Kap FL 5 Questions (Warning, spoiler!)

[Sammy1024]

So i'm going through my practice test that I took and i'm a little confused by a question.

(The picture for the passage, basically where P and Q are is a magnetic field.

Looking at the picture, I wold imagine that R1 would affect the magnetic field, but the answer says all 3 would affect the magnetic field. I would imagine that since R2 and R3 come after the magnetic field, they wouldn't be affecting the magnetic field?

And then for this one, using the same image above, can someone tell me where to point my thumb/ fingers? I'm confused because the wires are in a loop on the right, and then P/Q seem in random noman's land area.

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[Cawolf]

For Item 8 - the resistance in the entire circuit will effect the current passing through the loop - which generates the magnetic field at point P. You need to take the entire circuit into account.

For Item 9 - when the switch is closed the current will travel through the loop in a clockwise direction.

The majority of the field at point P can be modeled using the "loop rule". Take your right hand and curl your fingers in the direction of the current - you would be pointing your thumb into the page to curl your fingers clockwise, parallel to the screen. Your thumb points in the direction of the magnetic field within the loop. So point P has a field into the page.

The field at Q can be visualized as the other side of the field at point P - so it will be in the opposite direction so the flux in zero. Or if you use the "wire rule" - point your right thumb along the wire closest to point Q (in the direction of the current, or downwards) and your fingers curl around the wire in the direction of the magnetic field. This shows how the current goes out of the page at point Q - and conversely, in at point P.

 

[Sammy1024]

Oh wow I get it! I was wondering how you knew to use the wire rule vs. the loop rule since it's a 2d image?

 

[Cawolf]

They are really the same thing.

The "loop rule" is just a superposition of all the little sections of wire. It is useful to see the current in the middle of a loop.

The field at point Q will likely be made up of the current passing closest to it - tangent to loop. So you can just use the "wire rule".

Being able to use many tools and tricks is helpful is visualizing how a field will look - and knowing that they always start and end at the same spot is useful for determining the direction. This is particularly helpful for problems where to they want to know how two current carrying shapes will interact.

 

[Sammy1024]

Was there something that helped you master the rules? I understand how to use the rule but when I was doing the question, I wouldn't figure out which direction the magnetic field would be going, which is so much clearer now.

 

[Cawolf]

Nothing other than just taking a good electricity and magnetism course and doing plenty of practice problems - especially conceptual ones where you practice with the right hand rules.

I have not started my MCAT studying quite yet, so I can't speak to how to incorporate it there.

 

[justadream]

@Cawolf

Do you know the formula that relates the amount of current flowing through the loop to the strength of the magnetic field produced?

I don't think I came across such a formula from TBR/TPR.

 

[Cawolf]

The Biot Savart Law

The derived version for the point in the center, on a plane, with a symmetric current carrying loop - such as in the example problem.

B = (mu not)(I)/2R

This is for the magnitude.

 

[justadream]

@Cawolf

lol that was way more complicated that I expected.

Is that the same as:

 

[Cawolf]

That is the magnetic field generated by current traveling through a wire.

 

[The Brown Knight]

Item 8: the resistors and loop are part of a single circuit, so TOTAL resistance will determine how much current flows in circuit. If you are a part of the current, you already know in advance what you will be encountering along the circuit you are going to travel in. And since the current determines the B field, the total resistance impacts the strength of resulting B field.

Item 9: put your thumb in direction of current and wrap the rest of your palm around the wire. Direction of rest of your fingers determine B field direction. Q is out of page because when you completely wrap your palm around the wire, you fingers curl UP in region outside circuit.

 

[justadream]

@Cawolf

Isn't that what we are looking at here?

 

[Czarcasm]

@Cawolf

Isn't that what we are looking at here?

I'm a little confused myself, haha.

 

[Cawolf]

@Cawolf

Isn't that what we are looking at here?

B = (mu not)(I)/2piR is the magnitude of magnetic field that travels circularly around and out along a long, straight wire.

B = (mu not)(I)/2R is the magnitude of the magnetic field that is essentially staight and perpendicular in the center of a current carrying loop.