Q. 495.0( 3 Votes )

A ball of mass 100g and having a charge of 4.9 × 10–5 C is released from rest in a region where a horizontal electric field of 2.0 × 104 C NC–1 exists.

(a) Find the resultant force acing on the ball.

(b) What will be the path of the ball?

(c) Where will the ball be at the end of 2s?

Answer :

Mass of the ball : m = 100 g = 100× 10-3 kg =0.1 kg
Charge on the ball : q = 4.9 × 10–5 C
Horizontal electric field : E= 2.0 × 104 C NC–1
Initial Velocity of the ball: u = 0

Here, R is the resultant Force due to gravitational force Fg and electric force Fe.
Formula used:
Electric force Fe due to charge q and electric field E is
Fe = qE

Gravitational force Fg experience due to mass of the ball m and acceleration due to gravity g is:

Thus we see that Fe = Fg
The Resultant force R can be calculated by:

R2= (0.98)2+(0.98)2


R=1.3859 N
Hence the resultant force of 1.3859 N is acting on the ball.
We take tangent of the angle θ

But Fg= Fe,

Hence, the path of the ball is along a straight line and inclined at an angle of 45° with the horizontal electric field.
Here, we will be using one of the equations of motion.
Here s is the distance covered by the ball, u is the initial velocity of the ball, a is the acceleration of the ball and t is the time required to cover s.
We need to find s at t=2s
Firstly ,vertical displacement due to gravitational force is:a=g

Horizontal displacement due to electric force is:

Thus net displacement = (sv2+ sh2)1/2
Net displacement = ((19.6)2+(19.62))1/2
Net displacement = 27.71 m

Thus, the ball will be at a distance of 27.71m after 2s

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