KEYPOINTS

KEYPOINTS

1. POWER

   •    1.     Power is the rate of using energy and a power of one watt corresponds to energy being used at the rate of one joule per second.   

2. POWER

   • Power is the rate at which energy is converted from one form to another. A power of one watt is equivalent to one joule of energy being converted every second.

3. A MAGNETIC FIELD OF FLUX

   • Is the region in which the forces created by the magnet have influence. This field surrounds a magnet in all directions and is concentrated at the north and south poles of the magnet.

4. ELECTRIC CURRENT

   • Whenever an electric current flows in a conductor a magnetic field is set up in the space surrounding the conductor. The field spreads out around the conductor in concentric circles with the greatest density of magnetic flux nearest to the conductor.

5. CURRENT-CARRYING CONDUCTOR

   • If we place a current-carrying conductor in a magnetic field, the conductor has a force exerted on it. If the conductor is free to move this force will produce motion.

6. FLUX DENSITY

   • Flux density is found by dividing the total flux present by the area over which the flux acts.

7. E.M.F (current)

   • The induced e.m.f. tends to oppose any change of current and because of this we often refer to it as a back e.m.f.

8. E.M.F (magnetic flux)

   • An e.m.f. is produced when the magnetic flux passing through an inductor changes.

9. CURRENT INDUCED

   • The current induced in a conductor always opposes the change that produces it.

10. R.M.S

    • The root mean square (r.m.s.) value of an alternating voltage will produce the same amount of heat I a resistor as a direct voltage of the same magnitude.

11. CIVIL

    • A good way of remembering leading and lagging phase relationships is to recall the word CIVIL. Note that, in the case of a circuit containing pure capacitance ( C) the current ( I ) will lead the voltage ( V ) by 90° whilst in the case of a circuit containing pure inductance ( L ) the voltage (V ) will lead the current ( I ) by 90°.

12. CAPACITORS OR INDUCTORS

    • When alternating voltages are applied to capacitors or inductors the magnitude of the current flowing will depend upon the value of capacitance or inductance and on the frequency of the voltage. In effect, capacitors and inductors oppose the flow of current in much the same way as a resistor. The important difference being that the effective resistance (or reactance) of the component varies with frequency (unlike the case of a conventional resistor where the magnitude of the current does not change with frequency).

13. RESISTANCE AND REACTANCE

    • Resistance and reactance combine together to make impedance. In other words, impedance is the resultant of combining resistance and reactance in the impedance triangle. Because of the quadrature relationship between voltage and current in a pure capacitor or inductor, the angle between resistance and reactance in the impedance triangle is always 90°

14. AC CIRCUIT

    • In an AC circuit the power factor is the ratio of true power to apparent power. The power factor is also the cosine of the phase angle between the supply current and supply voltage.