Define an electric field
An electric field is the force per unit positive charge: It is a region of space in which an electric charge experiences a force, assuming there is already a positive charge in the field
Define the electric field strength:
The electric field strength is the force per unit charge experienced by a small stationary positive charge at a certain point within an electric field
How do you calculate the electric field strength?
E = F / q
F = the electric force on the charge
q = the magnitude of the charge
Note that the direction of the electric field strength depends on the direction that a positive charge would move if placed in the field
What are electric field lines?
Electric field lines represent the magnitude and direction of the electric field:
- They always point from a positive to a negative charge
- The closer the field lines are to each other, the stronger the electric field at that point is
What are uniform electric fields?
Uniform electric fields have equally spaced field lines at all points within the field, meaning the electric field strength is constant at all points
What are radial electric fields?
Radial electric fields have field lines that gradually become further apart with distance, meaning the electric field strength decreases with distance from the charge producing the field
What type of electric fields do point charges and conducting spheres have?
From afar, both point charges and conducting spheres have radial electric fields, but closer to their surfaces they are perpendicular, meaning they are uniform
Draw the electric fields between two opposite point charges:
Draw the electric fields between two same point charges:
What do the electric field lines between two parallel plates look like?
When a PD is applied between two parallel plates, they become charged, and the electric field between the two plates is uniform, but the electric field beyond the edges of the plates is non-uniform
How do you calculate the magnitude of the uniform electric field strength between two parallel charged plates?
E = ΔV / Δd
ΔV = the change in PD across the uniform field of the plates
Δd = the change in distance between the plates
What is the direction of the electric field between two parallel charged plates?
The electric field points from the positively charged plate (connected to the positive terminal of the battery) to the negatively charged plate
What happens if one of the two parallel charged plates is earthed?
There is a PD of 0V between the two parallel charged plates
Prove the derivation of the electric field strength between two parallel plates:
Draw the electric field lines around a point charge travelling between two parallel plates:
What happens if a stationary charge is placed in a uniform electric field?
The charge will move parallel to the field lines of the field, going either along or against them depending on the charge
What happens if a moving charge is placed in a uniform electric field?
The charge will experience a constant force and travel in a parabolic trajectory, where the direction of the parabola depends on the charge:
For a moving charge between two charge parallel plates, a positive charge will deflect towards the negatively charge plate, while a negative charge will deflect towards the positive plate
What is the trajectory of a moving charge in a uniform electric field affected by?
The amount of deflection depends on the mass, charge and speed of the particle:
- A greater mass causes smaller deflection
- A greater charge causes greater deflection
- A greater speed causes smaller deflection
Define Coulomb’s law:
The force between two charges is directly proportional to the product of the charges and inversely proportional to the square of the distance between them
How do you calculate the electric force between two charges?
F = (Q1Q2) / (4πer^2) or F = (kQ1Q2) / (r^2)
What is the permittivity of free space?
It is the measure of the resistance offered by a material in creating an electric field around it - the permittivity of air is taken to be the same as the permittivity of free space - all other materials have a higher value than the permittivity of free space
When can Coulomb’s law be used?
Coulomb’s law can only be used to calculate the force between charged spheres or point charges whose sizes are much smaller than their separation - it cannot be used to calculate the force between irregularly shaped charged objects
What is the relationship between the electric field strength and distance?
E ∝ 1 / r^2
Define electric potential:
Electric potential is defined as the work done per unit charge in taking a small positive test charge from infinity to a defined point
