electric fields

Question 1

electric field

Answer 1

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 sphere

Question 4

what do field lines show

Answer 4

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

Question 5

which direction do the field lines point

Answer 5

+ to -
the lines always point away from a positive charge and towards a negative

Question 6

what effect does distance 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

how close together the field lines are = the closer the lines the stronger the field

Question 8

electric field strength

Answer 8

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

Question 9

formula fro electric field strength

Answer 9

E = F / Q

Question 10

coulombs law

Answer 10

force between any two point charges 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 = Q1Q2/4piE0R^2

E0 = permittivity of free space
R^2 = distance between the charges

Question 12

define permitivity

Answer 12

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

Question 13

formula for the electric field strength of a point charge

Answer 13

E = Q/ 4piE0R^2

can be derived using E = f/Q and the formula for force (coulombs law)

Question 14

some similarities between gravitational and electric fields

Answer 14

• both follow inverse square law
• point masses and point charges both produce a radial field
• newtons law and coulombs law formulae for force are very similar
• field strength is defined by force per unit charge/mass

Question 15

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 coulombs law are different

Question 16

fromula for work done when moving a charge in an electric field

Answer 16

force x distance

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 point in the electric field

Question 18

formula for the potential at a point in an electric field

Answer 18

V = W/Q

W= work done
Q = charge
v = potential

Question 19

formula for the potential between two parallel plates

Answer 19

v = Ed

e = electric field strength
d = distance between the plates

Question 20

formula for the capacitance of a parallel plate capacitor

Answer 20

C = Ae0er/d

A = area of plates
e0 = permittivity of free space
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 twi 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 particles in a uniform electric field

Answer 22

• first calc the time the particle is in the field using d/s where the list = length of charged plates and speed is the velocity perpendicular to the field
• use a = f/m and f = Eq to calc the acceleration of the particle while it is in the field
  a = eq/m
• substitute these values into v = u+at where u is the initial parallel velocity and v is the final parallel velocity

Question 23

formula for the potential near a point charge

Answer 23

V = Q/4piE0r

Question 24

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

Answer 24

exponential decay curve
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

formula for electric potential energy near a point charge

Answer 26

E = Vq

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 classes as capacitors
using c = q/v and the formula for v you can derive capacitance

C = 4Pie0r

r = radius of sphere

Question 28

formula for electric potential energy near point charge

Answer 28

E = Q1Q2/4PiEor