A level Physics electric and magnetic fields

Force field

A force field is an area in which an object experiences a non-contact force

how to represent force fields

Force fields can be represented as vectors

electric fields

An electric field is a force field in which charged particles experience a force

Electric field strength

Electric field strength (E) is the force per unit charge experienced by an object in an electric field.

E = F/Q

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

\
Q1 = charge 1

Q2 = charge 2

ε0 = permittivity of free space

r = distance between the particles

attraction and repulsion

If charges have the same sign the force will be repulsive , and if the charges have different signs the force will be attractive .

Point charges

Point charges form a radial electric field

electric field strength formula

formula:

Absolute electric potential

at a point, it is the potential energy per unit charge of a positive point charge at that point in the field

absolute magnitude

greatest at the surface of a charge

as the distance from the charge increases, the potential decreases

so, the electric potential at infinity is zero

potential and charge

potential is positive and the charge is repulsive , when the charge is negative, potential is negative and the force is attractive

attraction and repulsion graphs

graphs :

Electric fields between parallel plates

You can form a uniform electric field using a pair of parallel plates with a potential difference across

electric field strength formula

formula:

Electric potential difference

is the energy needed to move a unit charge between two points

Radial electric fields diagram

Uniform field

A uniform field exerts that same electric force everywhere in the field

radial field

in a radial field the magnitude of electric force depends on the distance between the two charges

equipotential surfaces

The potential on an equipotential surface is the same everywhere, therefore when a charge moves along an equipotential surface, no work is done

Capacitance

Capacitance (C) is the charge stored by a capacitor per unit potential difference .

Energy stored by a capacitor

energy stored by a capacitor (W) is given by the area under a graph of charge against potential difference

Capacitor charging

the capacitor is connected to a power supply, current starts to flow and negative charge builds up on the plate connected to the negative terminal.

On the opposite plate, electrons are repelled by the negative charge building up on the initial plate, therefore these electrons move to the positive terminal and an equal but opposite charge is formed on each plate , creating a potential difference.

As the charge across the plates increases, the potential difference increases but the electron flow decreases due to the force of electrostatic repulsion also increasing, therefore current decreases and eventually reaches zero.

Capacitor discharging

When the capacitor is discharging the current flows in the opposite direction , and the current, charge and potential difference across the capacitor will all fall exponentially , meaning it will take the same amount of time for each of the values to halve.

Magnetic flux density

magnetic flux density (B) of a magnetic field, is a measure of the strength of the field, and it is measured in the unit Tesla

Magnetic flux

Magnetic flux ( ϕ ) is a value which describes the magnetic field or magnetic field lines passing through a given area

\
Φ = BA

Magnetic flux linkage

Magnetic flux linkage (N ϕ ) is the magnetic flux multiplied by the number of turns N , of a coil:

NΦ = BAN

Charged particles moving in a magnetic field

A force acts on charged particles moving in a magnetic field , this is why a force is exerted on a current-carrying wire in a magnetic field, because it contains moving electrons, which are negatively charged particles.

\
F = BQv sin θ

How find the direction of the force

fleming’s left hand rule :

Thumb - represents the direction of the Motion /force

First finger - represents the direction of the Field

Second finger - represents the direction of the Conventional Current (opposite direction to electron flow)

Solenoid

it is a coil of wire carrying an electric current. the chape of the magnetic field around a solenoid is similar to one around a bar magnet

\
the strength increases with:

the electric current

the number of turns in the coil

\

Motor effect

the force on side AB is acting up and the side CD has a force acting downwards. this makes the coil rotate 180 degrees in the clockwise direction. the split ring changes the direction of the current every half cycle. so AB is now acting down and CD is acting up allowing the coil to complete a full cycle.

Moving charged particle in a magnetic field

charged particle in magnetic field will have a circular motion

force is perpendicular to the charge particle

it will accelerate

speed stay the same but the direction changes so velocity changes

Hall effect

emf is set up across a current carrying conductor when a perpendicular magnet field is applied

\
at the start e- is attracted to 1 side of the conductor.

Eventually enough e- will build up on one side creating a magnetic force that produces an electrostatic force of repulsion

on the remaining free electrons attempting to move on 1 side of the conductor.

Electromagnetic induction

as the magnet approaches the coil the magnetic flux changes:

the magnetic flux changes and I is induced

\
as the magnet moves away from the coil the magnetic flux changes:

magnetic flux decrease and I is induced

\
\

Lenz’s Law

the direction of induced EMF is such that it opposed the change that produces it.

Faradays Law

induced EMF is proportional to the rate of change of flux linking through the coil

Explain why there is an induced EMF in a copper tube as a magnet passes through it

in the copper tube when the magnet is passed through the magnetic flux changes. so there is an induced current the produces an induced EMF . this happens when copper is a conductor

AC generators Vs Motor effect

difference is that AC isn’t connected to a power supply but motor effect is

AC generator

the coil starts in a position of 90 dgrs and the induced EMF is at 0v.

as the coil turns it cuts through the magnetic field change the mg flux inducing an EMF. the induced EMF is at its highest when the coil is at 180 dgrs

Transformers

a device that converts AC voltage of one level to AC voltage of another level

\

core of transformers

made up of laminated iron sheets. this is a magnetically soft material and magnetises and demagnetises easily

eddy currents

if fulx changes in the core then EMF is produced. so there will be an induced EMF all over the block which is the eddy currents

eddy currents produce heating effect so loss of energy

why is the core made of laminated sheets

the iron sheet are laminated to prevent electrons from moving layers which could cause short circuiting

step up transformer

change in I so a change in magnetic flux in the primary coil. so EMF is induced. the primary coil is linked to the secondary coil by the core. so current is induced in the secondary coil.

primary coil and secondary coil

if number of turn in primary < number of turn in secondary coil:

more change in flux in secondary coil

more induced EMF

more induced current