(3) The parallel resistance of

5

ohms

is

added

to

resistor A in series with

the parallel combination

to yield 10 ohms total.

(4) Using Ohm's law, you can

now determine the voltage

drop

across

any

one

resistor in question.

Figure 8. Sample series-

7.

ELECTRICAL POWER

Electrical power is the time

rate at which work is done by an

electrical

device.

Of

greatest

concern in electrical circuits is the

power loss which occurs when heat

is generated by current flowing through a resistance.

The heat is usually

dissipated into the air and lost, but can be utilized as in the case of light bulbs

to produce light, soldering irons to produce heat, or filaments of electron tubes

to produce electron clouds.

a. The Watt. The watt is the unit of electrical power and is equivalent to 1

ampere of current at a pressure (EMF) of 1 volt. In mathematical terms, power is

equal to current times voltage.

By the use of this formula, the power in an

electrical device can be determined if the current and voltage are known.

Other

formulas can be used to determine power if current and resistance are known.

By

simple algebraic conversions of these formulas, the third (unknown) quantity can be

determined if two quantities are known.

b. Sample Problems.

In using the formulas for finding the total power and

the power loss of a circuit, certain items should be noted.

To find the total

power in a circuit, we use the formula P = EI.

Thus, to find the power of the

entire circuit, the total circuit current and total circuit voltage must be known.

On the other hand, a combination of the power formula with Ohm's law will give the

power in a particular part of the circuit or the entire circuit. This combination,

therefore, gives us the formulas P = I2R and P = E2/R for computing the power in a

circuit. If it is desired to compute the power loss in only one part of a circuit,

the current and the voltage drop in that part of the circuit must first be known.

It is important to notice that a small increase in current (I) can give a large

increase in power (P).

For example, doubling the current yields four times the

power.

P = power in the circuit in watts.

E = EMF supplied to the circuit in volts.

I = current flowing in the circuit in amperes.

R = resistance of the circuit in ohms.

Example 1:

What is the power in a 10-ohm circuit that is drawing 2 amperes

of current?

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