electric field
region of space in which charged particles are subject to an electrostatic force
what shape of field do point charges have
radial fields
how can you model uniformly charged spheres
as a point charge at the centre of the sphere
what do field lines show
path a positive test charge would take when placed in a electric field
which direction do the field lines point
+ to -
the lines always point away from a positive charge and towards a negative
what effect does distance on the strength of the electrostatic force
the greater the distance the weaker the force
how is the strength of an electric field represented in a diagram
how close together the field lines are = the closer the lines the stronger the field
electric field strength
force per unit charge on a positive test charge placed in the field
formula fro electric field strength
E = F / Q
coulombs law
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
what is the formula for the force between two point charges
F = Q1Q2/4piE0R^2
E0 = permittivity of free space
R^2 = distance between the charges
define permitivity
ability of a material to transmit an electric field (how easily the atoms become polarised)
formula for the electric field strength of a point charge
E = Q/ 4piE0R^2
can be derived using E = f/Q and the formula for force (coulombs law)
some similarities between gravitational and electric fields
- 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
differences between gravitational and electric fields
- 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
fromula for work done when moving a charge in an electric field
force x distance
define potential at a point in an electric field
the work done per unit charge in moving a positive test charge from infinity to that point in the electric field
formula for the potential at a point in an electric field
V = W/Q
W= work done
Q = charge
v = potential
formula for the potential between two parallel plates
v = Ed
e = electric field strength
d = distance between the plates
formula for the capacitance of a parallel plate capacitor
C = Ae0er/d
A = area of plates
e0 = permittivity of free space
er = relative permittivity of dielectric
d = distance between the plates
what can the motion of charged particles in an electric field be modelled as
- 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
how do you calculate the parallel component of velocity for a charged particles in a uniform electric field
- 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
formula for the potential near a point charge
V = Q/4piE0r
what does the force-distance graph for a point/spherical charge look like
exponential decay curve
force is inversely proportional to the square of the distance
