Electric Energy and Power – Definitions & Derivations

Electric energy and power is an important concept in Physics. Students aspiring to pursue engineering or medical need to emphasize on this chapter. Electrical Energy is the source of energy on which a number of tasks in our day to day life depends significantly. Electricity plays a significant role in our lives these days. This article will discuss different aspects associated with electric energy and power.

Embibe offers a range of study materials that includes PDF of NCERT books, previous year question papers and solution sets. Students can follow these study materials to enhance their preparations and improve their scores in the long run. The NCERT syllabus is quite similar to that of different entrance examinations. Students need to cover all the topics included in the NCERT syllabus to be completely prepared for different entrance examinations as well.

Current Electricity: Definition

The flow of electric charge along a wire is known as current electricity or electric current. When we look around, we find countless things around us, which function and operate using current electricity. It is a major source of power for our civilisation. 

Terms Related to Current Electricity

To know more about electrical energy and power, we need to understand some important terms related to current electricity.

Charge

It is an inherent property of matter which is responsible for the electromagnetic force. There are two types of charges present in nature. A proton carries a positive charge and a negative charge carried by an electron. The magnitude of the charge carried by a proton and an electron is the same. It is \(e = 1.6 \times {10^{ – 19}}\) coulomb, where coulomb \(\left( C \right)\) is the SI unit of charge.

Electric Current

It is defined as the rate of flow of charge.
\(I = \frac{Q}{t}\)

The SI unit of current is Ampere \(\left( A \right).\) When a charge of \(1{\rm{C}}\) flows through a conductor in \(1{\rm{s}},\) the current through the conductor is \(1{\rm{A}}.\) Smaller units like milliampere \((mA)\), microampere \((μA)\) are used for smaller currents.

Potential Difference

The electric potential at a point in an electric field is defined as the work done in moving a positive charge from infinity to that point. When work is done in moving a charge from one point to another, it is called Potential Difference, \(V\).
\(V = \frac{W}{Q}\)

The SI unit for potential and potential difference is Volt \((V)\). The potential difference between two points is \(1{\rm{V}}\) when work of \(1{\rm{J}}\) is done in moving \(1{\rm{C}}\) of charge between these two points.

Electromotive Force (EMF)

An electric cell or battery is a device that can maintain a constant potential difference across a conductor and drives a current through a circuit. It is a device that converts chemical energy to electrical energy. It has a positive and negative terminal. When no current is drawn from it, the potential difference between the terminals of a cell is called the electromotive force or emf of the cell.

Resistance

The hindrance offered by a conductor to the flow of electrons through it is known as resistance. The SI unit of resistance is Ohm \((Ω)\).

Ohm’s Law

Ohm’s law gives the relationship between the resistance of a conductor, the current through it, and potential difference across it. It states that at a constant temperature, the potential difference across the ends of a conductor is directly proportional to the current through it.
\(V \propto I\)
The proportionality constant is the resistance of the conductor, \(R\).
\(V = IR\)

According to Ohm’s law, the potential difference is the product of the resistance of the wire and the current flowing through it.

Electric Power

Electric power is defined as the rate at which electrical energy is supplied by a source or consumed by a load.
\(P = \frac{W}{t}\)
We know that,
\(Q = It\)
and
\(W = VQ\)
Therefore,
\(W = VIt\)
This means that,
\(P = \frac{W}{t} = VI\)
This equation shows that electric power is the product of voltage and current.
Using Ohm’s law where \(V=IR\),
\(P = (IR) \times I = {I^2}R\)
In terms of voltage,
\(P = V\left( {\frac{V}{R}} \right) = \frac{{{V^2}}}{R}\)
SI unit for electric power is Watt \((W)\). A watt can be expressed in the following two ways,
\(1W = 1V \times 1A\)
\(1\;{\rm{W}} = \frac{{1\;{\rm{J}}}}{{1\;{\rm{s}}}}\)

Electrical Energy

Energy is the capacity to do work. Electrical energy is the energy that is obtained by moving charges. It is defined as the energy required in moving a unit positive charge between two points having a unit potential difference in unit time.
\(E = VIt\)
\(P = VI\)
\(E = P \times t\)
SI unit of energy is Joule. The commercial unit of electrical energy is kilowatt-hour \((kWh)\).
\(1{\rm{\;kWh}} = 1000{\rm{\;W}} \times 3600{\rm{\;s}}\)
\(1{\rm{\;kWh}} = 3.6 \times {10^6}{\rm{\;Ws}}\)
\(1{\rm{\;kWh}} = 3.6 \times {10^6}{\rm{\;J}}\)
\(1{\rm{\;kWh}} = 3.6{\rm{\;MJ}}\)

Conversion of Electrical Energy to Other Forms

Electricity is the most versatile form of energy compared to other forms. It is easy to store, stop, transfer, convert, increase, or decrease compared to others. The major application comes in the way it can be converted into many other forms of energy. Let us look at some conversions and their applications.

Electrical to Mechanical

The physical movement of objects is the result of mechanical energy. A motor is a device that converts electrical energy to rotational movement. We use motors in a wide range of machines in industries. At home, motors are present in fans, water pumps, refrigerators, mixers, hairdryers, and even for the vibrating mode of our cell phones.

The reverse conversion happens in electric generators where mechanical energy is converted to electrical.

Electrical to Light

Can we even imagine our lives more than a hundred years ago? We switch on the lights after darkness sets in. Since the time Thomas Alva Edison invented a way to get light out of electricity, there has been a steady improvement in lamps. From incandescent bulbs invented by Edison to fluorescent tubes, commonly called tube lights, we have come to CFLs (compact fluorescent lamps), halogen lamps, mercury vapour lamps and now LED (light-emitting diode) bulbs.

The reverse conversion happens in photovoltaic \(\left( {PV} \right)\) cells, commonly called solar cells, which convert light to electricity.

Electrical to Heat

The incandescent bulb or tungsten bulb converts electrical energy to heat energy. The extremely high temperature makes the thin filament glow and gives out light. The geyser, radiator, boiler, water heater coil, hot plate, iron box are a few devices that utilise this conversion.

The reverse conversion happens in a thermocouple where a potential difference is created due to temperature difference. Indirect conversions happen in steam power plants, geothermal plants, nuclear plants.

Electrical to Sound

In fact, this is also mechanical energy as sound is due to the vibration of objects and air particles. Speakers convert electrical energy to sound by using an electromagnet that moves according to the amount of current through it. A paper diaphragm is attached tightly to it, and this vibrates, producing sound according to the variation in current.

The reverse conversion happens in a microphone where sound waves are converted to electric current.

Electrical to Chemical

The charging of a rechargeable battery cell is a good example. We use rechargeable cells in our cell phones, laptops, and for powering cameras and a host of other devices.

The reverse conversion happens when a battery cell provides electric current to a circuit.