MCAT magnetism

[Mr. Z]

Thought I would post some things to know about magnetism. Feel free to add more...

1. Magnetic Force=qvBsin theta

2. Magnetic fields are caused by moving charges

3. Know how to use the right hand rule. Point your right
thumb in the direction that positive charge moves (opposite
electron flow) and point your fingers in the direction of the
magnetic field, the force caused by the field will be the
direction your palm faces.

4. The force due to a magnetic field is always perpendicular to the
velocity of the moving charge and the magnetic field.

5. The force of the magnetic field does no work , its
perpendicular to the velocity.

6. qvB = m v^2/r Magnetic force acts as a centripetal force.

7. a magnetic field in flux will generate an electric field.

I hate electricity and magnetism

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[limit]

You pretty much nailed them all. The only things I'd add would be to #3, use left hand rule for negative charge, not to get confused with reversing the direction of the force.

Also, it might help to know that the Earth's magnetic fields point north (unlike a magnet).

 

[Mr. Z]

Left hand rule?

 

[lady bug]

I only know of right hand rules when you point your thumb in the direction of the current....never heard of a left hand rule.

 

[missbonnie]

The good news is that any questions on Magnetism on the mcat are relatively easy..you don't need to know much.



-bonnie

 

[SuzyQ]

I just use the right hand role no matter the sign on the charge. If its positive then the force will be in the direction of your palm...and if the charge is negative then the force will be in the directino of the back of you hand.....switching hands would just confuse me!

 

[SuzyQ]

I know this has nothing to do with Magnetism and such....but does anyone understand torque really well? The whole concept just confuses me for some reason....the only thing I can remember about it is Torque = rFsin theta.....Any other important things that I should know?

 

[Lavndrrose]

Adding to the list, correct me if I'm wrong:

8. Opposite poles attract, like poles repel.

9. The electric field created by a changing magnetic field is non-conservative. Therefore, the induced electric field has no electric potential energy and its energy is dissipated as heat.

 

[freakazoid]

Could you explain number 9? With examples, hopefully . . .

 

[chandler742]

Z, I think you nailed it.

I would add one minor thing. That is that when a magnetic field is drawn the lines move from the "north" pole to the "south" pole. Also, line density is proportional to B strength.

Similarly, for an electric field line density is proportional to electric field strength. And by convention the electric field lines move from a + point charge to a negative charge.

 

[Mudd]

Awesome post Z!!!

Just to add to the mix, solenoids were on the MCAT a few times over the past few years. You should know that the B field depends on the coils and the current.

Also, a charged particle moving in a circular path generates a linear B field... this concept tripped people up on the solenoid.

Also, rail guns (electrical trains, etc...) have made their way onto quite a few MCAT exams. It is helpful that you know that F is propotional to current squared.

I wish everyone luck staying sane the next week and nailing this exam.

 

[Mr. Z]

Mudd, what exactly is a solenoid? could you describe one and kinda tie it in to your last post? any help would be appreciated, thanks

 

[Lavndrrose]

Freak,

Work done by an induced electric field is not conserved, therefore it is lost to thermal energy. But I don't know if you need to know this for the MCAT though.

 

[Mudd]

Basically, it's an electromagnet. Physically, it's a coiled loop of wire where the length of the solenoid is greater than the diameter, resulting in a fairly linear B field when current is passed through.

If it is AC, then the B field continually reverses and the solenoid can rotate to align with an external field. By repeatedly doing this, it rotates at a set frequency and thus is the mechanism behind an electric motor.

If it is DC, you can build an adjustable magnet, where the B strength can be altered by changing the current. A typical application is in NMR machines.

 

[Lavndrrose]

I think if a solenoid were to come up, it would be explained in a passage.

 

[Mudd]

Originally posted by Lavndrrose
I think if a solenoid were to come up, it would be explained in a passage.

Exactly! As far as I know, it has been on the exam twice. Once in its electric motor context and another time in MRI machines. The question centered around the dimensions and how far the person's head needed to go into the coil to get the best reading. They showed a picture and gave an explanation. Many of their passages are on unfamiliar topics, but they explain them and have generally easy questions. It's a great way to test your analytical skills.

 

[chandler742]

Mudd, i know you have good intentions. However, I must warn people. Solenoid and electric motors are beyond the scope for the MCAT.

For electric motor, it should suffice it is the conversion of electrical energy to mechanical energy. An engine is the conversion of thermal energy to mechanical energy.

 

[DrBravesgirl]

Originally posted by Mr. Z
Mudd, what exactly is a solenoid? could you describe one and kinda tie it in to your last post? any help would be appreciated, thanks

this year in physics lab, we had an experiment that involved a solenoid...we used a slinky. that's always how i'm going to remember, lol

 

[Anthropic]

Found this old thread. It has some good info.

 

[DieselPetrolGrl]

is that true about the Earths field being S->N whereas most fields are N->s?
can you clarify the direction of a magnetic field
all i an remember is electric is from + to -

 

[ifailedmcat]

is that true about the Earths field being S->N whereas most fields are N->s?
can you clarify the direction of a magnetic field
all i an remember is electric is from + to -

Yes the earth's field is actually backwards. We originally gave the north pole its name because the north pole of a magnet on a compass always pointed up. But remember opposites attract...so our north pole is actually a south pole for this law to be true and vice versa. And magnetic field lines are always drawn N to S.

 

[PRamos]

My exam had a passage on induced magnetic fields within a loop. It was explained in the passage, but had I reviewed it better, I think it would have been easier. Some of the people taking the exam with me were talking about an MRI passage, so it shows up.

For anyone who's been at this site for a while, are there any other threads like this that discuss topics? Are MrZ and Mudd SDN on-line tutors? Can you search for topics?

 

[N-toxicologist]

PRamos,

You can search for topics, but I don't know how efficient the search function is. About daily there's somebody asking a question, and of course, the 20 subsequent posts discussing it. If you don't turn up anything in the searches, you can always post your own question.

 

[tank you]

this is a great thread.
does anyone have anything to add to it?

 

[drguy22]

i think we should do one for every topic!...im gonna start wit bio lets see if the trend continues.

 

[MacGyver]

Once in its electric motor context and another time in MRI machines. The question centered around the dimensions and how far the person's head needed to go into the coil to get the best reading. They showed a picture and gave an explanation.

This doesnt make any sense; in an MR scanner you want the receive coil to be as close as possible to the samle object.