What is an electric field?
A region of space in which a charged particles experience an electrostatic force
Give an equation for electric field strength in a uniform field based on force and charge.
E=F/Q
What are the units of electric field strength?
Newtons per coulomb.
Give an equation for electric field strength in a uniform field based on electric potential difference and separation.
E=-dV/dD
What do electric field lines represent?
The path a positively charged test particle (of charge +e) would follow if it were to move freely in the electric field
Why can we model charged spheres as point charges?
Electric field strength is constant throughout the inside of the sphere, so we can assume an average ‘point charge’ at its’ centre given a regular shape.
Where might we find a uniform electric field?
Between 2 charged plates, e.g. in a capacitor.
if field lines run parallel, what can we deduce about the nature of the field strength?
It is constant throughout
What is an equipotential?
A line along electric field strength is always constant.
How do equipotentials differ between uniform and radial fields?
In uniform fields they are equally spaced given equal intervals of potential. But since potential varies by an inverse square relationship in a radial field, e.potentials become increasingly further separated for a given interval of potential.
Give a derivation for E=-dV/dD.
E=F/Q
E=(W/d)/(W/v)
E=V/D
E=-V/D correcting for direction, since field strength is the negative of the potential gradient.
State the equation that describes Coulomb’s law.
F=kqQ/r^2, where k=1/(rpiepsilon0)
What does Epsilon 0 represent in the Coulomb’s law equation?
The permittivity of free space, i.e. the degree to which a material resists the free flow of charge. This equals 8.85x10^-12 Fm^-2
Describe Coulomb’s law.
The force between 2 point charges is proportional to the product of their charges and inversely proportional to the distance that separates them.
What is electric field strength?
The electric force per unit exerted on a small test charge of charge +e at that point.
Give an equation for electric field strength in a radial field.
Since E=F/Q…
E=Q/(4piepsilon0*r^2)
Capacitors are an example of a uniform electric field. What three factors does capacitance depend upon?
- Distance between plates
- Cross sectional area of overlap of plates
- Permittivity of the dielectric material
Give an equation for the capacitance of a capacitor, given its dielectric permittivity, overall area and plate separation.
C = (epsilon*A)/d
- Where epsilon represents the permittivity of the dielectric…
How do we calculate the permittivity of a dielectric material?
epsilon0*epsilonR
(i.e. permittivity of free space * relative for the dielectric in question)
Describe the transfers of energy when charging a capacitor.
- Electrons experience a constant electrostatic force which opposes their separation from a plate.
- This increases the electric potential energy of the system
- This energy is stored in the electric field that forms between the separated charges
Why is an electric field produced between the parallel plates of a capacitor?
As electrons accumulate on the plates, an imbalance of charge builds up on the plates. This causes an electric field to form.
Where work is done to push the electrons onto the plate, they lose kinetic energy which is transferred to potential stores within the field itself, thus there is a ‘back’ potential difference.
This explains why current exponentially decays - overall p.d. is reduced as the capacitor charges so current falls.
Describe electron motion in the following circumstances…
- moving in the direction of the field
- moving against the direction of the field
- Decelerate
- Accelerate
Using equations, describe the transfer of energy for electrons due to their position in an electric field.
Electric potential energy = kinetic energy
QV = 1/2mv^2
eV = 1/2mv^2
What type of motion do electrons moving at 90 degrees to an electric field exhibit?
Parabolic motion - a little like projectile motion!
