6.2 Electic 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 feel a force due to the electric charge of the body

Question 2

what do electric field lines show?

Answer 2

the direction a small positive charge would move

Question 3

what is the equation for electric field strength?

Answer 3

E = F / Q

force per unit charge

Question 4

what is the field shape of a point charge?

Answer 4

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

Question 5

what is Coulomb’s law? (in words)

Answer 5

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

Question 6

what is the equation for Coulomb’s law?

Answer 6

F = Qq / 4πɛor^2
where Q and q = respective charges
ɛo = permittivity of free space
r = the separation between centre of charges

Question 7

what sign will an attractive force have?

Answer 7

negative, -

Question 8

what sign will an repulsive force have?

Answer 8

positive, +

Question 9

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

Answer 9

E = Q / 4πɛor^2

Question 10

how can you work out the resultant electric field strength when you have multiple electric fields?

Answer 10

VECTOR sum of all the electric fields

Question 11

what is a similaritiy between gravitational fields and electric fields?

Answer 11

both follow an inverse square law

Question 12

what are some differences between gravitational fields and electric fields?

Answer 12

• g is defines as the force per unit mass, E is defined as the force per unit charge
• gravitational field lines always act radially inwards whilst electric field lines for a radial source act radially outwards for a positive charge and inwards for a negative charge
• gravitational force is always attractive whilst electrostatic force can be repulsive or attractive
• 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 13

what is the equation for a uniform electric field?

Answer 13

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

Question 14

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

Answer 14

like projectiles

Question 15

why do charged particles in electric fields move like projectiles?

Answer 15

because of the combined effect of constant acceleration/force and constant velocity at right angles to one another is a curved path
(the component of the particle’s velocity at right angles to the field will remain constant)

Question 16

what is capacitance dependent on?

Answer 16

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

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 the definition for electric potential?

Answer 19

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 20

what is the definition for electric potential energy?

Answer 20

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 21

what is the equation for electric potential energy?

Answer 21

E = Vq or E = Qq / 4πɛor

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

Question 22

what is the equation for electric potential?

Answer 22

V = Q / 4πɛor, measured in volts

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

Question 23

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

Answer 23

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 24

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

Answer 24

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 25

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

Answer 25

C = 4πɛor
(because it is a charged sphere you can assume all of its charge is at its centre and treat it like a point charge)
because V = Q /C and V = Q / 4πɛor hence C = 4πɛor

Question 26

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 26

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