BASIC ELECTRONICS - OD1633 - LESSON 1/TASK 2
exists but current does not flow because the circuit is open, no work is
done. This is similar to the spring under tension that produced no motion.
When voltage causes electrons to move, work is done. The instantaneous rate
at which work is done is called the electric power rate, and is measured in
watts.
A total amount of work may be done in different lengths of time.
For
example, a given number of electrons may be moved from one point to another
in one second or in one hour, depending on the rate at which they are moved.
In both cases, the total work done is the same. However, when the work is
done in a short time, the wattage, or instantaneous power rate, is greater
than when the same amount of work is done over a longer period of time.
As stated, the basic unit of power is the watt. Power, in watts, is equal
to the voltage across a circuit multiplied by current through the circuit.
This represents the rate at which work is being done at any given instant.
The symbol P indicates electrical power. Thus, the basic power formula is P
= E x I, where E is volts and I is current in the circuit. The amount of
power changes when either voltage or current, or both voltage and current,
are caused to change.
In practice, the only factors that can be changed are voltage and
resistance.
In explaining the different forms that formulas may take,
current is sometimes presented as a quantity that is changed. Remember, if
current is changed it is because either voltage or resistance has been
changed.
Figure 16 (on the following page) shows a basic circuit using a source of
power that can be varied from 0 to 8 volts, and a graph that indicates the
relationship between voltage and power.
The resistance of this
circuit is 2 Ohms; this value does not change.
Voltage (E) is increased
(by increasing the voltage source), in steps of 1
volt, from 0 volts to 8
volts. By applying Ohm's law, the current (I) is
determined for each step
of voltage. For instance, when E is 1 volt, the
current is:
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