Electric Fields

Question 1

What does Coulomb’s Law state

Answer 1

The magnitude of the force between two points charges in a vacuum is -

• Directly proportional to the product of their charges
• And inversely proportional to the square of the distance between the charges

Question 2

What is the Equation for force between Electric Currents

Answer 2

F = (1 / 4π x ε0) x (Q1 x Q2 / r^2)

Force = (1 / 4π x the permittivity of free space) x (Charge 1 x Charge 2 / Distance^2)

Question 3

What are the assumptions made when using the electric charge force equation

Answer 3

• Air can be treated as a vacuum
• For a charged sphere, charge may be assumed to act at the centre of the sphere

Question 4

When is the electric charge force positive and when is the electric charge force negative

Answer 4

• If charges have the same sign the force they will be positive and will be repulsive
• If the charges have different signs the force will be negative and will be attractive

Question 5

What is stronger the gravitational force between subatomic particle or the electrostatic force between subatomic particles

Answer 5

• The electrostatic forces between subatomic particles is greater than the magnitude of gravitational forces
• This is because the masses of subatomic particles are incredibly small whereas their charges are much larger

Question 6

What is Electric Field Strength

Answer 6

The force per unit charge experienced by an object in an electric field

Question 7

What are the Electric Field Strength Equations

Answer 7

E = F / Q (General Case)

Electric Field Strength = Force / Charge

E = V / d (Unifrom Fields)

Electric Field Strength = Potential difference across the plates / distance between the plates

E = (1 / 4πε0) x (Q / r^2) (Radial Fields)

Force = (1 / 4π x the permittivity of free space) x (Charge / Distance^2)

Question 8

What direction do Electric Field Lines always go

Answer 8

Positive charge to negative charge

Question 9

What does the direction and the distance of the electric field lines show

Answer 9

• The direction of the lines on a electric field diagram represent the direction of the field - it shows the direction of a positive charge would move in the field
• The distance between lines represents the strength of the field - the closer the lines, the stronger the field

Question 10

Whats the difference between a unifrom electric field and a radial electric field

Answer 10

• A uniform field exerts that same electric force everywhere in the field, as shown by the parallel and equally spaced field line
• In a radial field the magnitude of electric force depends on the distance between the two charge

Question 11

Describe what happens to an electron when it enters a unifrom field that is directed at a right angle and when it is directed in the same direction as the electron’s velocity

Answer 11

Right Angle

• The electron will be accelerated in the perpendicular direction
• The inital horizontal velocity will stay the same
• It will form a parabolic path towards the positive

Same Direction

• The electron will be decelerated
• Direction of the motion will eventually be revered if the size of the force on the electron is great enough

Question 12

Describe the difference between a proton and electron’s acceleration within a uniform electric field

Answer 12

• The accleration of the proton is smaller than the acceleration of the elctron because the mass of the proton is much greater
• F = ma, F/m = a

Question 13

What is the Work Done Equation for Electric Fields

Answer 13

W = QV

Work Done = Charge x Electric Potential

Question 14

In uniform electric fields how can you find out if a particle is charged and if a particle is negative or positive

Answer 14

• If you the fire the particle at right angles to the field and observing its path:
• A charged particle will experience a constant electric force either in or opposite to the direction of the field (depending on its charge), this causes the particle to accelerate and so it follows a parabolic shape
• If the charge on the particle is positive it will follow the direction of the field, if the charge is negative it will move opposite to the direction of the field

Question 15

What is Electric Potential

Answer 15

The work done per unit charge of a positive point charge in moving the charge from infinity to the point

Question 16

What is Electric Potential at it’s greatest

Answer 16

• Electric potential is greatest at the surface of a charge
• As the distance from the charge increases, the potential decreases so electric potential at infinity is zero
• V = 1/4π x Q/r (As r goes to infinite the whole equation goes to zero)

Question 17

When is Electric Potential positive and negative

Answer 17

• When the charge is positive, potential is positive and the charge is repulsive
• When the charge is negative, potential is negative and the force is attractive

Question 18

Describe the Electric Potential against Distance Graph

Answer 18

The gradient of both graphs will give the Electric Field Strength

Question 19

What is an Equipotential Sufrace of Electric Fields

Answer 19

• The potential on an equipotential surface is the same everywhere
• Therefore when a charge moves along an equipotential surface, no work is done.
• Between two parallel plates the equipotential surfaces are planes which are equally spaced and parallel to the plates
• Whereas equipotential surfaces around a point charge form concentric circles