Electrical Calculations - Basic Mathematics for Water and Wastewater Operators - page 136

E

I R

FIGURE 10.3 Ohm's law circle.

E E

I R

R I

E
I = R

E
R = I E = I × R

FIGURE 10.4 Putting Ohm's law circle to work.

120

E

R

40

FIGURE 10.5 Ohm's law illustration for Example 10.4.

■ Example 10.4
Problem: Find I when E = 120 V and R = 40 W.

Solution: Place finger on I as shown in Figure 10.5. Use Equation 10.1 to find the
unknown I :

E
R

120
40

I == = 3amperes

■ Example 10.5
Problem: Find R when E = 220 V and I = 10 A.

Solution: Place finger on R as shown in Figure 10.6. Use Equation 10.2 to find the
unknown R :

E
I

220
10

R == = 22 ohms