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I thought I understood the right hand rule but I guess not. Here is how I learned it in regards to finding the direction of the magnetic field vector of a current carrying wire. Point your thumb in the direction of the current and curl your fingers around the current-carrying wire. I’m guessing the palm points in the direction of the magnetic field. I have two questions about the following problem:

  1. It seems like charge doesn’t matter in this problem. Why is that?

  2. The right hand rule is used twice to determine the direction of the magnetic field within and outside the loop. This confuses me because the book I have doesn’t explain this well at all. It never mentioned anything about inside and outside expect in this problem. How am I supposed to figure that out?

Example problem

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    $\begingroup$ It’s not your palm. The magnetic field curls around the wire just like your fingers. $\endgroup$
    – Ben51
    Commented Feb 15, 2021 at 22:35
  • $\begingroup$ Ok but how does that help me determine whether the magnetic field is into or out of the page? Also how would that help with knowing whether that direction is in the loop or out of the loop? $\endgroup$
    – Ibby
    Commented Feb 15, 2021 at 22:37
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    $\begingroup$ Magnetic field lines make circles around the wire. Think of your fingers as arrows pointing which direction the circulation of the B field is in. $\endgroup$
    – Ben51
    Commented Feb 15, 2021 at 22:39
  • $\begingroup$ I’m sorry but I still don’t get it. If the magnetic field points in the direction of circulation, which I’m assuming is the current, then isn’t this a no-brainer? Wouldn’t the magnetic field’s direction just be described as clockwise and the right hand rule be unnecessary? $\endgroup$
    – Ibby
    Commented Feb 15, 2021 at 22:58
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    $\begingroup$ The RHR is another way of saying that if you look down the length of a wire, with the current going away from you, the magnetic field is clockwise around the wire. $\endgroup$
    – Ben51
    Commented Feb 15, 2021 at 23:01

2 Answers 2

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You are given the direction of the current; current is by convention the motion of positively charged particles (negatively charged particles move in the direction opposite to the current).

Point your right hand thumb in the direction of the current; the magnetic field is given by curling your fingers in a circular motion (like turning a screw head). "Encircle" means "Curl" in the problem.

This approach is used twice because the current is up on the left side of the loop and down on the right side of the loop.

You also can use this approach at the top of the loop where the current is to the right. In all cases the magnetic field is into the page inside the loop and out of the page outside the loop.

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  • $\begingroup$ Thanks for the clarification but I’m unclear about one more thing: how I’m supposed to tell if the direction of the magnetic field is in or out of the loop? If I do it on the left side, I would see that the direction of the magnetic field is out of the page but how would I know that this is the direction of the magnetic field outside the loop? $\endgroup$
    – Ibby
    Commented Feb 16, 2021 at 1:24
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    $\begingroup$ With your right hand thumb in the direction of the current, if you curl (twist) your fingers , your fingers point in inside the loop and out outside the loop as you complete a 360 degree twist of your fingers, $\endgroup$
    – John Darby
    Commented Feb 16, 2021 at 1:42
  • $\begingroup$ That clears up the confusion I had. Thank you $\endgroup$
    – Ibby
    Commented Feb 16, 2021 at 2:32
  • $\begingroup$ You are welcome. $\endgroup$
    – John Darby
    Commented Feb 16, 2021 at 3:22
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The Right Hand Thumb Rule states that

If you imagine holding a conductor in your right hand with your thumb pointing out it in the direction of current, then the direction in which the fingers are curled gives the direction of magnetic field.

Think about it practically. Weren't the direction switch to clockwise and anticlockwise when the direction of your thumb becomes opposite?

And for the second question, let me tell you that the book says in the first line that "use the rule" and in the second line (or paragraph), it says "how to use it".

Hope this helps. Ask anything if not clear :)

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  • $\begingroup$ You explain this similar to how @Ben51 does but it just doesn’t make sense to me. I don’t see how my fingers are determining the direction of the magnetic field because they aren’t pointing into or out of the page. If I’m not mistaken the Xs are a 2D representation of the magnetic field pointing into the page and the points are a 3D representation of the magnetic field pointing out of the page. If I curl my fingers, aren’t they are pointing left or right not into or out of the page. $\endgroup$
    – Ibby
    Commented Feb 16, 2021 at 0:12
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    $\begingroup$ @Ibby: They are pointing in a circle with clockwise or anticlockwise direction. All you have to do is check the direction in and out the loop. Either the direction will make into the page (inside the loop) and out of the page (outside the loop) or vice versa, which basically forms a loop, or circle. $\endgroup$
    – ultralegend5385
    Commented Feb 16, 2021 at 1:36
  • $\begingroup$ Got it. Thanks for clarifying $\endgroup$
    – Ibby
    Commented Feb 16, 2021 at 2:31

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