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Physics 2: Magnetism/ Electeromagnetism (Magnetic Fields: (How do you know…
Physics 2: Magnetism/ Electeromagnetism
Interpret Diagrams of EM devices:
How does an Electromagnetic door lock work:
Switch is pressed - allows current thru a solenoid
Attracts the locking bar - allows door to open
When switch is released, current stops
The Bar springs back locking the door
Magnet:
Behaviour of magnets as you bring them together:
Exert non-contact forces (on each other)
2 diff' Attract : 2 same Repel
Induced Magnets:
2 facts:
Induced magnetism always causes a Force of A
When removed from field, it loses all magnetism
Induced magnet:
Becomes a magnet when put in a field
Where is the magnetic field strongest (on a magnet)?
At the poles
Permenant Magnet:
Has it's own Magnetic field
Magnetic Fields:
How do you know the direction:
Direction of the force that would act on another N-pole placed at that point
What is a Magnetic field line:
Line we draw around a magnet (Shows where field is)
lines go from North to South
What are Magnetic fields:
Region around a magnet, where the force acts on a magnet or another magnetic material
How does a compass work:
The magnetic compass needle (small bar magnet)
Earth has a Magnetic field (Core is Magnetic)
The Needle points in the direction of the magnetic field
4 Magnetic materials:
Iron
Nickel
Steel
Cobalt
Transformers:
Transformer Equation
Input p.d / Output p.d = turns of Primary coil / turns of Second coil
(Vp / Vs = np / ns)
What do we assume about them?
Transformers are 100% efficient
2 types of Transformers:
Step up - increase p.d & more turns on 2nd coil
Step down - Decrease p.d & more turns on 1st coil
Calculate power in Transformers:
p.d second coil X current second coil = p.d primary coil X Current primary coil
Vs X Is = Vp X Ip
Describe Basic Transformers:
Primary & Secondary coil of wire wound on an iron core
Change the size of the p.d and alt-Current
Generator effect, Induced p.d
& Microphones:
Moving the magnet
What if you move the Magnet/ Conductor in the opp' Direction?
The p.d/ Current will be reversed
What if you keep the Magnet/Coil moving Back & Forth?
Alternating current is generated
Generators:
What's the Generator effect:
Moving a conductor relative to a field,
Causes p.d to induce across the ends of the conductor
Causes current to flow
How do Generators Work?
Turn a magnet inside a coil
Or turn a coil in a magnetic field
How do Moving coil Microphones work?
Sound waves strike diaphragm
Diaphragm vibrates at same freq' as sound wave
This causes the coil to move Back & Forth in the field
Induces a changing current at the same freq' (as sound)
The changing current (signal) is sent to the Amp
The amp increases the size of the signal, to drive loudspeakers
Changing the size of Induced p.d:
Increase speed of movement
Increase strength of magnetic field
Electro-Magnetism:
What is produced around a wire when a current is passed thru?
Magnetic Field
What's a Solenoid:
Coil of wire acting as a magnet (with current)
What does the strength of a field depend on?
Distance (from wire)
Current
Force = Magnetic flux density X Current X Length
(F = B I L)
Electric Motors, Loudspeakers
& Headphones:
Loudspeakers/ Headphones:
How do they use the motor effect?
Convert changes in the current into vibrations, Causing soundwaves
How does the moving call Loudspeaker work? (headphones are the same)
Use a coil that slides back/ forth over 1 pole of a permanent magnet
Coil has a cone at the end
When the variable alt-current flows, The coil goes left to right
The cone then vibrates in/out at the same freq' as the coil & current
Producing rarefactions & compressions of sound waves (in air)
Electric motor rotation:
Current causes a field around the wire (in coil)
When at 90(degrees) the field of the coil is repelled by the field of the bar magnet
This Pushes the coil round & the split ring communicator swaps contact every 1/2 turn
(Happens over & over again)