There is a definite relationship between the three primary electrical characteristics: current, voltage and resistance. A German mathematician, George Simon Ohm, formulated this relationship in the 19th century. His law (Ohm's Law) stated that current is directly proportional to voltage and inversely proportional to resistance. The following formula was derived from that law:
Current = Voltage/Resistance or I = E/R
Current (I) in amps: Voltage (E) in volts: Resistance (R) in ohms
Figure 7. Ohm's Law
Ohm's Law is the basic formula used in all AC and DC electrical circuits. So if you know two of the three characteristics, you can calculate the third one.
Electrical designers use it to determine how much voltage is required for a certain load, like a motor, a computer, or even a house full of appliances.
DC Circuits
We can use a simple DC circuit here to demonstrate Ohm's Law. Before we do any calculations, however, let's briefly discuss the symbols that will be used in our circuit diagrams.
Voltage Symbol: The terminals of a battery are symbolically indicated on an electrical drawing by one or more pairs of lines. The longer line represents the positive terminal, and the shorter line the negative terminal.
Current = Voltage/Resistance or I = E/R
Current (I) in amps: Voltage (E) in volts: Resistance (R) in ohms
Figure 7. Ohm's Law
Ohm's Law is the basic formula used in all AC and DC electrical circuits. So if you know two of the three characteristics, you can calculate the third one.
Electrical designers use it to determine how much voltage is required for a certain load, like a motor, a computer, or even a house full of appliances.
DC Circuits
We can use a simple DC circuit here to demonstrate Ohm's Law. Before we do any calculations, however, let's briefly discuss the symbols that will be used in our circuit diagrams.
Voltage Symbol: The terminals of a battery are symbolically indicated on an electrical drawing by one or more pairs of lines. The longer line represents the positive terminal, and the shorter line the negative terminal.
Resistance Symbol: Resistance is represented in one of two ways: either an open rectangle or a zigzag line. Resistance in a circuit can take the form of many different components from light bulbs to motors. Most of these components have their own unique
Figure 9. Resistance Symbol (Resistor)symbols. For now, we will use the zigzag line symbol to represent the loads.
Series CircuitUsing the simple circuit shown, assume that the voltage supplied is 12 volts, and the resistor provides six ohms of resistance. To determine the current, use the following Series Circuit
Using the simple circuit shown, assume that the voltage supplied is 12 volts, and the resistor provides six ohms of resistance. To determine the current, use the following formula.
Using the simple circuit shown, assume that the voltage supplied is 12 volts, and the resistor provides six ohms of resistance. To determine the current, use the following formula.
E Voltage (volts) =I R
Another example of a simple DC circuit is a flashlight. Batteries in the flashlight provide the DC voltage source, the inside of the battery case usually acts as the conductor, and the lamp bulb is the load.
The flashlight has an ON and OFF switch which controls the flow of electricity. Because there must always be a complete path for current to flow, the switch stops the flow when it is in the OFF position. Why? Because the circuit is open when the switch is OFF. When the switch is ON, the circuit is complete and current flows, lighting the bulb.
The simple circuits above are called Series Circuits, which means all loads are connected one after another in a series. If a conductor or a load is broken, it opens the circuit. This condition does not allow the current to complete the circuit and makes the entire circuit dead. A good example of this is the old design for holiday lights. If one bulb was burned out, the entire string would not light. the entire string would not light.
Figure 12. Series Circuit
Figure 12. Series Circuit Take a look at the next series circuit. The voltage is unknown, but can be calculated using Ohm's Law, E = IR. The current (I) is four amps as shown, but the resistance has to be calculated. In a series circuit, when more than one resistance is in the circuit, the resistances are added together to get the total resistance (RT). The RT is 12 ohms. Given these two values and Ohm's Law, the voltage is 48 volts.
Now is a good time to talk about how current and voltage behaves in a series circuit. The current value is the same in every part of the circuit. An Ammeter can verify this.
Voltage, on the other hand, does not remain constant throughout the circuit. Voltage values can be measured across each resistor or load. This is called the Voltage Drop. The total voltage (VT) is equal to the sum of all the voltage drops in that circuit. A Voltmeter can verify this. The formula is:
Voltage, on the other hand, does not remain constant throughout the circuit. Voltage values can be measured across each resistor or load. This is called the Voltage Drop. The total voltage (VT) is equal to the sum of all the voltage drops in that circuit. A Voltmeter can verify this. The formula is:
(VT) = V1 + V2 + V3 ...
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Basic Fundamentals
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