Electric Fields

Question 1

Define electric field strength:

Answer 1

An electric field strength at a point is defined as the force per unit positive charge placed at that point.

Question 2

What’s the equation for defining electric field strength? Annotate it as well

Answer 2

E = F/Q
E = Electric field strength (NC^-1)
F = Force on the charged particle placed in the electric field (N)
Q = Charge on a particle in the electric field (C)

Question 3

What are the two main rules for drawing electric field lines?

Answer 3

• The field lines go from positive to negative
• The field lines leave and strike a surface at right angles to the surface

Question 4

Coulomb’s Law

Answer 4

The force between two electric charges is proportional to the product of the charges and inversely proportional to the square of the distance between them.

Law only applies for radial fields NOT uniform fields

Question 5

Define F = Qq/4piE_0r^2

Answer 5

F = Force between two electric charges (N)
Q = Charge of large particle (C)
q = Charge of small particle (C)
E_0 = Permittivity of free space (C^2N^-1m^-2)
r = distance between the centre of the charges (m)

Question 6

What are the similarities between electric and gravitational fields?

Answer 6

• Both of radial fields
• The force for both is inversely proportional to r^2

Question 7

What is a difference between a gravitational field and electric field?

Answer 7

• A gravitational field can only be attractive whereas an electric field can be both attractive or repulsive

Question 8

Define the equation for electric field strength for a uniform field between to parallel conducting plates:

Answer 8

E = V/d
E = Electric field strength (NC^-1)
V = p.d. between the plates (V)
d = distance between the plates (m)

Question 9

If a positive charge , Q is placed in a uniform electric field between two parallel, what force does it experience?

Answer 9

E = V/d
F = EQ therefore you get:

F = VQ/d
F= force on particle (N)
V = p.d. between parallel plates (V)
Q = Charge of particle (C)
d = Distance between plates (m)

Question 10

How to calculate the acceleration of a charged particle moving between plates and perpendicular to electric field lines?

Answer 10

• Determine electric field strength
  E = V/d
• Determine the force acting on particle F = EQ
• Now determine acceleration
  a = F/m

Question 11

How to calculate the time of a charged particle moving between plates and perpendicular to electric field lines?

Answer 11

-Since the motion horizontally is constant, as the electric force acts perpendicular to this direction, then there is no acceleration
- So s = ut +1/2at^2 which becomes s = ut
- Therefore, t = s/u

Question 12

How to calculate the vertical velocity of a charged particle moving between plates and perpendicular to electric field lines?

Answer 12

• Since v=u+at and u=0 in the vertical plane then v=at then work out acceleration and time and multiply together. So to find velocity acceleration of particle and time within field must be worked out first.

Question 13

How to calculate the vertical displacement of a charged particle moving between plates and perpendicular to electric field lines?

Answer 13

-Since s=ut+1/2at^2 and u=0 vertically we can do the following
s=1/2at^2
So to find vertical displacement: acceleration of particle and time within field must be worked out first.

Question 14

What does the area under a force(N) against separation(m) graph show?

Answer 14

The area between the line and x-axis is equal to the work done

Question 15

State the equation for electric potential energy and define the symbols:

Answer 15

Electric potential energy = Qq/4piE_0r

Q= charge of large mass (C)
q= charge of small mass(C)
E_0= permittivity of free space (C^2N^-1m^-2)
r=distance between charges (m)

Question 16

Define electric potential:

Answer 16

The electric potential at a point is the work done per unit charge in bringing a unit positive charge from infinity to a point

Question 17

Equation for electric potential and define symbols:

Answer 17

V=Q/4piE_0r

V=electric potential (V)
E_0 = permittivity of free space (C^2N^-1m^-2)
r=distance between charge and a point (m)

Electric potential can be positive or negative depending on the charge of Q.
Note that the work done in bringing a charge Q from infinity to r is W=VQ