7 Electric and Magnetic Fields Flashcards
Electric field
A region in which a charged object will experience a force
In which direction do electric fields exert a force?
Positive to negative
Radial field
A field which forms around a point charge
Uniform field
A field typically formed between two plates
What happens to field strength in a radial field?
As you go further from the centre the field becomes weaker because the field lines become more spread out
Field strength in a uniform field
The same throughout because the lines are equally spaced
How is electric field strength shown in a drawing
The closer the field lines are the stronger the field is at that point
Equation linking electric field strength, force on a positive charge and the charge
E = F/Q
Electric field strength = force acting on the charge/ charge
Equation to work out electric field strength between two plates
E = V/d
Electric field strength = voltage between two plates/ distance between plates
Lines of equipotential
Lines joining points of equal electric potential in an electric field
Equation for work done in moving an object of charge between two lines of equipotential (learn)
W= Qx (change in)V
Uses of electrostatics
- microwaves
- precipitators in chimneys
- crop spraying
- spray painting
Equation for electric field strength in a radial field
E = Qx 8.99x10^9/r^2
Electric field strength = charge of point/ permittivity of free space x 4pi x radius squared
Equation for force between two charges in a radial field
F = Q1 x Q2 x 8.99x10^9/r^2
Force between charges = charge of point1 x charge of point2 / permittivity of free space x 4pi x radius squared
Electric potential
The work done per unit charge when a particle is moved in a field
Equation for electric potential in a uniform field
V =Ed
Electric potential = electric field strength x distance
Equation for electric potential in radial field
V = Q x 8.99x10^9/r
Differences between G fields and E fields
•gravitational forces affect all particles with a mass, electrostatic forces only
affect particles carrying a charge
•gravitational forces are always attractive but electrostatic can attract or repel
•it isn’t possible to shield a mass from a gravitational field but this can be done in an electric field
What happens when you apply a p.d. to the terminals of a capacitor?
The plates become charged and an electric field is created between the plates
Equation linking capacitance, charge and voltage
C =Q/V
What does the area under a voltage against charge graph represent
Energy transferred
Equation linking work done, voltage and charge in a capacitor
W =1/2 Vmax x Q
or
W=Vaverage x Q
Equation linking work done, capacitance and voltage
W = 1/2 C x V^2
W= 1/2 Q^2/C
Uses for capacitors
- camera flash
- keys on a laptop
- inside the cable from a satellite dish to a TV
How do capacitors charge and discharge?
Exponentially
Equation linking I, Q, R and C in a discharging capacitor (learn)
I= Q/RC
RC = time constant
What gives the half life of a discharging capacitor?
RC x ln(2)
Time constant x ln(2)
Equation linking starting value of charge, voltage or current and and the value at any point when discharging
Q = Qo x e^(-t/RC)
V = Vo x e^(-t/RC)
I = Io x e^(-t/RC)
When a capacitor discharges for 1 time constant what value of the original charge, voltage or current is given?
37% of the original value
Q= 0.37Qo
V= 0.37Vo
I= 0.37Io
Magnetic field
A region in which a magnetic force will be experienced by a magnetic material
In which direction do magnetic field lines travel?
North to south
What force is felt at neutral points in a field?
Nothing since the forces cancel out
If current is travelling into the page which way is the magnetic field acting?
Clockwise
Equation linking force in current carrying wire, magnetic flux density, current in a wire, length of wire, angle between the field and wire
F = BILsin(x)
Equation linking force on a charged particle, magnetic flux density, charge of particle and velocity of particle
F = BQVsin(x)
F= force on charged particle B= magnetic flux density Q= charge of particle V= velocity of particle X= angle between magnetic field and movement of particle
Equation to work out magnetic flux density (learn)
B = I/A
B= magnetic flux density I= magnetic flux A= area of surface perpendicular to field
Equation for flux linkage (learn)
Flux linkage = N x I
Flux linkage = number of turns of coil x magnetic flux
Electromagnetic induction
The generation of a current in a conductor by the interaction of a changing magnetic field with the conductor
How can the magnitude of an induced electromagnetic voltage be increased
- stronger magnet
- moving the magnet faster
- increasing the turns if the coil
What happens when a magnet falls through a coil
- a current is induced in the coil
- the coil opposes the movement of the magnet due to Lenz’s law
- as the magnet falls out the other end of the coil the “poles” of the coil flip in order to oppose the direction of the magnet
- this causes the current to change direction
Faraday and Lenz’s law
E= -d(NI)/dt
E= -d(NBA)/dt
emf= - change in number of coils x magnetic flux/ change in time
Uses of electromagnetic induction
- large rotating coils generating electricity
- step up or step down transformers
- charging phones
- braking on trains
- braking on electric cars
Transformers
Transformers increase or decrease the voltage of alternating current electrical power supplies
How do transformers work?
- primary coil produces changing magnetic flux
- this magnetic flux is linked to the secondary coil by iron core
- therefore the magnetic flux linkage in the secondary coil varies continuously
- the change of this flux linkage induces a varying emf in the secondary coil
Transformer equation (learn)
Vsecondary/ Vprimary= Nsecondary/ Nprimary
Equation to work out average voltage from peak voltage
Vrms = Vo/ route 2
Irms = Io/ route 2
If three identical magnets are dropped through a cylinder of cardboard, copper and plastic, which will hit the floor first and why?
Cardboard and plastic first then copper
The copper opposes the direction of motion of the magnet so slows it down
