b. Circuits. To have a flow of current there must be a path, or conductor,

in which electrons can move. This conductor must be in the form of a closed loop

which provides a continuous path for the flow of current from the negative source

of EMF back to the positive side. This closed loop is known as a circuit. Current

flow will cease anytime the circuit is broken, but the electrical potential

difference will remain across the two points of the break.

In every electrical

circuit where there is movement of electrons, pressure, resistance, and current are

present and must be considered together to understand the circuit clearly. It must

be remembered that Ohm's law applies just as much to any particular part of a

circuit as it does to the entire circuit.

c. Application of Ohm's Law.

Figure 3 is a diagram of a simple series-

connected circuit.

A conductor connects a 12-volt battery, an ammeter, and a

resistor in this simple circuit. The ammeter is placed in series with the circuit

to measure current (I), and the voltmeter is placed in parallel with the battery to

measure voltage (E). The wire used to connect the various pieces of equipment is

considered to have negligible resistance in electrical problems unless a specific

value is given. The relationship of I, E, and R in figure 3 is illustrated in the

three examples that follow.

Figure 3.

Simple electrical circuit.

Example 1:

Find the current when

the

applied

voltage

is

12

volts

and

the

resistance is 3 ohms.

Example 2:

Find the resistance when the voltage is 12 volts and the current is 4

amperes.