Simple Electrical Circuit
A simple electrical circuit is one in which the current flows from the source to a load and reaches back the source to complete the path.
As shown in Fig 1, the electrical circuit should consist of the following.
• An energy source (cell) to provide the voltage needed to force the current through the circuit.
• Conductors through which the current can flow.
• A load (resistor ‘R’) to control the amount of current and to convert the electrical energy to other forms.
• A control device (switch ‘S’) to start or stop the flow of current.
In addition to the above, the circuit may have insulators (PVC or rubber) to confine the current to the desired path, and a protection device (fuse ‘F’) to interrupt the circuit in case of malfunction of the circuit (excess current).
Fig 2 shows a simple circuit which consists of a battery as the energy source and a lamp as the resistance. In this circuit, when the switch is closed, the lamp glows because of the electric current flows from the +ve terminal of the source (battery) via the lamp and reaches back the –ve terminal of the source.
Flow of electric current is nothing but the flow of free electrons. Actually the electrons flow is from the negative terminal of the battery to the lamp and reaches back to the positive terminal of the battery.
However direction of current flow is taken conventionally from the +ve terminal of the battery to the lamp and back to the –ve terminal of the battery. Hence, we can conclude that conventional flow of current is opposite to the direction of the flow of electrons. Throughout the Trade Theory book, the current flow is taken from the +ve terminal of source to the load and then back to the –ve terminal of the source.
The unit of current (abbreviated as I) is an ampere (symbol A). If 6.24 x 1018 electrons pass through a conductor per second having one ohm resistance with a potential difference of one volt causes one ampere current has passed through the conductor.
We know the electrons cannot be seen and no human being can count the electrons. As such an instrument called ammeter is used to measure the current in a circuit. As an ammeter measures the flow of current in amperes it should be connected in series with the resistance (Load).
For the decimal and decimal sub- multiples of the ampere we use the following expressions.
1 kilo-ampere = 1 kA = 1000 A = 1 x 103A
1 milli-ampere = 1 mA = 1/1000 A = 1 x 10–3A
1 micro-ampere = 1 μA = 1/1000000 A = 1 x 10–6A
Electro Motive Force (EMF)
In order to move the electrons in a circuit- that is to make the current to flow, a source of electrical energy is required. In a torch light, the battery is the source of electrical energy. The terminals of the battery are indicated in the circuit symbol by two lines, the longer line for the positive and the shorter for the negative terminal.
Within the battery the negative terminal contains an excess of electrons whereas the positive terminal has a deficit of electrons. The battery is said to have an electromotive force (emf) which is available to drive the free electrons in the closed path of the electrical circuit. The difference in the distribution of electrons between the two terminals of the battery produces this emf.
In Simple, Electromotive force (EMF) is the electrical force, which is initially available in elecrical source, cause to move the free electrons in a conductor
Its unit is ‘Volt’
It is denoted by letter ‘E’
It cannot be measured by any meter. It can be only
calculated by using the formula
E = Potential Difference (P.D) + V. drop
= p.d + V.drop
E = V + IR
Electromotive force is essential to drive the electrons in circuit
This force is obtained from the source of supply i.e. Torch lights, dynamo
System International (SI) unit of electromotive force is Volts (symbol ‘E’)
Potential Difference (PD)
The difference of volatge and pressure across two points in a circuit is called a potential difference (p.d) and is measured in volts.
In a circuit, when a current flows, there will be a potential difference across the terminals of the resistor/load. In the circuit shown in Fig 4, when the switch is in open conidition, the voltage across the terminals of the cell is called electromotive force (E) whereas when the switch is in the closed position, the voltage across the cell is called potential difference (p.d) which wil be lesser in value than the electromotive force earlier measured. This is due to the fact that the internal resistance of the cell drops a fer volts when the cell supplies current to the load.
The force which causes current to flow in the circuit is called emf. Its symbol is E and its unit is Volts (V). It can be calculated as
EMF = voltage at the terminal of source of supply +
voltage drop in the source of supply
or emf = VT + IR
Terminal voltage (p.d)
It is the voltage available at the terminal of the source of supply. Its symbol is VT. Its unit is also the volt and is also measured by a voltmeter. It is given by the emf minus the voltage drop in the source of supply, i.e.
VT = EMF – IR
where I is the current and R is the resistance.
Hence EMF is always greater than p.d [E.M.F>p.d]
Electrical voltage is measured with a voltmeter. In order to measure the voltage of a source, the terminals of the voltmeter must be connected to the terminals of the source. Positive to the positive terminal and negative to the negative terminal, as shown in Fig 5. The voltmeter connection is across or it is a parallel connection.
For the decimal or decimal sub-multiples of the volt, we use the following expressions.
1 kilo-volt = 1 KV = 1000 V
= 1 x 103 V
1 milli-volt = 1 mV = 1/1000 V
= 1 x 10–3V
1 micro-volt = 1 μV = 1/1000000
V = 1 x 10–6V
In addition to the current and voltage there is a third quantity which plays a role in a circuit, called the electrical resis- tance. Resistance is the property of a material by which it opposes the flow of electric current. The resistance is the property of opposition to the flow of the current offered by the circuit elements like resistance of the conductor or load is limit the flow of current
The unit of electrical resistance (abbreviated as R) is ohm (symbol Ω). For the decimal multiples or decimal sub-multiples of the ohm we use the following expressions:
1 megohm = 1 MΩ = 1000000Ω = 1 x 106Ω
1 kilo-ohm = 1 kΩ = 1000Ω = 1 x 103Ω
1 milli-ohm = 1 mΩ = 1/1000Ω = 1 x 10–3Ω
1 micro-ohm = 1 μΩ = 1/1000000Ω = 1 x 10–6Ω
Meter to measure resistance
Ohmic value of a medium resistance is measured by an ohmmeter or a Wheatstone bridge. (Fig 6) There is a provision to measure the ohmic value of a resistance in a multimeter. There are various methods to determine the ohmic value of resistance.
It is defined as that resistance offered to an unvarying current (DC) by a column of mercury at the temperature of melting ice (i.e. 0°C), 14.4521 g in mass, of constant cross- sectional area (1 sq. mm) and 106.3 cm in length.
One international ampere may be defined as that unvarying current (DC) which when passed through a solution of silver nitrate in water, deposits silver at the rate of 1.118 mg per second at the cathode.
It is defined as that potential difference which when applied to a conductor whose resistance is one international ohm produces a current of one international ampere. Its value is equal to 1.00049V.
The property of a conductor which conducts the flow of current through it is called conductance. In other words, conductance is the reciprocal of resistance. Its symbol is G (G = 1/R) and its unit is mho represented by . Good conductors have large conductances and insulators have small conductances. Thus if a wire has a resistance of R Ω, its conductance will be 1/R .
Quantity of electricity
As the current is measured in terms of the rate of flow of electricity, another unit is necessary to denote the quantity of electricity (Q) passing through any part of the circuit in a certain time. This unit is called the coulomb (C). It is denoted by the letter Q. Thus
Quantity of electricity = current in amperes (I)
x time in seconds (t)
or Q = I x t
It is the quantity of electricity transferred by a current of one ampere in one second. Another name for the above unit is the ampere-second. A larger unit of the quantity of electricity is the ampere-hour (A.h) and is obtained when the time unit is in hours
1 A.h = 3600 Asec or 3600 C