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Question

# A ball of mass 1g and charge 10âˆ’8C moves from A, where potential is 600V, to B, where potential is 0. The velocity of the ball at B is 20cm/s. The velocity of the ball at point A must have been

A
16.8cm/s
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B
22.8cm/s
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C
228cm/s
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D
168cm/s
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Solution

## The correct option is B 22.8cm/sGiven a ball of m=1g=10−3kg charge on ball, q=10−8C potential at A;vA=600v potential at B;vB=0 velocity of ball at \$ B;{ v }_{ B }=20cm/swe have to find velocity of ball at A ex.vA=? so; we use conservation of energy,change in kinetic energy= change in potential energy.12m(vA2−vB2)=Q(vA−vB)12×10−3(vA2−(0.2)2)=10−8×(600−0)vA2−(0.2)2=6×10−6+2×103=12×10−3vA2=12×10−3+4×103vA=22.8cms−1 hence the answer is option (b)

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