Q. 464.9( 7 Votes )

A pendulum bob of mass 80 mg and carrying a charge of 2 × 10–8 C is at rest in a uniform, horizontal electric field of 20 kVm–1. Find the tension in the thread.

Answer :

Mass of the bob: m = 80 mg= 80× 10-6 kg
Charge on the bob: q = 2 × 10–8 C
Electric field : E = 20 kVm–1 = 20× 103 Vm-1.

Here the electric field is horizontal.
The tension in the string is resolved into two components:
Horizontal component : Tsinθ
Vertical component : Tcosθ
As shown in the figure.
Formula Used:
Since electric field acts on the bob having charge q, it experiences an electric field in horizontal direction.
Electric force is given as:
F is electric force, q is the charge on the bob and E is the horizontal electric field. This electric force is balanced by the horizontal component of the tension.
Tsinθ = qE
Now the vertical component of tension is balanced by the weight of the bob.
Dividing we get,

Substituting the value of θ , we can calculate tension in the string.

Hence the tension in the string is 8.8×10-4 N.

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