Magnetic Fields and Electromagnetic Induction

Magnetic Fields

A magnetic field is a region where a magnetic force is experienced.

The two types of magnet are permanent magnets and electromagnets.

Permanent magnets are common and are made of iron, cobalt or nickel alloys.

Lines are drawn to represent magnetic fields. These lines are called lines of flux.

The arrows show the direction of the force.

Field direction always goes from north to south.

The spacing between the lines of flux tells you about the strength of the field. The closer together the lines, the 

stronger the field becomes (close to the poles).

Like poles repel, unlike poles attract.

Field lines never intersect.

Fleming’s left hand rule is used on electric motors and particles.

Magnetic flux density is defined as the force acting per unit current in a wire of unit length, which is perpendicular to the field.

The direction of the current is opposite to the direction of the electron.

If the magnetic field is not uniform, then the particles will move in a spiral.

A commutator can be used to prevent wires from twisting for a current in a loop. This allows the coil to keep 

rotating in one direction.

A cyclotron is a particle accelerator which uses a high frequency, alternating potential difference.

The 2 ways of varying magnetic flux density are the current (B is proportional to I) and the spacing of the coils (B is proportional to N / L)

Electromagnetic Induction

The motor effect is where the interaction of two fields (magnetic and electric) to produce motion.

Two methods of producing electricity are moving a magnet in and out of a solenoid (the dynamo (movement) 

effect) and transformers.

The dynamo converts mechanical energy into electrical energy. Electricity is passed along wires. The motor 

reverses the process and converts electrical energy into mechanical energy.

Fleming’s right hand rule is used on generators.

Faraday’s law states that the induced E.M.F is proportional to the rate of change of flux linkage.

Lenz’s law states that the induced current is always in such a direction as to oppose the motion or change causing it.

Step-up transformers have more turns in the secondary coil than the primary coil therefore the potential 

difference in the secondary coil will be greater than the potential difference in the primary coil.

Step-down transformers have less turns in the secondary coil than the primary coil therefore the potential difference in the secondary coil will be less than the potential difference in the primary coil.

Transformer rule is the ratio of the secondary potential difference to the primary potential difference is equal to the ratio of the number of secondary turns to the number of primary turns.

Transformer inefficiency is due to:

• Resistance heating in the current in each coil (this can be resolved by adding low-resistance windings).
• Induced currents which are caused by the heating effect of the eddy currents in the core (this can be resolved by adding a laminated core which consists of layers of iron separated by layers of insulator.
• Repeated magnetisation and demagnetisation of the core (this can resolved by adding a soft iron core so that it can be easily magnetised and demagnetised.

©2011 Grant Dwyer

Electric Fields and Capacitance

Electric Fields

Conductors have free electrons.

Insulators don’t have free electrons as they are attached to the atoms.

Like charges repel, unlike charges attract.

Equipotentials are lines of equal potential.

Equipotential surfaces are perpendicular to field lines.

Any electrical conductor is an equipotential surface.

Coulomb’ law states that any 2 charged particles exert a force on each other which is proportional to their 

charges and inversely proportional to the square of their distance apart.

Air is an insulator but during a thunderstorm, the insulating properties break down due to the electric field 

between the clouds and the ground.

ε₀ is the measure of how well a medium will permit an electric field to pass through it.

Electrical potential is the work done per unit charge on a small positively charged object to move it from 

infinity to that point in the field.

Electric fields can be attractive and repulsive, gravitational fields are only attractive. Both fields can be 

represented by inverse square laws.

Electric field strength at a certain point in an electric field is defined as the force per unit charge on a positive test charge placed at that point.

Electric field is equal to the negative of the potential gradient.

No work is done when a charged particle moves along a line of constant potential.

Electric potential is a scalar quantity.

Electric field strength is a vector quantity.

Potential gradient is a point in a field where the change in potential per unit change of distance along the field 

line at that point.       

Capacitance

Capacitors store electrical energy.

Charge a capacitor by connecting it to a battery.

Discharge a capacitor by disconnecting from the battery and attaching it to the circuit.

The time taken to charge or discharge depends on the capacitance (which affects the charge) and the 

resistance (which affects the current).

Dielectric is an electrical insulator which can be a solid (ceramic, mica, glass, plastics and oxides of various metals), liquid (distilled water), gas (dry air) or a vacuum.

©2011 Grant Dwyer