# OCR Advancing Physics: Electromagnetic Machines

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• Created by: Caraa
• Created on: 03-06-13 15:50
Which way do magnetic field lines go?
From north to south
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Which way do magnetic field equipotentials go?
Perpendicular to magnetic field lines
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What rule can you use to work out the direction of the magnetic field around a current carrying wire?
The right hand rule
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In the right hand rule what do your fingers represent?
Thumb is the current and fingers are direction of the field
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How can you tell how strong the magnetic field is from a diagram of field lines?
The field lines will be closer together where it is stronger
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What can Fleming's left hand rule be used for?
Working out the force on a current carrying wire in a external magnetic field
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If a current carrying wire is placed in a magnetic field such that the current and magnetic field lines are parallel, what is the force?
Zero
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In Fleming's left hand rule, what do your fingers represent?
Your thumb is motion (Force), first finger is field and second finger is current
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How does the direction of the force relate to that of the magnetic field and current?
Perpendicular
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Whats the equation for force on a current carrying wire?
F = BIL
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What is magnetic field strength also known as?
Flux density
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What is the equation for flux density?
B = Flux / A
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What is a motor?
Where a emf causes movement
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How can a loop in a magnetic field be used to make a motor?
There will be a force on both the side arms meaning it will rotate. Using a split ring communicator you can also reverse the current every half turn so the loop rotates all the way round.
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N.Flux
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How does a conductor moving through a magnetic field make a emf?
The electrons in the conducctor feel a force which means they accumulate at one end of the rod. This induces an emf across the ends of the rod. If the rod is part of a complete circuit then a current will also flow
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When is an emf produced?
When flux is cut
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emf = change in flux linkage / change in time
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What is lenz's law?
Incduced emf is always in such a direction as to oppose the change that caused it. Represented by putting a minus sign in Faraday's law. It agrees with conservation of energy
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Explain the parts of a transformer
2 coils and a iron core which is often laminated
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How do transformers work?
A alternating current in the primary coil produces a magnetic flux which passes through the iron core into the secondary coil. Here it induces a alternating voltage of the same frequency
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How would you step up the voltage?
By having more turns in the secondary coil
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What is the the permeance of a material?
Amount of flux induced in it for a given amount of current turns surrounding it (flux = permeance.N.I)
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What is the equation for permeance?
Permeance = permeability.A/L
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How would you make the ideal transformer?
Make the iron core short and fat and made from a high permeability material like iron
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What are the problems with the ideal transformer idea?
You also need the conductance of the coils to be as high as possible to limit energy loss, meaning you need small radius coils, but this doesn't really work with a fat core
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What do magnetic circuits do that electric don't?
They can go through air although this is not advisable as air has a low permeability
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What is the problem with a solid iron core and how is it overcome?
A solid core will have more eddy currents since iron is a good conductor of electricity. This means more energy loss (Due to the core heating up). This can be overcome by using a laminated core
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What are eddy currents?
The alternating field induces an emf in the core and as iron is a good conductor this causes electrical currents to flow. These currents induce a flux in themselves to oppose the flux that created them, which reduces the overall flux in the core.
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How else may energy be lost from the core and how is it minimised?
Heat from the resistance of the coils. Minimized by using copper coils which have a low resistance.
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Why are transformers used in the national grid?
Since power = current x voltage, if you increase voltage you decrease current. In the cables current must be low so they don't get too hot and voltage must be stepped down to go into houses.
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What is a generator?
It converts kinetic energy to electrical. An example is rotating a coil in a magnetic field which cuts flux so produces an emf
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How would you find emf from a graph of flux linkage against time?
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How would you find change in flux from a graph of emf against time?
The area
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## Other cards in this set

### Card 2

#### Front

Which way do magnetic field equipotentials go?

#### Back

Perpendicular to magnetic field lines

### Card 3

#### Front

What rule can you use to work out the direction of the magnetic field around a current carrying wire?

### Card 4

#### Front

In the right hand rule what do your fingers represent?

### Card 5

#### Front

How can you tell how strong the magnetic field is from a diagram of field lines?