magnetic field problem(urgent)

2011-05-07 1:12 am

For Q155,
1. What is meant by normal (CN)? Is it the supporting force or what? How is it determined by right hand grip rule? (which direction should the thumb point to and why)

2.The solution says the normal tends to point to B (the direction of magnetic field), why?

3. How is the coil going to turn? Does it turn anti-clockwise until N points to B with x-axis as its asix or in other direction?

For Q 157
4. How to calculate the momenton C2 ? Since the question doesn't give the direction of current of the coil, how can I know the direction of force acting on it when it is just released?

圖片參考：http://imgcld.yimg.com/8/n/HA01097219/o/701105060061113873434890.jpg

thanks

更新1:

D and A are the answers to Q 155 and 156 respectively.

回答 (1)

天同

2011-05-07 4:09 am

✔ 最佳答案

Q155
1. A coil carrying a current behaves like an electromagentic. You could use the "right hand grip rule" (curl the fingers along the current direction, the thumb points in the direction of the [north] pole) to find the direction of the magnetic field on the axis of the coil. The direction of "normal" to the coil is just the direction of the central magnetic field line.You may refer to the following web-page:

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

2. As said, the coil behaves like a magnet, with Noth and South poles. A magnet placed in a magnetic field tends to align with the field, i.e. to position with its axis along the magnetic field lines.

3. The coil will turn until its plane is perpendicular to the field lines. That is, the "normal" aligns with the field lines.

Q1574. Torque (or moment) T1 on C1 is,
T1 = 10 x 6 x (pi x 0.04^2) x Bsin(90)
Torque T2 on C2 is,
T2 = 5 x 3 x (pi x 0.02^2) x Bsin(30)hence , T1/T2 = 32: 1

No matter you use 30 degrees or (180-30) = 150 degrees in calculating T2 is of no importance. You would get the same answer, because only the magnitude of the torque is of interest.

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