 10^9 N m^2/ C^2

Question 8

Permittivity of Free Space

Answer 8

ε0 = 8.85 X 10^-12 C^2/ (N m^2)

Question 9

What happens when the distance between two charges doubles?

Answer 9

The force reduces to 1/4 of the original force because F = 1/r^2

Question 10

How do the gravitational and electrostatic forces compare for the electron and proton in a Hydrogen atom?

Answer 10

Gravitational Force = 3.63 X 10^-47 N
Electrostatic Force = 8.22 X 10^-8 N
Electrostatic Force stronger than Gravitational Force by approximately 10^40

Question 11

Electric Field

Answer 11

A vector quantity that every charge exerts on other electric charges defined by the force a stationary positive test charge feels. The units for electric field are N/C = V/m.
E = F/q0 = k q/ r^2

Question 12

What force does any charge feel due to an electric field?

Answer 12

F = qE, a conservative force (independent of pathway)

Question 13

Which direction would the force a positive charge feels due to an electric field point?

Answer 13

A positive charge in an electric field feels a force in the direction of the field, while a negative charge feels a force opposite of the direction of the electric field.

Question 14

Electric Field Lines

Answer 14

Imaginary lines that show the direction a positive test charge would be accelerated in the electric field.

Question 15

How are the electric field and electric field lines related?

Answer 15

The direction of the electric field is tangent to the electric field line at the given point, pointing the same direction. Furthermore, like topographical maps, the distance between field lines indicates relative strength, where closer field lines indicate a stronger electric field.

Question 16

What do you do for an electric field due to a collection of charges?

Answer 16

Do a vector sum of the electric fields of each of the charges.

Question 17

Electric Potential

Answer 17

V = k Q/ r = W/ q = U/ q
Units: J/ C = V
A scalar quantity

Question 18

Potential Difference

Answer 18

ΔV = Vb - Va = W(from a to b)/ q0

Question 19

Equipotential Lines

Answer 19

Equipotential lines cross the electric field lines perpendicularly and represent areas of the same potential, meaning that any two points on the same equipotential line will have a potential difference of 0. No work is done when moving a charge along an equipotential line, because work depends only on the potential difference, not on the path.

Question 20

Electric Potential Energy

Answer 20

U = W = qV = q ΔV {change in electric potential energy} = k qQ/ r = (1/ (4πε0)) qQ/ r
Units: Joules

Question 21

Electric Dipole

Answer 21

Two charges of equal magnitude and opposite sign separated by a distance

Question 22

What is the potential at any point P due to a dipole moment?

Answer 22

V = k q/ r1 - k q/r2 = k q ((r2 - r1)/ r1 r2)

When P&raquo_space;> d, V = k (q d/ r^2) cos θ = k (p/ r^2) cos θ

Question 23

What is the magnitude of the electric field along the perpendicular bisector of a dipole?

Answer 23

E = (1/ (4πε0))(p/ r^3), because θ is 90 degrees and this plane lies halfway between the two charge of the dipole

Question 24

Dipole Moment

Answer 24

A vector quantity that is equal to qd with units C m, with a direction that points towards the negative charge away from the positive charge.

Question 25

What happens when a dipole is placed in a uniform external electric field?

Answer 25

The net force on the dipole will be zero, because each of the charges feels an equal and opposite force, meaning that no translational movement occurs, but the dipole will spin about its center so that its dipole moment will align with the electric field.
τ = F d/2 sin θ + F d/2 sin θ = Fd sin θ = qEd sin θ = pE sin θ