Current & R Hand rule

[arc5005]

TBR Physics II Diagnostic Practice Set 2.2 Question

Click pic above to see the passage:

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Question 1:
In experiment I, after the bar was released from the top of the rails (points e and b), what was true of the current produced as the bar slid down the rails?


A. no current is produced
B. direct current is produced that travels from X to Y
C. direct current is produced that travels from Y to X
D. alternating current is produced
B) direct current is produced that travels from X to Y






In experiment I, the external magnetic field points vertically upward, so the direction of induce current can be obtained using the right hand rule. Let's eliminate choices first. The bar slide down the rails, so there will be a current, eliminate A. Because the bar slides down the entire time, the current will remain in 1 direction, so eliminate D.

When the bar slides down the rails, the magnetic flux thru the loop created by the rails and the bar decreases, because the area inside of the loop decreases. Lenz's law states that a current will be induced in this loop to fight the change in flux.

Using R hand rule, point thumb in direction the bar is sliding (angular and down) and put your index finger in the direction of the magnetic field (up), then your middle finger points from X to Y, so that is the direction of the current.


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So i got this question correct, by assuming, that since the cyclic path was going from f-> a -> X -> y -> f, that current would flow in that direction...but that wasn't their explanation...


I don't think I'm following their guidelines for R hand rule... because earlier they teach that you
1) Point thumb in the direction of the velocity
2) point fingers in the direction of the magnetic field
3) your palm represents the direction of the force for a (+) particle.

so using this approach:

1) bar is sliding down so thumb is pointed down
2) magnetic field is pointing up (??? I can't point my fingers up if my thumb is down)
3) still not following their process...




Question 2:

In experiment I, releasing the bar from a higher point on the rails will result in all of the following except:

A. a greater induced current when the bar reaches the bottom of the rails
B. a greater kinetic energy when the bar reaches the bottom of the rails
C. an increase in the external magnetic field when the bar reaches the bottom of the rails
D. an increase in the induced magnetic field when the bar reaches the bottom of the rails



C) an increase in the external magnetic field when the bar reaches the bottom of the rails


In Experiment 1, if the bar was released from a higher point on the rails, it would have more KE. In addition, there would be a larger induced current and thus induced magnetic field due to the fact that there was a greater change in flux through the loop than if the bar had been let go from a lesser height. There is no relationship b/w external magnetic field and height of the bar.


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What is the relationship here? Is there a way to visualize this in terms of equations?

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[sapientnarwhal]

For experiment 1, the passage notes that a uniform B-field is oriented upwards perpendicular to base of apparatus. Initially, the B-flux is at a max. Once the bar is released, the out-of-the-page B-flux is now decreasing. According to Lenz's law, an induced current will flow to compensate for the change in magnetic flux.

to conceptualize this, do a thumbs up! Point your thumb in the direction of the current, and the CURL of your other 4 digits is the direction of the induced magnetic field. In this problem, we want the curl of our 4 fingers pointing upwards from the base of the device. The thumb will then be pointing in the direction of the current.

Experiment I is not concerned with the movement of a charged particle through a magnetic field, rather the behavior of a conducting loop when subjected to an external B-field.


For question 2, the external mag field is not influenced by the device at all. You need to consider each component. The KE is in reference to the mechanical energy of the bar, and is not related to the induced current or magnetic fields.

A simple way to approach this problem is by simplifying the diagram.

imagine a square loop in the plane of the page with a moveable bar as it's right edge. This loop is placed in the plane of the page with an external magnetic field pointing out of the page. When the moveable bar is moved to the left (area of the loop is now decreasing), less of the "magnetic field lines" are passing through the loop. Thus an induced emf drives current flow in a CCW direction such that a magnetic field is generated out of the page to compensate for the loss of magnetic flux.

 

[arc5005]

For experiment 1, the passage notes that a uniform B-field is oriented upwards perpendicular to base of apparatus. Initially, the B-flux is at a max. Once the bar is released, the out-of-the-page B-flux is now decreasing. According to Lenz's law, an induced current will flow to compensate for the change in magnetic flux.

to conceptualize this, do a thumbs up! Point your thumb in the direction of the current, and the CURL of your other 4 digits is the direction of the induced magnetic field. In this problem, we want the curl of our 4 fingers pointing upwards from the base of the device. The thumb will then be pointing in the direction of the current.

Experiment I is not concerned with the movement of a charged particle through a magnetic field, rather the behavior of a conducting loop when subjected to an external B-field.


For question 2, the external mag field is not influenced by the device at all. You need to consider each component. The KE is in reference to the mechanical energy of the bar, and is not related to the induced current or magnetic fields.

A simple way to approach this problem is by simplifying the diagram.

imagine a square loop in the plane of the page with a moveable bar as it's right edge. This loop is placed in the plane of the page with an external magnetic field pointing out of the page. When the moveable bar is moved to the left (area of the loop is now decreasing), less of the "magnetic field lines" are passing through the loop. Thus an induced emf drives current flow in a CCW direction such that a magnetic field is generated out of the page to compensate for the loss of magnetic flux.

thank you. that helps!