Question on directions of currents and magnetic fields

[123yt]

How do I find the direction of these things? The right hand rule only works when I have the current and it's asking for clockwise/counter-clockwise, and the left hand rule only works when I have two of the three: Current, Magnetic Field, and Force.

Example:
[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/hrkc30f43.gif[/URL]

How do I find the direction of the Magnetic Field?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/31-33.jpg[/URL]

The magnetic flux going through this loop is increasing. How do I find the direction of the current?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/31-38.jpg[/URL]

The magnetic flux going through this loop is decreasing. How do I find the direction of the current?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/hr31p17.gif[/URL]

How do I find the direction of the Magnetic Field?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/31-43.jpg[/URL]

Magnetic Field is going out of the page, and is increasing. How do I find the direction of the current?

 

[Bill_K]

Fingers = current
Thumb = magnetic field

Given current, you point your fingers and see which way your thumb goes.
Given field, you point your thumb and see which way your fingers go!

 

[ZapperZ]

How do I find the direction of these things? The right hand rule only works when I have the current and it's asking for clockwise/counter-clockwise, and the left hand rule only works when I have two of the three: Current, Magnetic Field, and Force.

Example:
[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/hrkc30f43.gif[/URL]

How do I find the direction of the Magnetic Field?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/31-33.jpg[/URL]

The magnetic flux going through this loop is increasing. How do I find the direction of the current?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/31-38.jpg[/URL]

The magnetic flux going through this loop is decreasing. How do I find the direction of the current?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/hr31p17.gif[/URL]

How do I find the direction of the Magnetic Field?

[URL]http://homework.phyast.pitt.edu/res/sc/gblanpied/courses/usclib/hrw8/hrwpictures/31-43.jpg[/URL]

Magnetic Field is going out of the page, and is increasing. How do I find the direction of the current?

The problem here is that you did not reveal why and where exactly you are having trouble solving this. For example, how did you apply the right hand rule to the first situation? We don't know if you simply have zero clue on the application of the right-hand rule (this means you have a more fundamental problem than just these problems), or if you simply used it wrongly!

Try looking at this page, and see if you are able to duplicate the answer based on the instruction on how to use the right-hand rule:

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magfor.html

Zz.

 

[123yt]

I did. Every single source I've read tells me I need two of the three vectors to find the third.

The first one only gives me current, the second one doesn't give me anything, I just realized I can actually figure out the third one, the fourth one only gives me current, and the fifth one only gives me magnetic field.

 

[ZapperZ]

I did. Every single source I've read tells me I need two of the three vectors to find the third.

The first one only gives me current, the second one doesn't give me anything, I just realized I can actually figure out the third one, the fourth one only gives me current, and the fifth one only gives me magnetic field.

This makes no sense. First of all this is your original post, with MY ADDITION of the number of problems:

How do I find the direction of these things? The right hand rule only works when I have the current and it's asking for clockwise/counter-clockwise, and the left hand rule only works when I have two of the three: Current, Magnetic Field, and Force.

Example:


How do I find the direction of the Magnetic Field?



1 The magnetic flux going through this loop is increasing. How do I find the direction of the current?



2 The magnetic flux going through this loop is decreasing. How do I find the direction of the current?



How do I find the direction of the Magnetic Field?



3 Magnetic Field is going out of the page, and is increasing. How do I find the direction of the current?

So, what "fourth one"?

Secondly, why is it difficult to point your right thumb in the direction of the magnetic field, and the look at how your fingers curl?

However, I suspect something else is going on here and there are other parts to this question that we're not seeing. This is because of the changing magnetic field, and you are probably being asked for the INDUCED current and the application of Lenz's Law.

Zz.

 

[123yt]

5 pictures, 5 problems. Fourth one is the fourth picture.

Secondly, why is it difficult to point your right thumb in the direction of the magnetic field, and the look at how your fingers curl?

My book says I'm supposed to point my thumb in the direction of the current and look at how the fingers curl for magnetic field instead.

Using what you just told me, I can get the solution to the fifth problem, but it doesn't help me with the others. The second problem doesn't even give me the direction for anything.

However, I suspect something else is going on here and there are other parts to this question that we're not seeing. This is because of the changing magnetic field, and you are probably being asked for the INDUCED current and the application of Lenz's Law.

Yes, they're all asking for induced current, so if the flux is increasing, the induced current would be in the opposite direction as whatever I get for the right-hand rule. But I need to get this right-hand-rule problem settled first.

I posted what I thought was relevant to finding the direction, but here's the full information for the problems if you want them. I already know how to find the numbers, but I can't figure out the direction.

1: A long hairpin is formed by bending a very long wire, as shown. If the wire caries a current of 18 A, what is the magnitude and direction of the magnetic field at the point a? Take R = 12 mm. Take the sign of B to be positive if the direction is out of the paper and negative if B is into the paper.

What is the magnitude and direction of the magnetic field at the midpoint b? The distance between a and b are much larger than R (each straight section is "infinite"). Take the sign of B to be positive if the direction is out of the paper and negative if B is into the paper.

2. The magnetic flux through the loop shown in the figure increases according to the relation

φB = 5.0 t^2 + 8.0 t

where φB is in milliwebers and t is in seconds.

What is the flux at t = 4.0 s? (in webers = Wb)
What is the magnitude of the emf induced in the loop at t = 4.0 s?
What is the direction of the current, clockwise or counterclockwise?

3. A square wire with 2 m sides is perpendicular to a uniform magnetic field, with half the area of the loop in the field as shown in the figure. The loop contains a 20 V battery with negligible resistance. If the magnitude of the field varies with time according to

B = 1.2910 - 0.9000 t

with B in teslas and t in seconds, what is the total emf in the circuit?

What is the direction of the current through the battery?

4. A small circular loop of area 5.0 cm2 is placed in the plane of, and concentric with, a large circular loop of radius 1.0 m. The current in the large loop is changed uniformly from 200.0 A, counterclockwise, to -200.0 A, clockwise, in a time of 1.0 s, beginning at t = 0.0 s.

What is the magnetic field at the center of the small circular loop due to the current in the large loop at t = 0.0 s? Take B as positive if out of the paper.

What is the magnetic field at the center of the small circular loop due to the current in the large loop at t = 0.5 s? Take B as positive if out of the paper.

What is the magnetic field at the center of the small circular loop due to the current in the large loop at t = 1.0 s? Take B as positive if out of the paper.

What emf is induced in the small loop at t = 0.5 s? Take emf as positive if counterclockwise. Note that since the inner loop is small, you can assume that the B field due to the outer loop is uniform over the area of the smaller loop.

5. In the figure the square loop of wire has sides of length 2 cm. A magnetic field points out of the page; its magnitude is given by

B = 4.0 t^2y

where B is in teslas, t is in seconds and y is in meters. Determine the emf around the square at t = 1.50 s.

What is its direction? (clockwise or counterclockwise)