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Electromagnetism
How can we make an electromagnet?
Starter:Make a list of uses for magnets.
Too easy? What are magnets made of ?
16-18/21
Glossary Words
• Electromagnet – A magnet formed when a coil of wire with a current through it, its wrapped around an iron core.
• Electromagnetic induction – the process of inducing a potential difference in a wire by moving the wire so it cuts across magnetic field.
What do magnetic fields look like?
• Demo – magnet and compass.
• Demo - magnet and iron filings
• Demo making an electromagnet with a wire, nail and paper clips.
• How can we increase the strength of the magnet?
Notes - printed
• An electromagnet is made by coiling an insulated wire about an iron core.
• This produces a magnetic field around the bar making it temporarily magnetic.
• Switch off the current, and the iron will lose the magnetism.
• Steel is no good because it keeps its magnetism.
Uses.
• Read page 243 and answer these questions.
1) name 4 uses of electromagnets.2) What is the benefit of using electromagnets in a scrap yard crane, rather than a permanent magnet. 3) Briefly describe how a circuit breaker works.
A relay switch A relay is a switch.Pass a current through the EM the armature is pulled towards it. This closes a gap and allows something to be turned on.
Increasing the strength of an electromagnet
• 1) increase number of coils.• 2) increase current.
Mark the long answer question
• What is the mark out of 6?
• Now … correct it to make it a full mark answer using the marking criteria.
• 3. Quality of written communication: One mark for correct sequencing. 1 bolt out → plunger up → switch off / circuit broken
• any five from 5 • • high current flows • • electromagnet is stronger • • the iron bolt is pulled out • • the plastic plunger moves up • • the switch is lifted / open / off accept circuit is broken • • no current flowing • • to re-set the plunger must be pushed down
Electromagnetic induction
How can a current be induced in a coil?
Starter
What can you remember about how electricity is made?
18/21
Glossary
Generator – a system consisting of an iron core and coil of wire which will produce a voltage when moved.
TOP TIP- on an exam question if there is a power supply it is NOT electromagnetic induction.
Generators (notes)
• A voltage is induced across the ends of the wire when it cuts across a magnetic field.
• (The conductor has to cross the magnetic field).• This is electromagnetic induction. If the wire is in
a complete circuit it will make an electric current
DEMOs
• Coil and magnet
• Coils on a cardboard tube and iron core page 247
The Motor Effect
• Electric motors work because a force produced in a magnetic field can make things move. E.g. in an electric motor.
• This is the motor effect.
How can I work out the direction of the force?!
Which direction is the force?
• Your first finger points in the direction of the magnetic field (North to South).
• Your second finger points in the direction of conventional current (positive to negative).
• Your thumb points in the direction of the thrust or force on the conductor.
Use a pen to write these on your hand, and work out the direction of the force.
Force Factors
• Increase the size of the force in the motor by – Increase size of current– Use a stronger magnet.
Work your way through the questions.
