Fundamental Engineering Concepts - Mathematics and Statistics for the Environment - page 259

E

I

R

FIGURE 11.22 Ohm's law circle.

E E

I

R I R

E E

I = R =

E = I × R

R I

FIGURE 11.23 Putting the Ohm's law circle to work.

-

I = 0.5 A

Light bulb
R = ?

V = 120 volts

+

FIGURE 11.24 Simple circuit.

Solution: The first step in solving a circuit problem is to sketch a schematic diagram of the circuit
itself, labeling each of the parts and showing the known values (see Figure 11.24). Because I and E
are known, we can use Equation 11.20 to solve for R :

E
I

120
0.

R == = 240 ohms

11.7.4 e leCtriCal p oWer
Power , whether electrical or mechanical, pertains to the rate at which work is being done, so the
power consumption in a plant is related to current flow. A large electric motor or air dryer consumes
more power (and draws more current) in a given length of time than, for example, an indicating light
on a motor controller. Work is done whenever a force causes motion. If a mechanical force is used
to lift or move a weight, work is done; however, force exerted without causing motion, such as the
force of a compressed spring acting between two fixed objects, does not constitute work.

Note: Power is the rate at which work is done.

11.7.4.1 Electrical Power Calculations
The electric power ( P ) used in any part of a circuit is equal to the voltage ( E ) across that part of the
circuit multiplied by the current ( I ) in that part. In equation form: