magnets & motors 3rd July - 14th July

Field- the area around an object in which a field is felt

• iron or steel can be magnetised or demagnitised
• cobalt & nickel are harder to enforce a field on
• permanent magnets are made of steel

To magnetise an object:

• holding a magnetic object near a magnet
• this will induce a magnetic force on it 

force = magnetic flux density x current x length

Magnetic materials:

• nickel
• iron
• cobalt

> Around a current carrying wire there is a circular magnetic field

> Around a coil carrying current we get a simular patteren to a permanent magnet

Uses:

• scrapyard cranes
• circuit breakers
• electric bell
• relays

• create a simple circuit
• wrap part of the wire around an iron core
• turn on battery to complete circuit
• the object has now been electrically induced and will have an electric field

> to increase the electric field you must increases the electric current or number of coils around iron core

> if you reverse the current the magnetic field lines will become reversed

> the greater the current the closer the magnetic field lines as there aee more of them

The current is flowing through a wire with an anticlock wise magnetic field flowing around it. Which way is the current flowing?

Use your right hand and hold as is you're riding a motorbike and stick your thumb out. Twist with the direction of the magnetic field. The way your thumb points is the direction of the current.

The current is coming out of the page. 

Circuit breaker:  

The switch is held closed by a spring, if the current becomes too large the switch is pulled open by the spring which becomes an electromagnet and will stay open until manually re-set.

Electric bell:

When connected to a battery the iron ammature is pulled on to the electromagnet which opens the make-and-break switch and the elctromagnet is turned off. This makes the aramture spring back and the make-and-break switch close again and the cycle will repeat.

Relay:
Used to switch on electric machines on & off. A small current passes through the coil of an electromagnet magnetcally inducing the iron core, pulling the iron armuture on to the elcetromagnet.This closes the switch gap and turns the machine on. By using a small current with an electromagnet we can turn something on with a much larger current. 

Tells us the direction that a current carrying wire is forced when it is in a magnetic field

To increase motor effect:

• increase length of wire
• increase the current
• increase the magnetic field

Electric current is coming out of page:

Electric current is going in to page: