Electric Fields

Question 1

What do the symbols represent in Coulomb’s law?

Answer 1

Q1 and Q1 are charges in C, r is distance, eo is permittivity of free space.

Question 2

What are the rules of electric force?

Answer 2

Can be attractive or repulsive, depends on the permittivty of the material between them, follows an inverse square law, prop to product of charges.

Question 3

How do you find the radial electric field strength?

Answer 3

Combining E = F/Q with F = Q1Q2 / 4 pi r^2 e0 gives the field strength for a radial field as a distance r from a charge Q.

Question 4

What does E = F / Q represent?

Answer 4

The force per unit charge on a positive test charge placed at a point.

Question 5

How do electric field diagrams work?

Answer 5

Arrows point in direction of force on a positive charge at that point. Separation of lines shows strength of field at that point.

Question 6

Where do the field lines point in a uniform electric field?

Answer 6

Positive to negative

Question 7

How do you derive the formula for uniform electric field strength?

Answer 7

F = EQ on a charge.
W = Fd
so W = EQd
W = QV
so V = EQd / Q
so E = V/d

Question 8

What is the work done in moving a charge Q through a p.d V?

Answer 8

W = QV

Question 9

What is the force F on a charge Q due to an electric field E?

Answer 9

F = EQ

Question 10

When do charged particles experience a constant force in an electric field?

Answer 10

When they enter it perp. to the field. Force is in the same direction as the field lines if pos. charged.

Question 11

What paths do particles follow in an electric field when they enter it perp. to the field?

Answer 11

Parabolic trajectories - can be modelled using SUVAT, mirrors a particle under gravity.

Question 12

What is electric potential defined as?

Answer 12

The work done to bring a small positive test charge from infinity to that point.

Question 13

When is electric potential positive?

Answer 13

Around a positive charge; it’s negative around a negative charge. As it is modelled for a positive test charge.

Question 14

What is the work done in moving a charge though a changing electric field?

Answer 14

deltaW = Q x deltaV

Question 15

What are differences between electric and grav fields?

Answer 15

Grav fields are caused by and act on objects with mass. Electric fields are caused by and act on objects with charge.
Grav forces are always attractive. Electric forces can be attractive or repulsive.

Question 16

What are similarities between electric and grav fields?

Answer 16

Size of force in both follows an inverse square law.
Field strength in both follows an inverse square law.
Field strength is defined similarly in both, as force per unit charge or unit mass.
Both electric and grav. potential vary with 1/r.
Field lines can be used to represent both, are parallel and equally spaced for uniform fields of both types.
Potential is the potential energy per unit charge or unit mass for both, and is zero at infinity for both positive charges and grav. fields.

Question 17

Where are electric equipotentials?

Answer 17

Perp. to field lines.

Question 18

What is the unit for electric field strength?

Answer 18

NC^-1

Question 19

How can you find the electric field strength from a graph?

Answer 19

From the gradient of the V against r graph at that point.