TrueTears question thread

[TrueTears]

Magnetism is so fun

[TrueTears]

Another question: see attached.



How do you work out the the direction of the change in magnetic flux? (In this case upwards)

And when it says "the induced magnetic field will then be down at this point" At what "point" is it down? How do you work out the direction of the current?

Thanks!

[Mao]

Lenz's law: the direction of magnetic field generated by the induced current is in the opposite direction to the direction of change in flux.

i.e. for the above case figure a
in that position, as it rotates flux to the right decreases
hence the direction of magnetic field generated by the induced current points to the right [since it is a coil, treat it as a solenoid].
by the right hand rule, current flows from A to B.

[TrueTears]

Ah cool thanks!

Also when it says in part (a) the magnetic flux is entering the loop from above and in part (b) it enters from below. What does "entering from above" and "entering from below" mean?

[Mao]

Ah cool thanks!

Also when it says in part (a) the magnetic flux is entering the loop from above and in part (b) it enters from below. What does "entering from above" and "entering from below" mean?

it means a weird explanations that you should pay no attention to

always consider it as a change in flux.

in part b, as the coil rotates there is a net increase in flux to the right, hence by lenz's law, the induced current will have a magnetic field opposing this, i.e. to the left.
by the right hand screw rule, current flows from C to D.

[TrueTears]

Thanks Mao, I get it now XD

[TrueTears]

How much charge, in coulombs, flows in a loop of wire of area and resistance 0.2 ohms when it is totally withdrawn from a magnetic field of strength 3.0 T?

Thanks.

[Mao]

Faraday's Law:

[TrueTears]

Thanks Mao.

Yeap, that's what I did and I got = 24000 C. But book says 0.0024 C :S. Wrong?

[cns1511]

I have a feeling it should be 1.6*10^{-3} m^2

-Mao

[TrueTears]

Heh, yeap that works out to be the right answer

stupid shittycaranda XD

[TrueTears]

Just out of interest, when an electromagnet is formed normally it would be iron and you would put copper wires around it with a current flowing through it, some of the electron domains in the iron would line up with the magnetic field hence become an electromagnet. But why can't all metals become electromagnets? (Or not work as well as iron). Don't all metals have electrons?

Thanks.

[NE2000]

Just out of interest, when an electromagnet is formed normally it would be iron and you would put copper wires around it with a current flowing through it, some of the electron domains in the iron would line up with the magnetic field hence become an electromagnet. But why can't all metals become electromagnets? (Or not work as well as iron). Don't all metals have electrons?

Thanks.

I think it something to do with iron, nickel and cobalt having electrons that tend to align in the same direction. All metals have electrons, all electrons spin and all spinning electrons generate magnetic fields. In the case of many metals these magnetic fields cancel out very well so the metals don't have the ability to be turned into electromagnets. In the case of these three metals nearby on the periodic table the magnetic field seems to add together in these domains which allows them to be permanently or temporarily magnetised. It's something like that anyway.

[TrueTears]

Ah cool I think I get the gist of it

[TrueTears]

Also, split rings are used in motors and slip rings are used in generators. What are the differences between the 2? (besides the fact one is used only for DC motors the other is used in generators) What are the functions of each? How do they exactly work?

Many thanks!

EDIT: Okay, I think I know what they do now, can anyone check if my understanding of these 2 devices are correct?

1. Split rings
Split rings are used in DC motors. They are connected to the coils so they can spin with them. Their purpose is to connect to the opposite terminal, so if one half of the split ring was connected to the -ve terminal and the other half was connected to the +ve then each time the coil is rotated into 90 degree position (ie, when it's vertical), the 2 halfs of the split ring would now be connected to its opposite terminals. As a result the current is now reversed so the net torque is still in its original direction (If it was clockwise, it would continue to be clockwise) hence the coil would still rotate in its original direction.

2. Slip rings
Slip rings are used in generators. They act like "terminals" of a battery. During a period of time when the coil is being rotated, one slip ring (let's call this P) would act as the +ve terminal and the other slip ring (let's call it Q) would act as the -ve terminal. Then as the coil continues to rotate, P would act as the -ve terminal and Q would act as the +ve terminal. What kind of terminal the 2 slip rings would act can easily be determined using Lenz Law and finding the direction of current flow (a diagram would be much easier to explain this). This means the current is reversed after a certain period of time hence it also explains why generators only produce AC current (due to the reversal of current).

Thanks