# A pendulum bob of mass 80 mg and carrying a charge of 2X10 -8 C is at rest in a uniform, horizontal electric field of 20 kVm -1 . Find the tension in the thread.

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by NutspradhaPa

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by NutspradhaPa

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See diagram.

From the equilibrium of concurrent forces,

T / Sin 90 = q * E / Sin (180-α) = m g / Sin(90+α)

or, T = q * E / Sin α = m g / Cos α

tan α = q E / mg

= 2 * 10⁻⁸ * 20 * 10³ / [80 * 10⁻³ * 9.81 ] = 0.000509

α = 0.292 deg.

T = mg / cosα = 0.080 * 9.81 / 0.999987 = 0.7846 Newtons

From the equilibrium of concurrent forces,

T / Sin 90 = q * E / Sin (180-α) = m g / Sin(90+α)

or, T = q * E / Sin α = m g / Cos α

tan α = q E / mg

= 2 * 10⁻⁸ * 20 * 10³ / [80 * 10⁻³ * 9.81 ] = 0.000509

α = 0.292 deg.

T = mg / cosα = 0.080 * 9.81 / 0.999987 = 0.7846 Newtons