The image below shows a coil of wire immersed in an external magnetic field. The
ID: 1446880 • Letter: T
Question
The image below shows a coil of wire immersed in an external magnetic field. The wire is not attached to a battery nor any power supply that could create a current in it. The coil is attached to a meter. When the current flows in the coil the needle deflects. The needle deflects in opposite directions, depending on the direction of the current flow.
Part A
Which of the following statements are true? Check all that apply.
Which of the following statements are true? Check all that apply.
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When there is no current in the electromagnet, so that B = 0, the meter shows no current. When the electromagnet is turned on, there is momentary current through the meter as B increases. When B levels off at a steady value, the current drops to zero. With the coil in a horizontal plane, we squeeze it so as to decrease the cross-sectional area of the coil. The meter detects current only during the deformation, not before or after. When we increase the area to return the coil to its original shape, there is current in the opposite direction, but only while the area of the coil is changing. If we rotate the coil a few degrees about a horizontal axis, the meter detects current during the rotation, in the same direction as when we decreased the area. When we rotate the coil back, there is a current in the opposite direction during this rotation.Explanation / Answer
The true options are:
1. When there is no current in the electromagnet, so that B = 0, the meter shows no current. [ because current wpould be produced only if there is a flux change through the coil, faraday's law]
2. When the electromagnet is turned on, there is momentary current through the meter as B increases. [ because change in B would cause flux change]
3. When B levels off at a steady value, the current drops to zero. [ no flux change, so no emf produced]
4. With the coil in a horizontal plane, we squeeze it so as to decrease the cross-sectional area of the coil. The meter detects current only during the deformation, not before or after. When we increase the area to return the coil to its original shape, there is current in the opposite direction, but only while the area of the coil is changing. [ the flux is product of B and A, so any change in A also produces change in flux and hence the emf. Now increase in A is +ve and decrease in A is -ve, and hence both would have opposite direction of emf produced and hence different direction of current]
5. If we rotate the coil a few degrees about a horizontal axis, the meter detects current during the rotation, in the same direction as when we decreased the area. When we rotate the coil back, there is a current in the opposite direction during this rotation.
[ same as 4th]
Hence all of them are correct
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