6.2 Electric fields

Question 1

what is an electric field?

Answer 1

an electric field is the region around a body in which other charged particles will experience a force due to the electric charge of the body

Question 2

How can you create an electric field by yourself

Answer 2

By rubbing a glass rod with a silk cloth. Friction transfers electrons from the glass to the silk, making the cloth negative and the rod positive. The rod is then surrounded by an electric field - it will attract small pieces of a paper or water.

Question 3

what do electric field lines show?

Answer 3

the direction of the force on a small positive charge (point charge)

Question 4

what is the equation for electric field strength?

Answer 4

E = F / Q

force per unit +ve charge

Question 5

Define electric field strength

Answer 5

The electric field strength of an electric field at a point in space is defined as the force experienced per unit positive charge at that point

Question 6

How can you test the presence of an electric field using basic equipments

Answer 6

Using a thin strip of gold foil attached to the bottom of an insulator. The gold foil is given a constant +ve charge by momentarily touching it to the charged ball. The charged foil experiences an electrostatic force when close to the able. This force is smaller the further away the foil is from the charged ball.

Question 7

what is the field shape of a point charge?

Answer 7

a point charge or any body which behaves as if all its charge is concentrated at the centre - has a radial field

Question 8

what is Coulomb’s law? (in words)

Answer 8

the electrical force between two point charges is directly proportional to the product of their charges an inversely proportional to the square of their separation

F = k Qq/r^2

K = 1/4pie x epsilon

F = Qq/4pie x epsilon x r^2

Question 9

what is the equation for Coulomb’s law?

Answer 9

F = Qq / r^2

K =1/4πɛo

F = Qq / 4πɛor^2

where Q and q = respective charges
ɛo = permittivity of free space
r = the separation between centre of charges

Question 10

what sign will an attractive force have?

Answer 10

negative, -

Question 11

what sign will an repulsive force have?

Answer 11

positive, +

Question 12

what is the electric field strength strength equation for a point charge?

Answer 12

E = F / q = Qq/ 4πɛor^2q

E = Q / 4πɛor^2

Question 13

what is a similaritiy between gravitational fields and electric fields?

Answer 13

both follow an inverse square law

Question 14

A graph of E again 1/r^2 will produce

Answer 14

A straight line through the origin

Question 15

what are some differences between gravitational fields and electric fields?

Answer 15

-g is defines as the force per unit mass, E is defined as the force per unit charge

-gravitational field lines always attractive so the direction of field always act radially onwards towards the point of mass of object
For electric field: +ve point charges produce a repulsive force where the direction of field is away from the object
-ve point charges produce an attractive field where the direction of field is towards the object

• objects cannot be shielding from gravitational fields however objects can be shielded from electric fields
• the medium between the masses makes no difference on the gravitational force however the medium between the charges does effect the size of the electrostatic force

Question 16

what is the other equation for a uniform electric field in a capacitor?

Answer 16

E = V / d (like a capacitor)
where V = voltage between the plates
d = distance between them

Question 17

what is the other equation for capacitance?

Answer 17

C = ɛo x A / d
where ɛo = permitivity of free space (if dielectric is air)
A = area of the plate overlap
d = separation of the plates

Question 18

what if the plates of the capacitor have a material between them or a different dielectric, what equation should you use for the permittivity?

Answer 18

ɛ = ɛr x ɛo

new permitivity = relative permitivity x permitivity of free space

Question 19

what is capacitance dependent on?

Answer 19

permittivity of the dielectric, area of overlap (between plates) and plate separation

Question 20

Explain the experiment that works out the permitivity of any insulator places between the plates

Answer 20

The charged stored on a capacitor can be measured directly by discharging it into a coulombmeter

Using the flying lead, the capacitor is charged to a p.d. of V. The charge Q is measured by tapping the flying lead to the plate of the coulombmeter

Then draw a graph of Q against V

Gradient will give you capacitance

Then work out the permitivity using equation
C = Epsilon x A / d

Question 21

Derive the equation E = V / d

Answer 21

When a charge accelerates from the +ve plate to the -ve plate it gains energy. Now between the two plates there is a p.d.

W = Fd
Or

VQ = (EQ)d 
V = Ed 
E = V/d

Question 22

An electron is fired from a +ve capacitor plate towards the -ve plate along the direction of the electric field, with a velocity of 1.0 x 10^7 m/s. The p.d. across the plates is 600 V and their separation is 3.0 cm. Calculate the maximum distance the electron will travel

Answer 22

s = 1.4 x 10^-2 m

the electron will travel about 1.4 cm from the +ve plate before it turns back

Question 23

how do charged particles move through uniform electric fields? similar to what?

Answer 23

like projectiles

Question 24

why do charged particles in electric fields move like projectiles?

Answer 24

Because there is no acceleration in the horizontal content, hence the horizontal velocity of the particle remains constant

The time spent in the field is given by the equation
t = L/v

There is however, acceleration in vertical component of the particle

Which gives the equation

a = F/m = EQ/m

Initial certainly velocity u = 0
Vv = u + at = 0 + EQ/m x L/v = EQL/mv

Question 25

what is the definition for electric potential?

Answer 25

electric potential is the work done per unit POSITIVE charge to move that that from infinity (where the potential is zero) to a point in an electric field, measured in volts

Question 26

what is the definition for electric potential energy?

Answer 26

electric potential energy of a body of charge q, is the work done to move that charge from infinity to a point in an electric field

Question 27

what is the equation for electric potential energy?

Answer 27

E = Vq or E = Qq / 4πɛor

where charge q is at a distance r from Q (point charge)

Question 28

what is the equation for electric potential?

Answer 28

V = Q / 4πɛor, measured in volts

because V = E / q, look at electric potential equation

Question 29

what does the graph of electric potential against distance look like for a point positive charge moving closer to a positive source charge?

Answer 29

exponential downwards, because you have to put in work against the field to bring it closer, there is a repulsive force so there is an increase in electric potential as you get closer to the source charge

Question 30

what does the graph of electric potential against distance look like for a point positive charge moving closer to a negative source charge?

Answer 30

exponential upwards starting from negative axis, because electric potential will decrease as seperation decreases, electric potential gets bigger further away because its now an attractive force not a repulsive one

Question 31

what is the equation for the capacitance of an isolated charged sphere? how can you get to it?

Answer 31

C = Q/V =4πɛorV/V = 4πɛor

Question 32

what does the force-distance graph for a point or spherical charge being moved in a repulsive radial (two like charges) electric field look like? and what does the area underneath equate to?

Answer 32

an exponential downwards, force decreases with distance, the area underneath is equal to the work done or the electric potential energy

Question 33

A football of diameter 22cm is covered with aluminium foil and suspended from a nylon string. A high-tension supply is set to 5.0kV. It’s +ve electrode is used to give a +ve charge to the ball. Calculate the charge stored on the ball

Answer 33

6.1 x 10^-8C

Question 34

In terms of work explain when a +ve charged is pushed against another +ve charge

Answer 34

The charges experience an electrostatic force of attraction so they repel each other. So you have you do work to decrease the seperation between the charges. All the work done is stored as electrical potential energy. This stored energy is recoverable — all you need to do is let go

Question 35

Define electric potential difference

Answer 35

It is the work done per unit charge between two points around the particle of charge Q