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## AP®︎/College Physics 2

### Course: AP®︎/College Physics 2>Unit 5

Lesson 3: Electric motors

# Electric motors (part 2)

Sal shows that a commutator can be used in order to keep the loop of wire rotating in the magnetic field. Created by Sal Khan.

## Want to join the conversation?

• Sal specifically calls the direction of the magnetic force (I cross B) "upwards" , but he denotes it with a circle and dot that I take to mean out of the page towards you- Am I safe in assuming that he means upwards to be out of the page? When I am doing the right hand rule with these directions I get that my thumb is out of the page...
• Yes. I was confused by this also (Video Part 2: & ). Usually "up" and "down" are parallel with the page/screen. He drew the into/out of notation and it would be clearer if he used the same vocabulary as what he drew (apparently "up" means out of the page to Sal). I got the same result as you when I used the right hand rule.
• Why are split ring commutators used?

• the commutator or split ring is fixed to the coil and rotates with it and when the coil is nearly vertical the forces cannot turn it much further but when the coil overshoots the vertical the commutator changes the direction of the current through it so the forces change direction and keep the coil turning
• Whats the point of having a brush in the motor?
• the brushes are two contacts which rub against the commutator and keep the coil connected to battery or motor they are usually made of carbon
• Why are commutators used instead of split rings?
• commutator are called split ring and it is used in DC motors to rotate the coil in vertical direction
A split - ring commutator (sometimes just called a commutator)
is a simple and clever device for reversing the current direction
through an armature every half turn
The commutator is made from two round pieces of copper,
one on each side of the spindle. A piece of
carbon (graphite) is lightly pushed against the copper
to conduct the electricity to the armature. The carbon
brushes against the copper when the commutator spins.
As the motor rotates, first one piece of copper, then the next
connects with the brush every half turn. The wire on the
left side of the armature always has current flowing in
the same direction, and so the armature will keep turning
in the same direction.
The pieces of copper are held apart in the centre
and do not touch each other. They look like a
ring of copper which is split down the middle
This is why it is called a split - ring commutator.
• whats the difference b/w ac and dc motor?
• Hello Erij,

The DC motor requires a commutator. Think of this is a mechanical switch that activates the correct coils in the motor. This may be done using old school brushes or with electronics. Ref:

https://en.wikipedia.org/wiki/Commutator_(electric)

The AC motor does not require a commutator because the applied AC voltage coupled to physically displaced coils is seen as a rotary magnetic field. Ref:

https://en.wikipedia.org/wiki/Induction_motor

Please leave a comment below if you would like to continue the conversation. Know that it takes awhile to visualize what is happening in these motors...

Regards,

APD
• When we talk about the area where the flux goes through, the flux goes through the empty space inside the square, what happens if you have a coil shaped like a cylinder. How do you measure the area? is it the surface area of the wire?
• at isn't the rotation supposed to be in the the opposite direction? If it's going into the page on the left, the rotation should be flipped.
• What is the moment arm?
• moment arm is simply the length between a joint axis and the line of force acting on that joint.
Every joint that is involved in an exercise has a moment arm. The longer the moment arm is the more load will be applied to the joint axis through leverage. As an example, think of trying to get a nut and bolt apart. If you can’t do it by hand because the moment arm is small, you use a crescent (as shown) which provides you with a much larger moment arm and allows less force (applied by you) to result in much more torque (rotational force) being applied at the nut. This is because torque at an axis is:
Force x Moment arm = Torque
In the exercise examples that follow you'll see the moment arms that work on the hip and knee joints with some common squat variations. Understanding these moment arms will enable you to determine which variations are safe or dangerous and what muscles are working most/least with each variation.