Direction and Magnitude of the Force 
A small metal ball is placed on a pair of electrified rails, as seen in this picture, it's in the presence of a magnetic field so that it is free to roll back and forth. So it's free to sort of move forward and backwards depending on the forces acting on it.
So if the current passing through the metal bowl is 90 A finds the direction and magnitude of the force on it. Now, at first, this might seem a bit large, but remember that we actually have quite a short circuit over here. So we, in reality, we might even get less than 90 A of resistance. But that's what the questions asking us. So these that.
Now, the first thing to find is the direction and that should be fairly easy. If we have the current flowing in this direction around the circuit, which we can see from the power supply, then we can just use the righthand rule in order to figure out the direction of the force. Current moves that way, magnetic fields into the board. So the force will be upward and will be propelled toward the end of the rails.
So what's the magnitude of the force? Well, we have the strength of the magnetic field here 2T. This is a fairly strong field, but not the strongest you can get. The distance between the Rails is 1 cm. So that will be the length of the conductor inside the magnetic field. So if we have our equation, F=B I L sin θ, we can substitute in some of these numbers. We know that B is 2T, I is 90 A, and L is 1 cm, we can see from the direction of the magnetic field that is going to be perpendicular to the direction of the cart.
So we can ignore the sin θ, it'll just take on its maximum value. So shooting in those numbers, we have 2T x 90 A, x 1 cm, which gives us 1.8 N, and it will be away from the voltage source and toward the end of the rails.
So there's a force 1.8 N. Now if the ball weighs 5 g, find its speed offered as travelled along 70 cm of the rails. And that seems fairly accurate. I think, if we have a 1 cm diameter sphere, and it's made of metal, then it will tend to have a mass of between 5 and 10 g. So we can assume that the rails are 70 cm long. And during all of this, the ball will experience the acceleration due to this force.
So now we can use our equations of motion to figure out exactly how fast the ball ends up going. So in this question, we're given a displacement, r. and acceleration, we can figure out from a force and our initial velocity is of course zero, or in trying to find is V, the velocity of the ball after it's been accelerated over 70 cm.
So we know that F = ma, we have the mass, 5g or 5 times 10 to the minus three kilogrammes [sysco]. And we have the force so we can calculate acceleration.
Substituting in our numbers and rearranging the equation, we have v^2 = 2ar, or taking the square root, v^2 = 2ar. Of course, a is equal to f/m. And you can see I've already made that substitution here.
So now we're taking our numbers, F is going to be 1.8, M is going to be 5g. R is going to be 0.7 because we have to measure that in meters.
So now our equation looks like this. And of course, we can use a calculator to figure out exactly what that value will be. It turns out to be about 22.45 m/s, which is roughly the speed of a car.
So we've actually propelled this little pellet quite fast off the rails. If we used our equations of motion to figure out the time that it would take, it turns out that it would take about a 20th of a second for it to travel this 70 cm, obviously, we could increase the speed of the pellet going off the rails. Simply by increasing the strength of the magnetic field, increasing the current throw it or increasing the distance of the rails. In fact, if we get it fast enough, then we could be firing this little pelletlike a bullet from a gun.

Describe the amount of torque on the coil of an electric motor 
Which equation can we use to describe the amount of torque on the coil of an electric motor?
Well, by now you should know this. F equals NBIA cosine theta. Of course, that’s the torque, not the force.
So it would be force times distance where the distance is the radius of the coil.
So the equation in part A is sometimes zero because cosine theta will sometimes be zero, right? So why is it that the coil keeps spinning even though the torque on it is sometimes zero?
Well, the answer is momentum or inertia. When the torque is zero, the angular momentum of the coil is usually sufficient to make it keep spinning.
If the force or the torque suddenly shuts off, it’s not going to stop moving instantly. It’s going to keep moving a bit and it will move far enough that the torque will actually stop being zero and become a positive number again.
So that means that it will keep spinning.
If the motor isn’t spinning, then it doesn’t have any of that angular momentum. So, in fact, it won’t actually start and you will need a little push to get it going.

Motors and Generators 
Question 23
A small metal ball is placed on a pair of electrified rails in a
magnetic field, so that it is free to roll back and forth.
(a) If the current passing through the
metal ball is 90 A, find the
direction and magnitude of the
force on it.
(b) If the ball weighs 5 g, find its speed
after it has travelled along 70 cm
of the rails.
Question 24
Two parallel wires separated by 3 cm are carrying currents of
magnitude 4.5 A in opposite directions. What is the force per
metre on each wire?
Question 25
A 12 V 24 W globe is connected to the secondary coil of a
transformer. The primary coil is attached to a 240 V power
supply.
(a) What is the current in the globe, assuming it runs at its
recommended power and voltage?
(b) What is the current in the primary coil, assuming perfect
efficiency?
Question 26
What is the difference between ideal transformers and other
transformers?
In the operation of an ideal transformer, the device can step
up or step down the voltage of a signal without losing any
energy.
For all other transformers, a small amount of energy will be
lost during the process.
This is the case for all transformers in the real world; there is
no such thing as an ideal transformer.
Question 27
Three discs are dropped through a horizontal magnetic field
that points perpendicular to the plane of the discs.
In which order do they land? Briefly explain why.
The insulator lands first, because no eddy currents can be
induced in an insulator.
The disc with a gap will land second, because its structure
can support small eddy currents that oppose its motion.
The solid metal disc will land last, because large eddy
currents that oppose its motion can circulate around the disc.
Question 28
Briefly outline how AC and DC generators power AC and DC
motors.
AC generators create an alternating current which changes
directions with a certain frequency. AC can be transformed
to different voltages, and is used to power devices which
require an alternating power source, like AC motors.
DC generators create a direct current in a single direction,
like a battery. The voltage of this form of electricity cannot
be transformed. It is mostly used in lowpower electronics,
and other devices which require a DC power source, like DC
motors.
Question 29
(a) Which equation can be used to describe the amount of
torque on the coil of an electric motor?
(b) The equation in part (a) is sometimes zero in an electric
motor when it is operating normally. Why does the
motor continue to spin at these times?
When the torque is zero, the angular momentum of the coil
is usually sufficient to keep it moving.
If the motor is not yet spinning, it will not start spinning if
the torque stays at zero.
