Question on induced current..no clue what its saying please help?

Question

A long straight wire lies on a horizontal table and carries an ever increasing current northward. Two coils of wire lie flat on the table, one on either side of the wire. When viewed from above, the induced current circles:

a)counterclockwise in both coils
b)clockwise in both coils
c)clockwise in the east coil and counterclockwise in the west coil
d)clockwise in the west coil and counter clockwise in the east coil

WHAT??? i have no clue what this means! please help

Answer 1

Here you have to use the Right Hand Rule where the current and Magnetic field lines are perpendicular to each other. To do this you simply place your RIGHT thumb aligned with the middle current carrying wire. Your fingers will tell you the direction of the magnetic field lines, what you do is keep your thumb aligned with the middle line and then "curl" your fingers. If you do this, the the one to the east of the middle wire will have your fingers go through the middle, and the fingers will come out or away from the wire to the west.

Then using these directions as the Magnetic field lines, you now do the same Right Hand Rule, but now the fingers will represent the current and the thumb is aligned to each of the directions that you found for the Magnetic field lines. In other words, the East wire will have (using the Right hand rule again) the thumb pointing inwards, and the fingers go or curl CLOCKWISE - but the current goes COUNTERCLOCKWISE due to Lenz Law. In the west wire the thum points out of the plane of the wire and the fingers curl COUNTERCLOCKWISE, so the current actually goes CLOCKWISE, again according to Lenz Law. So the answer is D.

The key is knowing that you have to apply the RIGHT HAND RULE twice, and that the thumb can represent the current or the Magnetic field, and the fingers point in the direction of the other, and need to use Lenz Law correctly to get the right answer.

This is a very good question and I think you genuinely want to understand it - most people just get frustrated and give up. Keep the questions coming and ask for further clarification if you need more info.

Answer 2

Robert B suggests to use the right-hand rule to find the magnetic field caused by the wire, but comes to an incorrect answer. You must also use Faraday's Law and Lenz's Law which both tell us how a change in magnetic flux through a closed circuit will induce an emf and a current within that circuit.

We will look at just the coil to the East of the straight wire first. As the current in the wire increases the magnetic field also increases. By the right-hand rule the magnetic field is going down into the plane on the East and up out of the plane on the West. This increase in current causes an increase in flux down into the plane on the coil to the East of the wire. This flux induces an emf on the coil and must induce a current to oppose the change in magnetic flux (Lenz's Law). This means that a counter-clockwise current must be induced on the coil to the east.

If we look at the West coil, magnetic flux up out of the plane increases inducing an emf and a current that is in the clockwise direction by Lenz's Law.

d. Must be the correct answer

I would strongly suggest that you review your right-hand rule and Lenz's Law as these make excellent conceptual questions that should be relatively simple if you understand and can apply the material.

Answer 3

Right hand rule for direct current

If you grasp a long current-carrying wire with your right hand, holding your thumb extended in the direction of the current, then your fingers would curl round the wire in the direction of the magnetic field.

The answer is b)
clockwise in both coils