WEBVTT

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Hi, I'm Steve Jones and I'm going to explain
how an electric motor works. Now I've drawn

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this diagram, it looks rather complicated
but it isn't really that complicated. There

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are two main elements. The first is this pair
of magnets, one with a north here, the other

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with a south here. These are usually permanent
magnets on small motors, but on big motors

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they are not. The second main element is this
coil here, although I've shown just a single

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piece of wire, actually it starts here, it
goes around, around, around, around, around,

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around and then comes out here. So although
it looks like one piece of wire, it's actually

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several hundred turns on a coil. And then
maybe several coils. Simply how it works,

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we have two things, a magnetic field going
from north to south represented by these arrows

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and if I use my left hand, I can say my first
finger is that direction is the field direction.

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Now the second thing we have is an electric
supply going from plus to minus so the electric

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supply goes up this wire through what is a
carbon brush, this is a graphite carbon brush

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into this that we call a commutator. The commutator
is made of two pieces of copper, this copper

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disc is connected to this side, this copper
disc to this side. And these are fixed so

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as the coil rotates, this rotates. And as
you can imagine, if this rotates half a turn,

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the insulating part is going to be against
the brushes and the electricity will not flow.

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So what happens when we get a flow of electricity?
We've got our field. C stand for current,

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center finger, current, and if I use my left
hand and put it this way, this is called Flemming's

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left hand rule and my thumb represents the
direction of a force which is exerted on the

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coil. They must be at right angles. So my
first finger is the field going that way.

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Now the electric current is coming in here,
it's going around here and it's going down

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there. So if I put my center finger in the
direction of the current, I can see that in

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fact I will get a force down here and if I
do the same this side, because this is going

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this way, the opposite way in fact I'll get
a force going upwards. So that's my force

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there. And what will happen is the coil will
rotate in that direction. If I didn't have

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this device, it would rotate until it was
upright and then it would stop and it would

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stop because as soon as it went over, again
the force would still be going the same way

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and it would just hold it upright. So what
happens is when it reaches the vertical in

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fact these swap over, the current goes the
opposite way around and in fact it makes sure

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that the current in this side of the coil
that is, even when this side reaches here,

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the current is always going that way and therefore
the force is always making it continue to

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rotate in a circle. Obviously this is going
to be a very uneven device, it's going to

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accelerate until it gets vertical, it's going
to slow down and then it's going to sweep

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around quickly and slow down again when it's
vertical. So what normally happens, we have

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at least three and very often six, nine or
twelve separate coils, each put at a different

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angle with separate connections on this side.
This makes a very smooth electric motor where

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three or four coils are working at once. So
this is very simply how an electric motor

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works.