Review Question

Electrostatics

Calculate the electrostatic force between two charges of +6 nC and +1 nC if they are separated by a distance of 2 mm.

ANSWER:

Fe=kQ1Q2r2=(9,0×109)(6×10−9)(1×10−9)(2×10−3)2=1,35×10−2 N

This force is repulsive since it is between two like charges.

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If vector F is the force between two charges q1 and q2, then what is the force between the charges 2q1 and -3q2 located at the same position as q1 and q2 respectively?

ANSWER:

Use Coulomb's law to write that the force F between two charges q1 and q2 is given by

F = k q1 q2 / r² u, where k is a constant, r is the distance between the charges q1 and q2 and u is a unit vector.

We now substitute q1 by 2q1 and q2 by by -3q2 in the above formula to obtain the new force F₂

F₂ = k (2q1)(-3q2) / r² u = -6 F

What is the work done on 5 electrons moving through a uniform electric field from a potential of 0 volts to a potential of 10 millivolts (mV). The movement is parallel to the electric field. ( charge of electron = - 1.6 × 10^-19 C)

ANSWER:

The potential difference V = 10 mV - 0 = 10 mV is the work done per unit charge. Each electron has a charge of - 1.6 × 10^-19 C. Hence the total work W done on the 5 electrons is given by

W = 5 × 10 mV × (- 1.6 × 10^-19 C); = - 8 × 10^-21 J

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Capacitors

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Electromagnetism

The primary of a transformer is connected to a source of voltage that has two components: an alternating current (AC) component of 120 volts and a steady direct current (DC) component of 5 volts. The number of turns of the primary is 300 and the secondary is 6000. What is the voltage at the output of the secondary?

ANSWER:

Transformers block steady Dc currents and let only AC current through and therefore the output voltage will have an AC component only. The relationship between secondary voltage Vs, the primary voltage Vp, the numbers of turns in the secondary coil Ns and the number of turns in the primary Np is

Vs / Vp = Ns / Np
Vs / 120 = 6000 / 300
gives Vs = 2400 v (AC only)

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