Electromagnetism Three lessons of fun with motors and generators

1. Motors

Motor Effect ► If a current passes through a wire in a magnetic field, it feels a force ► The force is at right angles to both the current and the field lines

Motor Effect ► The size of force increases if the current, field strength or length of wire increases ► The direction of force reverses if the current or field is reversed ► Electrical energy is converted to kinetic

Making a motor ► By coiling the wire we can  Increase the wire length (and force)  Get a force on both sides

Making a motor ► We need to solve a problem …

Making a motor ► Solution:  a commutator to reverse the current  and brushes to maintain contact

A real motor Spring wire touches contact for outer coil Other end of spring touches one ‘brush’ in here Other end of brush pokes out here Commutator has 14 elements – each attaches to a separate inner coil Input mains lead connects here Outer coil makes an electromagnet – wire looks bare but is coated in plastic Electromagnet core is laminatedto reduce laminated to reduce heat loss

2. Generators

Starter Question ► Which direction is the force felt? Wire & current Field lines NONE!! They are not at right angles

Electromagnetic Induction ► If a conductor cuts through magnetic field lines a p.d. is generated ► Either the wire or the magnet can move (we need relative motion) ► If the wire is part of a complete circuit we get a current generated ► All our electricity is generated this way

Electromagnetic Induction ► The p.d. is reversed if …  the movement is reversed (in – out)  the field is reversed (N-S) ► For a coil of wire, the p.d. increases if …  the field is stronger (field lines are closer)  the speed of movement is more  the number of coils is more  the area of the coils is bigger

Practical Use ► Kinetic energy (turning the handle) becomes electrical energy ► Slip rings and brushes prevent the external connections from tangling up

Practical Use ► The generator produces a.c. ► As the coil turns the direction of current reverses as shown …

3. Transformers

Starter 1. How do you increase the voltage from an a.c. generator (four things)? 2. Explain the purpose of the slip rings and brushes

Transformers ► A transformer is just an a.c. electromagnet (primary coil) next to a generator (secondary coil) ► The a.c. electromagnet makes a changing magnetic field ► This hits the generator coil making an a.c. current flow in that coil

Transformers ► Why use electricity to make electricity? ► The size of the generated output voltage can be changed (more coils etc.) ► A transformer is used to change an a.c. voltage

Uses ► Step-up  Car HT coil – increases battery voltage for spark plugs  National grid – raise voltage for transmission ► Step-down  Adaptors – reduce mains a.c. voltage  National grid –reduce voltage for supply to homes

Demo ► Note: the laminated soft iron core  Core – helps ‘concentrate’ the field into the secondary  Laminated – reduces heat build-up  Soft iron – can magnetise and demagnetise better than, say, steel

Transformer Formula Coils on primary Voltage on primary Coils on primary Voltage on primary = Coils on secondaryVoltage on secondary VV Primary coils Secondary coils

Homework ► Revise for test