From Electrical Equipment Handbook: Troubleshooting and Maintenance

FUNDAMENTALS OF ELECTRIC SYSTEMS (CHAP. 1)

1.

Why does the current lead the voltage in a capacitor?

2.

What are the right-hand rules associated with the magnetic field?

3.

Explain Faraday's law.

4.

Explain the power factor.

Answers

1.

A capacitor is an energy storage device. It stores energy in an electric field. The current goes through the capacitor before the voltage can be established across it. Thus, the voltage must lag the current. The relationship between the current and the voltage in the capacitor is given by

where i is the current, V is the voltage, and C is the capacitor.

The voltage is normally given by

where V m is the maximum value of the voltage and ? = 2 ? f( f is the frequency, measured in hertz).

Thus,

where cos ? t is 90 ahead of (leading) sin ? t. Therefore, the current in the capacitor leads the voltage by 90 .

2.

There are two right-hand rules associated with magnetic fields. The first deals with the force exerted on a charge moving through a magnetic field. The force is given by

where F is the force, q 0 is a positive charge, V is the velocity of the charge, and B is the magnetic field.

The force F will always be at a right angle to the plane formed by V and B. The right-hand rule will specify the orientation...

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