Electric Fields

Question 1

Define electric field

Answer 1

A region of space in which charged particles are subject to an electrostatic force

Question 2

What shape of field do point charges have

Answer 2

Radial fields

Question 3

How can you model uniformly charged spheres

Answer 3

As a point charge at the centre of the shape

Question 4

What doe field lines show?

Answer 4

The path a positive test charge would take when placed in an electric field

Question 5

Which direction do the field lines point

Answer 5

POS to neg - always point away from a positive charge & towards a negative charge

Question 6

What effect does distance have on the strength of the electrostatic force

Answer 6

The greater the distance, the weaker the force

Question 7

How is the strength of an electric field represented in a diagram

Answer 7

By how close together the field lines are - the closer the lines, the stronger the field

Question 8

Define electric field strength

Answer 8

Force per unit charge on a positive test charge placed in the field

Question 9

What is the formula for electric field strength?

Answer 9

E = F/Q
E= electric field strength (N/C)
F = Force (N)
Q = charge (C)

Question 10

What is Coulomb’s law?

Answer 10

The force between any two point chargers is proportional to the product of their charges and inversely proportional to the square of the distance between them

Question 11

What is the formula for the force between two point charges?

Answer 11

F = QQ / (4 pi E0 r^2)

Question 12

Define permittivity

Answer 12

The ability of a material to transmit an electric field (how easily the atoms become polarised)

Question 13

What is the formula for the electric field strength of a point charge?

Answer 13

E = Q / (4 pi E0 r^2)

Question 14

Name some similarities between gravitational and electric fields

Answer 14

• both follow inverse square law for the force
• point masses and point charges both produce a radial field
• newton’s law and Coulomb’s law formulae for force are very similar:
• field strength is defined by force per unit charge/mass

Question 15

Name some differences between gravitational and electric fields

Answer 15

• gravitational fields are always attractive, electric fields can be attractive or repulsive (depending on the charge)
• THe constants of proportionality in Newton’s law and Coulomb’s law are different

Question 16

What is the formula for the work done when moving a charge in an electric field?

Answer 16

Work done = force x distance moved

Question 17

Define potential at a point in an electric field

Answer 17

The work done per unit charge in moving a positive test charge from infinity to that pint in the electric field

Question 18

What is the formula for the potential at a point in an electric field?

Answer 18

V = W/Q
V = potential
W = work done in moving the particle
Q = charge of the particle

Question 19

What is the formula for the potential between two parallel plates?

Answer 19

V = E x d
V = potential (V)
E = electric field strength (N/C)
D = distance between plates (m)

Question 20

What is the formula for the capacitance of a parallel plate capacitor?

Answer 20

C = (A E0 Er)/d

C = capacitatance
E0 = permittivity of free space (formula book)
Er = relative permittivity of dielectric
D = distance between the plates

Question 21

What can the motion of charged particles in an electric field be modelled as?

Answer 21

Projectile motion: the two components of velocity are independent of each other
Velocity perpendicular to the field is not affected, velocity parallel to the field is.

Question 22

How do you calculate the parallel component of velocity for a charged particle in a uniform electric field?

Answer 22

1 - First, calculate the time the particle is in the field (using time = distance/speed, where distance = length of charged plates and speed = velocity perpendicular to the field)
2 - Use a = F/m and F = Eq to calculate the acceleration of the particle while it is in the field (a = Eq / m)
3 - substitute these values into V = u + at where u is the initial parallel velocity and V is the final parallel velocity

Question 23

What is the formula for the potential near a point charge?

Answer 23

V = Q / (4 pi E0 r)

Question 24

What does the force-distance graph for a point/spherical charge look like?

Answer 24

Force is inversely proportional to the square of the distance

Question 25

What does the area under a force-distance graph for a point/spherical charge represent

Answer 25

The work done in moving the charge

Question 26

What is the formula for electric potential energy near a point charge?

Answer 26

E = Vq E = electric potential energy (J) V = potential (V) q = charge of the point charge (C)

Question 27

What is the formula for the capacitance of an isolated sphere?

Answer 27

Isolated spheres can store charge, so technically they can be classed as capacitors. Using C = Q/V and the formula for V in terms of Coulomb’s law, you can derive the formula for capacitance: V = Q / (4 pi E0 r) & C = Q/V so C = Q / Q / (4 pi E0 r) so C = 4 pi E0 r

Question 28

What is the formula for electric potential energy near a point charge? (Coulomb’s law)

Answer 28

E = QQ / 4 pi E0 r