Electricity

Electricity is the stationary movement of Electrical Charges. It is the most convenient and desirable source of energy. The primary sources like coal, natural gas, nuclear power, oil, etc., are converted to form Electrical energy. Electricity is now an indispensable requirement for our households and offices.
Read further to learn how electricity flows through our home, is generated, and how an electric circuit works.

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Electric Current and Circuit

The flow of electric charges through a conductor constitutes the electric current in a conductor. But the flow of electric current through a conductor is restricted by a gate called a switch. But unlike the gates in our homes, the current flows when the switch is closed, and if the switch is open, the current does not flow through the electric circuit- for example, it is when you switch on the fan in your room, it starts rotating, but if you push the switch off, the fan stops. Similarly, you switch on a bulb, it starts glowing, and it stops glowing the minute you switch it off. Thus, a switch can be defined as a conducting link between the source of electric current and an electrical device that needs to be powered. 
The current flows through conductors(wires) to power our devices; it follows a certain path that guides it. The continuous and closed path followed by an electric current is known as an electric circuit. If the circuit is broken anywhere or if the switch is turned off, the current will not flow.

Electric current can be defined as the flow rate of electric charges or the amount of charge flowing through a specific area in a unit amount of time. Since electrons were discovered after electricity, electricity was initially assumed to be the flow of positive charges. The direction of electricity was understood to be along the direction of the flow of positive charges. As per convention, the direction of flow of electricity in an electric circuit is opposite to the direction of flow of electrons, the negative charge. Mathematically, Current \(I\) flowing through a cross-section is:
\(I = \frac{Q}{t}\)

The SI unit of electric charge is the coulomb \((\rm{C})\), and one coulomb is the charge carried by nearly \(6 \times {10^{18}}\) electrons. The SI unit of current is ampere \((\rm{A})\) named Ander-Marie Ampere, a French scientist; One ampere is the amount of current that flows through a conductor due to the motion of a coulomb of charge per second.

Current flowing through a circuit is measured using a device called Ammeter. In an electric circuit, through which we wish to measure the current, Ammeter is always connected in series.

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The figure above shows a basic electric circuit. It consists of a battery (power source), a bulb (electric device). The current flows through the bulb and Ammeter from the positive terminal towards the battery’s negative terminal. 

Electric Potential and Potential Difference

The flow of electrons in a circuit is much like the flow of air in the atmosphere; they flow from a region of high pressure into a region having low pressure. Thus, a difference in the electric pressure is required to ensure the flow of electrons through a conductor, which is termed a potential difference.

This potential difference is supplied in an electric circuit by a battery or a combination of cells. In a cell, chemical reactions produce a potential difference across its terminals. When this cell is connected into a circuit, it sets the electrons into motion within the conductor, generating a current.  To maintain this flow of current, the cell uses the chemical energy stored inside it.

Thus, an electric potential difference is defined as the amount of work done to carry a positive unit charge from one point to another in a circuit. Mathematically, the potential difference \((V)\) can be given as \(V = \frac{{{\rm{ Work done }}(W)}}{{{\mathop{\rm Charge}\nolimits} (Q)}}\).

The SI unit of electric potential difference is volt \((\rm{V})\) named after Italian scientist Alessandro Volta.
The potential difference across two points in a circuit is said to be one volt when one joule of work is required to move a coulomb of charge through them. Voltmeter, a device used to measure the electric potential difference, is connected in parallel across the points between which we wish to measure the potential difference.

Components of an Electric Circuit

The main components present in an electric circuit, along with the symbol with which we present them while drawing a circuit, are given in the table below:

Ohm’s Law

The relationship between the current flowing through a wire and the potential difference applied across its terminals was given by German physicist Georg Simon Ohm. He stated that the potential difference \(V\) measured across two points in a metal wire is directly proportional to the current I flowing through it, given that the temperature remains constant. Mathematically,
\(V \propto I\)
\(V = IR \cdots (1)\)
Here, \(R\) is the constant of proportionality called resistance. The resistance of a conductor is its property to oppose the flow of charges through it. It is constant for a given conductor at a given temperature. Equation \((1)\) is known as ohm’s law. The SI unit of resistance is the ohm, and its represented by the letter \(\Omega \)  Resistance across a conductor is said to be \(1\,\Omega \) if a potential difference of \(1\;\rm{V}\) is applied across it and a current of \(1\;\rm{A}\) flows through it.

It is often required to regulate the flow of current through a circuit without changing the voltage source; it can be achieved by changing the resistance across the circuit. The device which is often used to change the resistance in a circuit is called a Rheostat.
For a good conductor, the amount of resistance offered by it to the motion of electrons will be less. For a poor conductor of the same size, the amount of resistance offered will be more, and it will be even higher in the case of an insulator.

