# Physics, topic 7, electromagnetism

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• electromagnetism
• electromagnets and magnets
• magnets
• wires
• charge going through a wire creates a magnetic field
• solenoid: coil of wire
• increase strength of solenoid by increasing coils, increasing current
• electromagnet
• put an iron bar in a solenoid
• an be switched on and off
• can be used in alarms
• the motor effect
• when a current in a wire is perpendicular to a magnetic field there is another perpendicular force.
• F=BIL
• F=force (newtons, N)
• B=magnetic flux density (tesla, T)
• I=current (amps, A)
• L=length (metres, m)
• uses
• electric motors
• an entire coil goes through a magnetic field
• one side goes up, one goes down
• the two ends of the wire alternate between split rings and keep it going
• loud speakers
• the magnet and wire do motor effect and the solenoid pushes outward
• this moves the paper cone, creating vibrations and sound
• generator effect
• creating a PD in a wire which is moving in relation to a magnetic field
• backwards motor effect
• pass a magnet through a solenoid, to produce a PD
• spin magnet in solenoid to produce AC
• increase PD by increasing speed or strength of magnet
• uses
• generator
• microphone
• backwards speaker
• set up speaker with no PD
• sound waves push paper cone, causing magnet to move
• creates PD, linked to speaker
• transformers
• step up transformer
• increase PD
• decrease current
• more turns in secondary coil
• step down transformer
• decrease PD
• increase current
• more turns in primary coil
• VsIs=VpIp
• Is= secondary current
• Untitled
• Vp/Vs=Np/Ns
• Np=turns in primary coil
• Ns=turns in secondary coil
• Vp= PD in primary coil
• VsIs=VpIp
• Is= secondary current
• Untitled
• Vs=PD in secondary coil