electric fields

• charge is a fundamental physical property of matter that causes it to experience a force when placed in an electric field
• like charges repel
• opposite charges attract
• air can be treated as a vacuum when calculating force between charges
• for a charged sphere, charge may be considered to be at the centre
• the magnitude of the electric force between two objects is much greater than the gravitational force between them
• the constant ε0 known as the permittivity of free space
• an electric field can be represented by electric field lines
• electric field lines show the direction and relative magnitude of the force on a positive test charged placed in the force field
• the direction of an electric field lines at a point gives the direction of the force that would be felt by the positive test charge at that point
• electric field strength is a vector quantity
• the electric field of a proton is an example of a radial field
• the electric field strength at a point in the field is defined as the force per unit charge on a positive test charge placed at that point
• the electric field strengths at a point in a field is independent of the charge placed there - it is a property of the field
• two objects of different charge placed at the same point in an electric field will experience the same field strength, but will feel different forces
• electric field strength is measured in newtons per coulomb
• a uniform electric field exerts the same force per unit charge everywhere in the field. this is shown by the field lines being parallel and evenly spaced
• an example of a uniform electric field is the field between two charged parallel plates
• the equation E=V/D can be derived from considering the work done moving a charge between the two plates (Fd=QΔV)
• if a charged particle enters a uniform electric field at right angles to the field it will feel a constant force parallel to the electric field lines. this causes the particle to accelerate at right angles to its original motion and hence it follows a curved path (a parabola)
• if a positive particle enters a uniform electric field initially at right angles the particle will accelerate in the direction of the field lines
• if a negative particle enters a uniform electric field initially at right angles the particle will accelerate in the opposite direction to the field lines
• the resultant electric field strength is the vector sum (i.e direction must be considered) of the individual electric field strengths
• the electric potential (V) at any point in an electric field is the work done per unit charge on a positive test charge when it is moved from infinity to that position
• the value of electric potential is zero at infinity
• electric potential is a scalar quantity
• electric potential is measured in joules per coulomb, or more commonly volts
• the electric potential difference (ΔV) between two points is the energy needed to move a unit charge from one point to the other
• the work done in moving a charge across a potential difference is given by: $ΔW=$$QΔV$
• equipotential are surfaces of constant potential
• equipotentials are always perpendicular to field lines
• no work is done when moving a charge along an equipotential surface
• the potential gradient at a point in an electric field is the change of electric potential per unit distance
• the electric field strength is equal to the negative of the potential gradient
• the quantity ΔV/Δx is known as the electric potential gradient
• electric field strength can be found from the gradient of a V-r graph
• ΔV can be determined graphically by finding the area under a E-r graph
• the force that acts between two point charges in a vacuum is directly proportional to the product of their charges and inversely proportional to the square of their separation