# P3 Electromagnetism

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If there is no current what else is there none of?
Magnetic field
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What kind of current should there be if there is a magnetic field around the wire?
Direct current (DC)
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If we change the direction of the current then what else should change?
Direction of the magnetic field (Alternating current - AC)
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If we can turn the current on and off what do we call this?
An electromagnet.
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How do we increase the magnetic field of a wire?
Coil the wire, increase current, magnetically soft iron core
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Where are electromagnets placed?
In cranes (iron/steel), loud speaker, door bell
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What are the steps for a loudspeaker to work? Step 1
Current passes through coil. Magnetic field produced.
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Step 2
Magnetic field now interacts with the permanent magnet. Attraction/repulsion
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Step 3
Produces a resultant force for the cone to move.
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Step 4
AC causes direction of force to change.
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Step 5
Changing direction of field, force means cone in/out=>sound waves.
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What are the steps for a doorbell to work?
Switch closed=complete circuit and current flows.
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Step 2
Electromagnetic field is produce.
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Step 3
Iron contact attracted to magnet, clapper hits bell.
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Step 4
Current is broken=>Current stops=> electromagnet=off
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Step 6
iron contact no longer attracted. Springs back.
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Step 7
Circuit is now complete and the process repeats.
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What other devices is this process used for?
Circuit breakers/other bells
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What direction is the magnetic field?
North to south
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What will happen to a wire carrying current if you place it in a magnetic field?
It will experience a force and will move.
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Why does this happen?
Two magnetic fields interacting
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Therefore, what is this effect called?
The motor effect
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How do we increase the size of the force?
Increase:Strength of magnetic field/Size of current
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What do the fingers of Fleming's left hand rule mean?
Thumb: Motion, First: Field (N-S), Second: Current (+ to -).
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What will happen to the wire if it is parallel to the magnetic field?
It will not experience a force and therefore not move.
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If there is no SRC, then what should happen at 90o?
It will stop rotating due to no moment
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How do we solve this problem?
Switch direction of current
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In commercial uses, what do we use for this?
A split-ring commutator.
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How does the direction of rotation continue?
Momentum carries it on, contacts are swapped every half turn
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What pole does the wire act as when it goes towards the field?
North
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Therefore, what does the wire act as coming away from the pole?
South
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How do we induce a p.d?
Conductor through field, field through conductor
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What does AC allow the magnetic field to do?
Change direction
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Why can a DC battery not induce a p.d?
Constant magnetic fields don't work
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If there are the same number of coils on each side of the iron core what does that mean?
Primary coil p.d = Secondary coil p.d
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In a step-up transformer, where should the be the most turns?
Secondary coil
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In step-down transformer, where should there be the most turns?
Primary coil
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What is the equation of a transformer?
Vp/Vs = Np/Ns
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What is the equation to find power?
P=IV
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If we were to find the power of primary and a secondary coil?
VpIp=VsIs
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Why must power in transformers be conserved?
No more energy goes out than comes in.
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Why is power always lost?
Heating of iron core
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What does a switch-mode tranformer do?
Operates at high frequencies (50Hz-200kZ
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What are the advantages of switch-mode tranformer?
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## Other cards in this set

### Card 2

#### Front

What kind of current should there be if there is a magnetic field around the wire?

#### Back

Direct current (DC)

### Card 3

#### Front

If we change the direction of the current then what else should change?

### Card 4

#### Front

If we can turn the current on and off what do we call this?

### Card 5

#### Front

How do we increase the magnetic field of a wire?