Author: S.Ashok
Electric Circuits
Wherever two charges are connected by a conductor, a pathway for current flow exists; and if the charges are unequal, current flows from the negative to the positive charge. The amount of current flow depends on the voltage difference of the charges and the resistance of the conductor. If two charged bars are connected by a copper wire, for example, current will flow from the more negative to the more positive bar, but only long enough to cause each bar to have an equal charge. Although current flows briefly, this kind of connection is not an electrical circuit.
An electric circuit is a completed electrical pathway, consisting not only of the conductor in which the current flows from the negative to the positive charge, but also of a path through a voltage source from the positive charge back to the negative charge. As an example, a lamp connected across a dry cell forms a simple electric circuit. Current flows from the (-) terminal of the battery through the lamp to the (+) battery terminal, and continues by going through the battery from the (+) to the (-) terminal. As long as this pathway is unbroken, it is a closed circuit and current flows; but, if the path is broken at any point, it is an open circuit and no current flows.
A closed loop of wire is not always a circuit. Only if a source of emf is part of the loop do you have an electric circuit. In any electric circuit where electrons move around a closed loop, current, voltage and resistance are present. The pathway for current flow is actually the circuit, and its resistance controls the amount of current flow around the circuit.
Direct current circuits consist of a source of DC voltage, such as batteries, plus the combined resistance of the electrical equipment connected across this voltage. While working with DC circuits, you will find out how the total resistance of a circuit is changed by using various combinations of resistances, how these combinations control the circuit current and affect the voltage.
You have found out how cells are connected in series or parallel, and now you will find that resistances are connected in the same manner to form the two basic types of circuits, series and parallel, circuits. No matter how complex a circuit you may work with, it can always be broken down into either a series circuit connection or a parallel circuit connection.
Simple circuit connections
Only the resistances in the external circuit, between the terminals of the voltage source, are used to determine the type of circuit. When you have a circuit consisting of only one device having resistance, a voltage source and the connecting wires, it is called a SIMPLE circuit. For example, a lamp connected directly across the terminals of a dry cell forms a simple circuit. Similarly, if you connect a resistor directly across the terminals of a dry cell, you have a simple circuit since only one device having resistance is being used.
Ohm’s Law in Simple Circuits
You have found out that voltage and resistance affect the current flow in a -circuit, and that voltage drops across a resistance. The basic relationships of current, voltage and resistance are as follows:
- Current in a circuit increases when the voltage is increased for the same resistance.
- Current in a circuit decreases when the resistance is increased for the same voltage. These two relationships combined are Ohm’s Law, the -most basic law of electric circuits, usually stated as follows:
THE CURRENT FLOWING IN A CIRCUIT CHANGES IN THE SAME DIRECTION THAT THE VOLTAGE CHANGES, AND IN THE OPPOSITE DIRECTION THAT THE RESISTANCE CHANGES.
You have seen that if a certain current of electricity flows in a circuit, it flows because a certain electromotive force, or voltage, forces it to flow, and that the amount of current is limited by the resistance of the circuit. In fact, the amount of current depends upon the amount of electrical pressure, or voltage, and the amount of resistance. This fact was discovered by a man named George S. Ohm and is expressed by the now famous Ohm’s Law which is the fundamental equation of all electrical science. Since it was first stated in 1827, this law has been of outstanding importance in electrical calculation. One of the most common ways of expressing Ohm’s Law is that THE CURRENT FLOWING IN A CIRCUIT IS DIRECTLY PROPORTIONAL TO THE APPLIED VOLTAGE, AND INVERSELY PROPORTIONAL TO THE RESISTANCE. When you put this word statement into a mathematical relationship you get
This enables you to find the voltage when you know the current and resistance.
If you know the voltage and the current, you can find the resistance then by simply applying the following form of Ohm’s Law.
Ohm’s law is used in electric circuits and parts of circuits to find the un- known quantity of current, voltage or resistance when any two of these quantities are known. In its basic form Ohm’s law is used to find the current in a circuit if the voltage and resistance are known. To find the current through a resistance, the voltage across the resistance is divided by the resistance.
As you know, the current in a circuit increases if the voltage increases and the resistance remains the same. By giving values to V and R, you can see how this works. Suppose that R is 10 ohms and E is 20 volts. Since the current equals 20 divided by 10, the current is 2 amperes.
Establishing total resistance in series circuits
In the previous topic you learned that the total resistance in a series circuit is equal to the sum of the individual resistances in that circuit. Total resistance in a series circuit,may be established by using Ohm’s Law if the amount of current in the circuit and the impressed voltage are known.
Note that the total impressed voltage, V, is 100 volts, and that the total current in the circuit is two amperes. Note also, that there are three resistors in series. This fact will not cause any difficulty in solving the problem if you remember that the total current flowing in a circuit is the result when the total voltage is applied across the total resistance in the circuit. Using Ohm’s Law, then, the total resistance is equal to the total voltage divided by the total current.
In the previous topic you also learned that when the voltage drops in a series circuit are added together, the total value is equal to the total im- pressed voltage, or
This is true even though the various resistors in the series circuit may all be of different values.
What is power
Power-whether electrical or mechanical means the rate of doing work. 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 with-out causing motion-such as the force of a spring under tension between two objects which do not move is not work.
Previously you found that electrical force is voltage and that voltage causes current flow-movement of electrons. Voltage existing between two points without causing current flow is similar to the spring under tension without moving, and is not doing work. Whenever voltage causes electron movement, work is done in moving electrons from one point to another. The rate at which this work is done is called electric power.
The same total amount of work may be done in different amounts of time. For example, a given number of electrons may be moved from one point to another in one second or in one hour, depending on the rate at which they are moved; and the total work done will be the same in each case. If all the work is done in one second, more electrical energy will be changed to heat or light per second than if the total amount of work is done in an hour.
Unit of electric power
The basic unit of power is the watt, which equals the voltage multiplied by the current-electrical force fines coulombs of electrons moved past a point per second. This represents the rate at which work is being done in moving electrons through a material. The symbol P indicates electrical power.