Voltage, Current, and Resistance

Every electrical circuit has current, voltage, and resis- tance. The movement of electrons along a wire or con- ductor is referred to as current.

Voltage is the electrical pressure that pushes elec- trons through a resistance. Voltage is measured in volts (V). Electrical pressure, electromotive force (EMF), difference of potential, and voltage are all terms used to designate the difference in electrical pressure or poten- tial. For example, a battery is a common power source. It furnishes the energy needed to cause electrical devices to function. A battery has a difference of potential be- tween its terminals. This difference of potential is called voltage.

Current is the flow of electrons. Current is mea- sured in amperes (A). A coulomb is 6.28 ´ 1018  elec- trons. When a coulomb is standing still, or static, it is referred to as static electricity. Once the coulomb is in motion, it is referred to as current electricity. The movement of 1 C past a given point in 1 s is 1 A. At times, it is necessary to refer to smaller units of ampere. The milli-ampere is one-thousandth of an am- pere (0.001 A or 1 mA). A microampere is one-millionth of an ampere (0.000001A or 1 mA). These smaller units are commonly used in working with transistorized circuits.

Resistance is the opposition offered to the passage of electrical current. Resistance is measured in ohms (W). The ohm is the amount of opposition presented by a substance when a pressure of 1 V is applied and 1 A of current flows through it.

OHM’S LAW

Ohm’s law states the relationship among the three factors of an electrical circuit. A circuit is a path for the flow of electrons from one side of a power source or potential difference to the other side. See Fig. 3-1.

Ohm’s law states that the current (I) in a circuit is equal to the voltage (E) divided by the resis- tance (R). Ohm’s law is expressed by the following three formulas:

Fig. 3-1   A simple circuit.

I = E

R

R = E

I

E = I ´ R

The best way to become familiar with Ohm’s law is to do a few problems. If two of the factors or quanti- ties are known, it is easy to find the unknown. Since the size of the wire used in a circuit is determined by the amount of current it is to handle, it is necessary to find the current and check a chart to see what size the wire should be. See Tables 3-1 and 3-2.

Problem: The voltage available is 120 V. The resis- tance of the circuit is 60 W. What is the current? What size of wire will handle this amount of current? See Fig. 3-2 and Tables 3-1 and 3-2.

I = E

R

I = 120

60

I = 2A

Now that you know the amount of current: 2 A refer to Table 3-2 to find the size of wire that would be used to handle 2 A. The table says that a No. 1 8 wire handles 2.32 A. Thus, there is a safety factor of 0.32 A, or 320 mA.

SERIES CIRCUITS

A series circuit consists of two or more consuming devices connected with one terminal after the other. Figure 3-3 shows that the current through the circuit is the same in all devices. However, the total resistance is

Table 3-1   Current-Carrying Ability of Copper Wire with Different Types of Insulation Coating

*Types “RHW,” “THW,” “TW,” or “R” are identified by markings on outer cover.

Actual size of copper conductors. Note the larger the gage number, the smaller the diameter of the wire.

Table 3-2   Wire Size and Current-Carrying Capacity

Wire Size                                  Current-Carrying

A.W.G. (B & S)                        Capacity at 700 cm/A

 found by adding the resistances. Thus, RT = R1 + R2 + R3 + ….Therefore, if a resistance of 1 W and a resistance of 4 W are connected in series, the total resistance is 5 W. To find the total current, divide the total resistance into the voltage (in this case 10 V). That gives Ohm’s law another use—finding the total current in the circuit.

Fig. 3-2   A circuit with one resistor.

Fig. 3-3   A series circuit with two bulbs. 

Since the total current in a series circuit is the current through each resistance, the individual light bulbs will have the same current through them. Or,

I = 10 V = 2 A 5 W

There are three basic laws regarding series circuits:

• Current is the same in all parts of the
• Voltage drop across each resistance varies according to the resistance of the individual device.
• Resistance is added to equal the Or, RT  = R1 +

R2 + R3 +×××

Another example of how series circuit laws and Ohm’s law can be of assistance follows:

In a circuit with 120 V and a current of 5 A, what is the resistance?

R = E

I

R = 120

5

R = 24 W

Fig. 3-4   A parallel circuit.

There  are  three  basic  laws  regarding  parallel circuits:

1. The voltage is the same across each
2. The current divides according to the
3. There are two ways of finding total This formula can be used for only two resistors:

In a circuit with 20 A and 40 W, what is the voltage needed for normal operation?

This formula can be used for any number of resistors:

Suppose you have a series circuit for which you know the voltage (120 V), the current (4 A), and the resistance of one of the two resistors (20 W). How do you find the value of the other resistor in the circuit?

Use Ohm’s law and the laws of a series circuit:

Current in a Parallel Circuit

The current divides according to the resistance. For example:

R1 = 10 W

R2 = 20 W

R = E

I

R = 120

4

R = 30 W

Subtract the known resistance of 20 W from the total of

R3 = 30 W

If the voltage is 60 V, then the following method is used to determine the current through each resistor.

Voltage across each resistor is the same (60 V). Therefore, the resistance and the voltage are known. Use Ohm’s law and find the current through each resistor:

30 W. This gives 10 W for the missing resistor value.

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