topic 7 Flashcards
what is an electric field
a region where a charged particle experiences a force
electric field strength formula
E = F / Q
define electric field strength
force per unit charge experienced by an object in an electric field
electric field strength in uniform and radial field
- uniform - E is constant
- radial - E varies
Coulomb’s law
- the magnitude of the force between two point charges is directly proportional to the product of their charges and inversely proportional to the square of the distance between them
Coulomb’s law formula
F = Q1Q2 / 4πε0r^2
what makes the force repulsive?
- if charges have the same sign
what makes a force attractive?
- if charges are oppositely signed
what do point charges form
a radial field
electric field strength in a radial field formula
E = Q / 4πε0r^2
absolute potential energy
- the potential energy per unit charge at a positive point charge at that point in the field
at which point is the absolute magnitude of electric potential the highest?
- at the surface of the charge
- as distance from charge increases, potential decreases
- hence, electric potential at infinity is zero
positive charge - what is potential?
- potential is positive
- charge is repulsive
negative charge - what is potential?
- potential is negative
- charge is attractive
how can you form an electric field
- two parallel plates
- potential difference across them
how to calculate electric field strength formed between the plates
E = V / d
- V is the p.d. across the plates
- d is the distance between the two plates
electric potential in a radial field
V = Q / 4πε0r^2
what is electric potential difference
- the energy needed to move a unit charge between two points
what do the field lines show
- the direction of force acting on a positive charge
field lines - uniform vs radial
- uniform - same electric force is exerted everywhere (equally spaced parallel lines)
- radial - depends on the distance between the two charges
what does the distance between the field lines represent
the magnitude of the force
what surfaces to electric fields have
equipotential surfaces
what happens when a charge moves along an equipotential surface
- no work is done
- this is because the potential on an equipotential surface is the same everywhere
magnetic flux denisty
measure of the strength of the field
magnetic flux linkage
magnetic flux multiplied by the number of turns in a coil
Flehming’s left hand rule - charged particles
- thumb - direction of motion/force
- first finger - direction of field
- second finger - direction of conventional current
what direction is the force
- force is always perpendicular to the motion of travel
- hence charged particles travel in a circular motion in magnetic fields
calculating force on a charged particle
F = Bqv sinθ
Flehming’s left hand rule - current carrying conductors in a magnetic field
- motion
- field
- current
what happens when a current passes through a wire
- a magnetic field is induced
- field lines of induced magnetic field form concentric rings around the wire
calculating magnitude of force on the wire
F = BIl sinθ
how is an emf induced in a coil
- conducting rod moves relative to magnetic field
- electrons in rod experience a force
- build up of electrons on one side of the rod
- e.m.f. is induced
when is a current induced
- if the coil forms a complete circuit a current is induced
Lenz’s law
direction of induced current is such as to oppose the motion causing it
induction of emf in a coil through the change in current of another coil
- magnetic field is induced around a coil current-carrying wire
- if the current through the wire changes, magnetic field will also change
- emf will be induced in a second coil
- induced emf in the second coil is proportional to the change in current in the first coil
- if second coil completes a full circuit, a current is also induced
factors affecting the emf induced in the second
- magnetic flux density
- distance between the two coils
- number of turns in the second coil
- cross-sectional area of second coil
- time taken for change in current
