Electric Fields

Question 1

What is an electric field?

Answer 1

the region around a body in which other charged bodies will feel a force due to the electric charge of the body

Question 2

What do electric field lines show?

Answer 2

the direction of the field at a point is the direction in which a small positive charge would move when placed at that point

Question 3

How can the strength of a field be shown on its field lines?

Answer 3

in a strong electric field , the lines are drawn close together, whereas in a weaker field they are drawn further apart

Question 4

Define Electric field strength

Answer 4

force per unit positive charge (NC^-1)

Question 5

Equation for electric field strength

Answer 5

E = F/Q

E- electric field strength (NC^-1)

F- size of force (N)

Q- size of positive charge (C)

*F is the force acting on the charge Q, which is within the electric field.Here,Q IS NOT causing the field

Question 6

State Coulomb’s law

Answer 6

the electrical force between two point charges is proportional to the product of their charges and inversely proportional to the square of their seperation

Question 7

State Coulomb’s law in equation form

Answer 7

F= Qq/4 x π xεo x r²

(for the force between two point charges)

εo- permitivitty of free space (8.85x10^-12)

Q and q are the charges

r is the distance between Q and q

Question 8

Keeping Coulombs law in mind, if the charges are opposite what will the force be?

Answer 8

the force will be attracitve but F will be negative

Question 9

Keeping Coulombs law in mind, if the charges are like charges what will the force be?

Answer 9

the force will be repulsive and F will be positive

Question 10

State the equation to find the electric field strength for a point charge

Answer 10

E = Q/ 4πεor²

Question 11

State 2 similarities and differences between the gravitational field of a point mass and the electric field of a point charge

Answer 11

-they both have an inverse square law with distance

g=GM/r² , E = Q/4 x π x Eo x r²

• g.f.s is the force per unit mass and e.f.s is the force per unit charge
• electric field lines for a radial source act radially outwards from a positive charge and inwards for a negative charge whereas gravitational field lines always act radially inwards
• electrostatic force can be attractive or repulsive whereas gravitational force is always attractive

Question 12

What do electric, magnetic and gravitational fields have in common?

Answer 12

they are regions where a force can be exerted on an object at a distance

Question 13

what is a neutral point?

Answer 13

the place between two charged objects where the fields exactly cancel and the net force is zero

Question 14

State the equation to find the electric field strength of a uniform field

Answer 14

E= V/d

V- potential difference between the plates (V)

d- distance between plates (m)

Question 15

State the equation to find the capacitance of a parallel capacitor

Answer 15

C = EA / d

C- capacitance ( F)

E- permitivity of the material between the plates

A- area of the overlap of the plates

d- the seperation of the plates

* if the material between the capacitor plates is air then you use : C= EoA / d

Question 16

How do you find the permitivity E, if you are given the relative permitivity Er, of the material?

Answer 16

E = Er x Eo

E- permitivity

Er- relative permitivity

Eo - permitivity of free space ( 8.85x10¯¹² C²m¯²N¯¹)

Question 17

What would occur if you placed a stationary charged particle within a uniform electric field?

Answer 17

the charged particle would accelerate either from the +ve plate to the -ve plate or vice versa depending on its charge

Question 18

What would occur if you were to inject a moving charged particle into a uniform electric field , initially at right angles to the field

Answer 18

the particle’s vertical velocity component will remain unchanged and the velocity in this direction will be uniform but there will be an acceleration in the direction of the field which results in a parabolic path

Question 19

Define electric potential

Answer 19

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

*the electric potential at infinity is zero

Question 20

State the equation for electric potential

Answer 20

V= Q/4πEor

Question 21

State the equation to find the capacitance of an isolated sphere and how you derive it

Answer 21

C= 4πEoR

Here , R is the radius of the sphere

Question 22

Demonstrate how a force-distance graph for a point/spherical charge looks like and state what the area under the graph is

Answer 22

work done = force x distance therefore area under the force-distance graph is numerically equal to the work done in moving the charge from one point in the field to another

Question 23

State the equation for electric potential energy

Answer 23

E=VQ

or

E = Qq/4πEor

where v is the electric potential

q is the size of charge in electric field

Question 24

Sketch the elctric field near two parallel plates

Answer 24

there is a uniform electric field between the plates except at the edges

Question 25

State an example where a uniform field can be created

Answer 25

two parallel conducting plates that have been charged by connection to the terminals of a battery

the uniformity of the field is shown by the constant seperation of the field lines

*remember the plates carry equal and opp charges so there is a uniform electric field between them except at the edges

Question 26

What does a capacitor consist of?

Answer 26

two parallel conducting plates which are seperated by a gap of size d - the gap can be filled with air or an insulating material called a dielectric

Question 27

Describe the similarities and differences between a radial electric field and a uniform electric field

Answer 27

similarities : field lines show the direction in which a positive unit charge would move if placed at that point in the field

difference : for a radial field , field lines diverge as we move further away from the source and field strength decreases with distance

for a uniform field the field lines are parallel and the field strength is the same at every point

Question 28

What is a point charge

Answer 28

a charge which exists at a single point in space e.g. an electron or proton