Power refers to the rate at which energy is used or converted from one form to another, measured in watts, where one watt equals one joule of energy per second. In electrical systems, the behavior of capacitors and inductors under alternating voltages is influenced by their capacitance or inductance values and the voltage frequency.
3) Circuits with multiple branches can be solved using a combination of Kirchhoff’s laws and Ohm’s law.
2) Metals such as copper and silver are good
conductors of electricity. Good conductors have
low resistance whilst poor conductors have high
resistance.
1) Current is the rate of fl ow of charge. Thus, if more
charge moves in a given time, more current will
be fl owing. If no charge moves then no current is
flowing.
key Point 1.3
2) In a primary cell the conversion of chemical energy to electrical energy is irreversible and so these cells cannot be recharged. In secondary cells, the conversion of chemical energy to electrical energy is reversible. Thus these cells can be recharged and reused many times.
1) Conventional current fl ows from positive to negative whilst electrons travel in the opposite direction, from negative to positive.
Key point 1.2
2) significant amount of charge can build up
between conducting surfaces when they are insulated from one another. Where this might be a problem steps are taken to dissipate the charge instead of allowing it to accumulate uncontrolled.
1) Charged bodies with the same polarity repel one
another whilst charges with opposite polarity will
attract one another.
Key point 1.1
2) Metals such as copper and silver are good conductors of electricity and they readily support the flow of electric current. Plastics, rubber and ceramic materials on the other hand are insulators and do not support the flow of electric current.
1) Electrons each carry a tiny amount of negative electrical charge.
Key Point 1.7
5) 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.
4) 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°.
3) When alternating voltages are applied to capacitors or inductors the magnitude of the current fl owing will depend upon the value of capacitance or
inductance and on the frequency of the voltage. In
effect, capacitors and inductors oppose the fl ow 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).
2) A good way of remembering leading and lagging
phase relationships is to recall the word CIVIL, as
shown in Fig. 1.42 . 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°.
1) The root mean square (r.m.s.) value of an alternating voltage will produce the same amount of
heat in a resistor as a direct voltage of the same
magnitude.
Key Point 1.6
7) The current induced in a conductor always opposes the change that produces it.
6) An e.m.f. is produced when the magnetic fl ux passing through an inductor changes.
5) 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.
4) Flux density is found by dividing the total fl ux
present by the area over which the fl ux acts.
3) 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.
2) Whenever an electric current fl ows in a conductor
a magnetic fi eld is set up in the space surrounding
the conductor. The fi eld spreads out around the
conductor in concentric circles with the greatest
density of magnetic fl ux nearest to the conductor.
1) A magnetic fi eld of fl ux is the region in which the
forces created by the magnet have infl uence. This
fi eld surrounds a magnet in all directions and is
concentrated at the north and south poles of the
magnet.
Key Point 1.5
2) 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.
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.
