Question 3 and 4 electromagnetism TBR

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dahmsom

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I'm confused on how the rod changes its area once it's inside the magnetic field. Can someone clarify this?


For number 4 Also since it's an electron and it the force is likely to point downward , i don't get how the direction is pointing counterclockwise . The Berkeley review says current is a flow of positive charge and I know electrons are negative . I'm just having a hard time seeing direction of currents

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3. Read 1st paragraph last line. You have to move the bar to the right to induce the emf and hence the current; hence, when you move the bar, while the vertical height remains constant the horizontal length increases, thereby increasing area (sidenote: emf is energy, but you can't get energy for free; you have to exert effort to turn your chemical energy in muscle to electric energy in the circuit).

4. Current is by definition positive (even though we intuitively know electrons not cations are flowing). B field point downward, but because area is increasing, the magnetic flux also points down; based on law of induction, this flux will be opposed by the induced current, so the induced current should create a hypothetical B field up. If you curl your fingers with thumb up, you'll get the CCW current.
 
@basophilic thanks for the response; but the passage says "dragging a conducting rod through a magnetic field induces an EMF between the ends of the bars. doesnt it mean that the area is changing vertically, not horiztonally :(? or am i misinterpreting the paragraph? but its still changing the area loll..


also im using my left hand for this cause its an electron.. i use my thumb to point the direction of the velocity, and i use my fingers to point where the magnetic field is going which is towards the page .. and my palm tells me where there current is flowing lol.. im having a hard time.. XD . i guess if i orient it straight it goes counter clockwise
 
In Fig. 2 you can see the v (velocity) pointing to the right which is why I said it's moving horizontally. But either way recognize that the area is increasing.

As for the hand rule (and if you really want to follow the electron), hold the circuit with left hand say at the top, curl fingers upward (direction of hypothetical B field caused by induced current), your thumb will point right - hence the electrons will flow clockwise. BUT current is by definition flow of POSITIVE CHARGE (make sure to understand this; don't wanna lose easy points), so you have to reverse current direction to CCW.
 
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