1. Explain why it is necessary to align the plane of the coil with the magnetic
ID: 3163250 • Letter: 1
Question
1. Explain why it is necessary to align the plane of the coil with the magnetic field of the Earth as shown in Fig. 8. lb. 2. If you reverse the current through the coil, how would the deflection angle change? Explain. 3. If you plotted current vs. Theta (instead of current vs. tan 0) would you expect to get a straight line? Why or why not? 4. If the coil consisted of fewer turns (say 10 instead of 15), how would the value of K change? 5. If the coil consisted of fewer turns (say 10 instead of 15), would the value of Be be affected? Explain.Explanation / Answer
1) The plane of the coil must be aligned with the magnetic field of the earth. This is becaue only in such a position, there will be two mutually perpendicular magnetic forces Bc and Bh acting on the compass(coil of tangent galvanometer). So the relation between Bc and Bh can be given by the expression 8.3. Or else this will not be valid as the two magnetic forces are not mutually perpendicular and thereby tangent of the angle will not give their ratios.
2) When the angle of the coil is reversed, the Bc reverses its direction. The angle will therefore be -ve with respect to the positive x axis. However, the modulus of the angle will remain the same as before.
3) From equation 8.5, if is clear that the equation obeys the general form of a y = mx line. hence, a straight line curve is expected between I and tan(theta)
4)From equation 8.2, we see that K is directly proportional to N (the number of turns). So if N decreases, then K will also decrease and vice versa.
5) No, even if the number of turns of the coil is changed the value of Be, that is the horizontal component of earth's magnetic field will not be affected.
This is because with decreasing N, the current and thereby Magnetic field Bc will decrease. This affects the tan(theta) value. Also from the previous question we see that K will decrease. These two effects cancel each other out (as seen from equation 8.5).
However with decreasing tan(theta) value it might be practically very difficult to conduct the measeurement and hence it is advisable to use a large enough value of N.
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