Factors Affecting Resistance

The value of resistance offered varies from a conductor to the other. For a given conductor, the resistance offered by it varies:

1. Directly with the length of conductor or \(R \propto l\)
2. Inversely with Its area of cross-section or \(R \propto _A^1\)
3. Nature of its material

Mathematically, \(R \propto _A^1\)
Or, \(R = \frac{{ol}}{A}\)
Here, \({\rm{\rho }}\) (rho) is the constant of proportionality called the electrical resistivity. It is a characteristic property of a given material. Higher is the value of resistivity of a material; higher will be its resistance. Thus, their resistivity is very low for good conductors, but for insulators, the value of resistivity is quite high. The values of both resistance and resistivity vary with temperature. Alloys do not get oxidised easily at high temperatures; hence even though they have higher resistivity than their constituent metals, alloys are used in heating devices called iron or toasters etc. In fact, filaments of the electric bulb are made with tungsten, while transmission lines are made from copper or aluminium.

Combination of Resistors

To reduce the amount of current flowing through the circuit, the resistors are connected in series. To increase the amount of current flowing through the circuit, the resistors are connected in parallel. In both these combinations, we need to calculate the equivalent resistance of the circuit.

Resistors in Series

If resistors \({R_1},\,{R_2},\,{R_3},\,{R_4},{R_5}\,……\) are connected in series, then the equivalent resistance of the combination will be:
\({R_{eq}} = {R_1} + {R_2} + {R_3} + {R_4} + {R_5}………….\)

Resistors in Parallel:

If resistors \({R_1},\,{R_2},\,{R_3},\,{R_4},{R_5}\,……\)  are connected in parallel, then the equivalent resistance of the combination will be:
\(\frac{1}{{{R_{eq}}}} = \frac{1}{{{R_1}}} + \frac{1}{{{R_2}}} + \frac{1}{{{R_3}}} + \frac{1}{{{R_4}}} + \frac{1}{{{R_5}}} \ldots  \ldots \)

Heating Effect of Electric Current or Electricity

Battery or a cell that supply energy to an electric circuit gets exhausted after a certain amount of time. A part of the energy supplied by the battery is used to sustain the flow of current through the circuit while some part of it gets converted into heat; this is called the heating effect of current or electricity. Due to this effect, electric devices (for example, our phone or laptop chargers) get hot after some time.
If a current \(I\) is flowing through a resistor of resistance \(R\) and if the potential difference across its terminals is \(V\),
Here, \(I = \frac{Q}{t}\)
Where \(Q\) is the charge flowing across the terminals of the circuit in time \(t\)
The work done in moving this charge through the applied potential difference, \(W\, = \,V\,Q\)
Thus, the power supplied by the battery, \(P = \frac{{VQ}}{t} = VI\)
The energy supplied by the battery in time \(t, E = P \times t\)
This energy gets expended across the resistor in the form of heat, and this \(H\) generated due to the current will be equal to,
\(H\, = \,VIt\)
Applying ohm’s law, we get:
\(H = {I^2}Rt\)
This is the joule’s law of heating. According to this law, the heat produced is:

1. For a given resistance, it is directly proportional to the square of the current
2. For a given current, it is directly proportional to the resistance
3. Directly proportional to the time for which current flows through the resistor.

Practical Applications of Heating Effect of Current or Electricity

1. The electric devices like laundry iron, electric toaster, electric oven, electric kettle and electric heater are based on the principle of joule’s heating.
2. An electric bulb generates light due to joule’s heating; although most of the energy is dissipated in the form of heat, a part of it is radiated in light.
3. Electric fuse, which is used to protect the devices in our homes from the damage caused by the flow of a large amount of current, are based on joule’s law of heating. An electric fuse is a wire having an appropriate melting point made from an appropriate alloy of copper and aluminium. As soon a current greater than the specified value flows through the wire, it melts, protecting the devices from getting damaged. It is placed in series with the devices.

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Electric Power

Electric power is defined as the rate of dissipation or consumption of electric energy in an electric circuit.
Mathematically,
\(P\, = \,V\,I\)
\(P = {I^2}R = \frac{{{V^2}}}{R}\)
\(SI\) unit of power is Watt \(({\rm{W}})\) One watt is defined as the power dissipated when one ampere of current flows through a circuit when operated at a potential difference of one volt.
Other units of power and units of energy in terms of power:

1. Kilowatt: \(1\;{\rm{k W}} = 1000\;{\rm{W}}\)
2. Watt-hour: energy consumed when one watt of power is consumed for one hour.
3. Kilowatt-hour: Energy consumed when one kilowatt of power is consumed for one hour,
   \(1\;{\rm{kWh}} = 1000\;{\rm{Wh}} = 1000 \times 3600\;{\rm{Ws}} = 3.6 \times {10^6}\;{\rm{Joule}}\)

Frequently Asked Questions on Electricity

Q1. Define ohm’s law
Ans: Ohm’s law states that the current flowing through a conductor is directly proportional to the potential difference applied across the conductor.

Q.2. What is the SI unit of power?
Ans: SI unit of power is watt.

Q.3. What is the electric potential difference?
Ans: Electric potential difference is the amount of work done required to take a positive unit charge from one point to another in a circuit.

Q.4. What are the factors affecting the resistance of a wire?
Ans: For a given conductor, the resistance offered by it varies
1. Directly with the length of conductor or \(R \propto l\)
2. Inversely with Its area of cross-section or \(R \propto _A^1\)
3. Nature of its material

Q.5. Define one ampere.
Ans: One ampere is the amount of current that flows through a conductor due to the motion of a coulomb of charges per second.

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