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# Biot-Savart Law watch

1. Hello,

I have been reading a book to see how the magnetic field due to a straight conductor is derived using Biot-Savart law. I have a little question from the attached (pdf) page. It looks from figure 30.3 that the angle theta is positive, so how can we take (highlighted text), irrespective of the fact that we have dx in the negative x-axis? Taking negative sign with x would give a negative value of theta, which seems quite wrong. I'm sort of confused. A little explanation would help.

Many thanks.
Attached Images
2. Physics for Scientists and Engineers, 6th Edition 929.pdf (109.0 KB, 114 views)
3. It's tan theta that is negative, not theta.
Because the tan of an angle is negative, that does not imply that the angle is negative.
Angles can be in different quadrants, giving positive or negative values of sin, cosine and tangent. The angle itself is positive.
For example, sin 90 = +1, sin 270 = -1
270 is not negative, is it? It's just in a different quadrant.
4. (Original post by Stonebridge)
It's tan theta that is negative, not theta.
Because the tan of an angle is negative, that does not imply that the angle is negative.
Angles can be in different quadrants, giving positive or negative values of sin, cosine and tangent. The angle itself is positive.
For example, sin 90 = +1, sin 270 = -1
270 is not negative, is it? It's just in a different quadrant.
Oops. Many thanks. Btw could you recommend me a good University Physics Book?
5. (Original post by Zishi)
Oops. Many thanks. Btw could you recommend me a good University Physics Book?
I think this question has been asked recently here.
Does a search bring it up?
6. (Original post by Stonebridge)
I think this question has been asked recently here.
Does a search bring it up?
Nope, the search brings up A Level books or books on specific topics (quantum mechanics) only.
7. (Original post by Zishi)
Nope, the search brings up A Level books or books on specific topics (quantum mechanics) only.
State whether you are asking for a general university physics text or a specific area such as mechanics.

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