Electric and magnetic fields- Paper 1

Force field

an area in which an object experiences a non-contact force

Electric field

a force field which charged particles experience a force

Electric field strength (E)

the force per unit charge experienced by an object in an electric field

Electric field strength (E) equation

E=F/Q

Describe the Electric field strength in a uniform field

It is constant

Describe Electric field strength in a radial field

it varies

Coulombs 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

Electric field strength in a radial field

E=Q/4pieE0r²

Absolute electric potential

at a point is the potential energy per unit charge of a positive point charge

Equipotential surfaces

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

Describe the equipotential surface lines between two parallel plates

straight lines perpendicular to the charge

Describe the equipotential surface on a radial field

form concentric circles

Capacitance (C)

the charge stored by a capacitor per unit charge

Describe how a capacitor charges

• Capacitor is connected to 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 and opposite charge is formed on each plate, creating a potential difference

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

Describe how a capacitor discharges

• The current flows in the opposite direction, and the current, charge and potential difference across the capacitor will all fall exponentially

• it will take the same amount of time for each of the values to halve

What is the time constant

the time taken for a capacitor to charge and discharge

Magnetic flux density (B)

a measure of the strength of a magnetic field

Magnetic flux

describes the magnetic field lines passing through a given area

Magnetic flux linkage

the magnetic flux multiplied by the numbers of turns of a coil

Electromagnetic induction

When a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force (as they are charged particles), and build up one side of the rod, causing an emf to be induced in the rod, this is known as electro-magnetic induction

When does an electromagnetic induction occur?

When you move a bar magnet relative to a coil of wire, if the coil forms a complete circuit, a current is also induced

What do Faraday’s law and Lenz’s law effect?

electromagnetic induction

Faraday’s law

the magnitude of induced emf is equal to the rate of change of flux linkage

Lenz’s law

the direction of induced current is such as to oppose the motion causing it

What are the factors that effect the emf induced in a coil when there is relative motion between the coil and a permanent magnet?

• Number of turns in a coil

• Magnetic flux density

• Cross sectional area

• The time taken for the motion