Page 135 - Physics - XII

PROJECT - 6

Aim

To estimate the charge induced on each one of the two identical styrofoam (or pith) balls suspended in a
vertical plane by making use of Coulomb’s law.

Apparatus and Materials Required
Two small sized identical styrofoam balls, physical balance or electronic balance, metre scale, cotton
thread, glass rod, silk/wood cloth, and clamp stand/rigid support

Principle/Theory
Suppose the point charge Q and Q are separated by a distance r in vacuum. According to Coulomb’s law,
2
1
the magnitude of Force (F) between them is given by
1 | QQ |
F = 1 2
4 r 2
0
In Fig. 6.1, the weight of each ball is Rigia support
W = mg O
The restoring force on each ball = mg sin θ l θ θ l
a T
From ∆OBC, sin θ = 2 = a a T
l 2l 2 q F
Let the charge on each ball is be q A B
Q = Q = Q B
2
1
At equilibrium, a θ mg cos θ
1 QQ 2 mg sin θ
mg sin θ = a
4 a 2 mg mg
0
a 1 Q 2 Fig. 6.1: Determination of charge
mg× = induced on each styrofoam ball
2l 4 a 2
0
2 mga 12/
3
So, Q = 0
l
where ε = Permittivity of free space
0
= 8.85 × 10 C /N–m 2
–12
2
By substituting the value of m, a, and l, we can fi nd Q.
Procedure
1. Measure the mass of each ball using physical balance.
2. Now, tie the ball with two cotton threads suspended at a point on a stand or a rigid support.
3. Measure the length of threads by using a metre scale and ensure that it should be equal.

4. Now, rub a glass rod with silk/cloth.
5. Touch the charged glass rod to both the balls together so the balls acquire equal charge.
6. The balls suspended freely from a rigid support stay a certain distance apart due to repulsive force
acting between them.

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