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## AP®︎/College Physics 2

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

Lesson 3: Electric motors

# Electric motors (part 3)

Sal finishes the explanation of how a commutator will allow a loop of wire to continue spinning in a magnetic field, thereby allowing it to work as an electric motor. Created by Sal Khan.

## Want to join the conversation?

• Aren't the rotational arrows supposed to be going in the opposite direction? Sal corrected himself in the first Electric motor video but he forgot to do it here. • in the generator the coil is moved,this induces current in it.on the other hand this current may also be responsible for generating a force on the coil.according to me , one situation will use right hand rule another the will use left hand rule.
if the magnetic field and current remains constant the the direction of force will be opposite in the two cases.is this the effect of the lenz's law which states that the induced current opposes the cause that produces it • can any one tell me hoe to measure magnetic flux??
basically wats a proper definition and explanation of magnetic fulx..??
thnk u • Richard is right. The units are Webers. 1 Weber= 1 Tesla times 1 square meter. It tells you how much of a magnetic field passes directly through a surface (i.e. what component is perpendicular). Conceptually, this isn't difficult, but it changes how we calculate magnetic flux. Instead of just taking the product of area and field strength, we need to take the product of area and the component of field strength normal to that area. For this we use the dot product. Here are some ways of finding magnetic flux mathematically (in decreasing difficulty).

If you have been through a multivariable calculus class then you know how to take the surface integral. Magnetic flux is defined mathematically as the surface integral of the dot product between the magnetic field vector and the differential area vector, over the whole surface. In physics and calculus the area vector has a direction normal to the surface (which means it points straight out). It looks like this (\Phi)=int(B dot dA). The math can get tricky, but this definition will work no matter how ugly your problem looks.

Another way, which is what you see in most examples, assumes a uniform magnetic field, and flat surface. If this is true, then you can pull the magnetic field vector outside of the integral as a constant. Then you have (\Phi)=B dot int(dA). The integral of dA is then just A (the area vector), and (\Phi)=B dot A. If you do not know the dot product, this is equivalent to (\Phi)=B*A*cos(/theta), where we treat B and A as scalars (values without direction), and /theta is the angle between the direction normal to the area and direction of the magnetic field. This is the first formula I learned for magnetic flux, and I always found it easy to remember because the equation looks like "Oh: BAcon"
• i knw this could be silly but when you are using the right hand rule for the right hand side of the circuit, do u have to invert your hand? • Why is the rotation counter-clockwise. If the force on the left is acting into the screen, and the force on the right is acting out of the screen, shouldn't it be rotating clock-wise? I mean, in the previous video, it was rotating clockwise. • I'm a little confused. Does the commutator switch the direction of the current? If so, does it make the DC motor a AC motor? • Can I get to know what a Cyclotron is, how does it works and what it does? • A cyclotron is a way to accelerate particles with a strong static magnetic field and a changing electric field
It goes like this:
The cyclotron is composed of two semicircles(a and b) with a distance d separating both of them, and there's an electric field between them that imposes a potential difference of V. The proton starts at a's surface facing b, with a potential V and its accelerated by the electric field until b, gaining a kinetic energy of V.q
When it gets to B, the magnetic field of intensity B takes over and the proton realizes a circular motion of radius r
magnetic force F = q.v.B = mv²/r , r = mv/Bq
Since mv²/2 = Vq , then v = sqrt(2Vq/m) , so r = m.sqrt(2Vq/m)/Bq = sqrt(2Vm/q)/B
So, r = sqrt(2Vm/q)/B , and the time it takes for the proton to come to b until it realizes half of a circle and reaches b again is f = pi.r/v = pi.m/Bq
Now, when the proton reaches b, the electric field must have changed (it needs to be set up to do so), in such a way that now b is at a potential V and a at a potential 0. This way, the proton, once again, gains a kinetic energy Vq from the electric field. Only this time, the radius increased, according to the equation written above
r = mv/Bq , where v is, now, sqrt(4Vq/m)
The ratio r/v, however, remains constant and equals to m/Bq , so f is constant and always equal to pi.m/Bq
So, the electric field must be always changing at this rate, in order for the proton to keep drawing semicircles with a growing radius and speed. At some point, the particles may get so fast you need to account for relativistic effects, too.
• I still dont understand the difference between AC and DC motor • I don't get how the commutator solves the problem of the wires twisting I get that it reverses the current so that it can keep the torques steady but how does it untwist the wires then? • Whats the differnece between SLIP AND SPLIT RINGS ??
(1 vote) • Split Rings are like half rings and have a brush connected to them.(the brush just connects it to the circuit and slides along it)there are two in generators and motors and are aligned in a way that they don't touch each other and after a 90 degree turn, exchange brushes. In a generator or motor, once the loop or armature (fancy word for loops in a motor) spins more than 90 degrees on the axel, these split rings exchange brushes and the current in the loop is reversed in a motor and in a generator the current is kept going in the same way (watch the Generators video for more info). This, in motor keeps it spinning and in a generator gives a d.c. output.

Slip Rings are complete rings found in generators that give an a.c. output. They just keep the wires from rolling along the axel or whatever. they also have brushes which connect them to the rest of the circuit but the brushes do not switch rings so the current will not be kept going the same way (see the Generators video to understand). they have no use in motors because since the current isn't exchanged, the motor cannot be kept spinning in one direction.

Google picture these to get an understanding of how they are arranged. Kinda hard to describe