32: Tactics Box 32.1 Right-Hand Rule for Fields

Figure 1

Figure 2

INTRO:
TACTICS BOX 32.1 Right-hand rule for fields

1. Point your right thumb in the direction of the current.(Figure 1)
2. Curl your fingers around the wire to indicate a circle.
3. Your fingers point in the direction of the magnetic field lines around the wire.

PART A: The following sketches show a wire carrying a current I in the direction indicated. Which sketch correctly shows the magnetic field lines around the wire?
a)b)c)

SOLUTION:
First off, we know that the magnetic field can't be going two different directions, such as it is in b), which rules out that answer. Next we must use the right hand rule to determine the direction it goes. Pointing our right thumb downward, in the direction thatI points, our fingers wrap counterclockwise around the bar. Since a) demonstrates this, we know that the correct answer is a). 

PART B: The magnetic field lines around a current-carrying wire are shown in the figure. In what direction is the current flowing in the wire? (Figure 2)

SOLUTION: For this one, pay special attention to which of the blue lines are in front of the diagram of the rod and which ones are behind, because this determines the direction of the magnetic field and also how to determine the direction of the current. So, that being said, place a right hand in the air and curl your fingers in the direction of the field, towards you as if you're looking at your fingernails. This results in the thumb pointing to the right. This means that this is the direction that the current is flowing in the wire. So the final answer for this part is 'from left to right'