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Basic of Energy, Electrical and Thermal Energy, Types of Energy - Concept, Definition, Formula, Unit

What is Energy or Definition of Energy

Energy is the ability to do work and work is the transfer of energy from one form to another. In practical terms, energy is what we use to manipulate the world around us, whether by exciting our muscles, by using electricity, or by using mechanical devices such as automobiles. Energy comes in different forms - heat (thermal), light (radiant), mechanical, electrical, chemical, and nuclear energy.

What are the Various Forms of Energy

There are two types of energy - stored (potential) energy and working (kinetic) energy. For example, the food we eat contains chemical energy, and our body stores this energy until we release it when we work or play.

Potential Energy

Potential energy is stored energy and the energy of position (gravitational). It exists in various forms such as: Chemical Energy, Nuclear Energy, Stored Mechanical Energy, Gravitational Energy, etc.

Kinetic Energy

Kinetic energy is energy in motion- the motion of waves, electrons, atoms, molecules and substances. It exists in various forms such as: Radiant Energy, Thermal Energy, Electrical Energy, etc.

Electrical Energy Basics

Direct Current (DC):

A non –varying, unidirectional electric current (Example: current produced by batteries)


>Direction of the flow of positive and negative charges does not change with time.

>Direction of current (direction of flow for positive charges) is constant with time.

>Potential difference (Voltage) between two points of the circuit does not change polarity.

Alternating Current (AC):

A current which reverses in regular recurring intervals of time and which has alternately positive and negative values, and occurring a specified number of times per second. (Example: Household electricity produced by generators, Electricity supplied by utility).


  >Direction of the current reverses periodically with time.

>Voltage (tension) between two points of the circuit changes polarity with time.

  > In 50 cycles AC, current reverses direction 100 times a second (two times during one cycle).

Ampere (A):

Current is the rate of flow of charge. The ampere is the basic unit of electric current. It is that current which produces a specified force between two parallel wires, which is 1 meter apart in a vacuum.

Voltage (V):

The volt is the International System of Units (SI) measure in electric potential of electromotive force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through the resistance. 1000 V = 1 Kilo Volts (kV)


Resistance = Voltage/Current                                      The unit of resistance is ohm (Ω)

Ohm’s Law

Ohm’s law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature and other external conditions remain constant.


The supply frequency tells us the cycles at which alternating current changes. The unit of frequency is hertz (Hz: cycles per second)

Kilovolt Ampere (KVA)

It is the product of kilovolts and amperes. This measures the electrical load on a circuit or system. It is also called the apparent power.

For a single phase electrical circuit , Apparent power (KVA) = (Voltage X Amperes)/1000

For a 3 phase electrical circuit, Apparent power (KVA) = (√3 X Voltages X Amperes)/1000

Reactive Power (KVAr):

KVAr is the reactive power; reactive power is the portion of apparent power that does no work. This type of power must be supplied to all types of magnetic equipment, such as motors, transformers etc. Larger the magnetizing requirement, larger the KVAr.

Kilowatt (KW) (Active Power):

KW is the active power or the work –producing part of apparent power.

For single phase, Power (KW) = (Voltage X Ampere X Power factor)/1000

For 3 Phase, Power (KW) = (√3 X Voltage X Ampere x Power factor)/1000

Power factor:

Power factor (PF) is the ratio between the active power (KW) and apparent power (KVA).

Power factor (cos ɸ)     = Active Power (KW) / Apparent Power ( KVA)                                       = KW/{√(KW)2 + (KVAr)2}                                       = 1.0 (when KVAr = 0)

When the current lags the voltage like in inductive loads, it is called the lagging power factor and when the current leads the voltage like in capacitive loads, it is called leading power factor.

Inductive loads such as induction motors, transformers, discharge lamp, etc. absorb comparatively more lagging reactive power (KVAr) and hence, their power factor is poor. Lower the power factor; electrical network is loaded with more current. It would be advisable to have highest power factor (close to 1) so that network carries only active power which does real work. PF improvement is done by installing capacitors near the load centers, which improve power factor from the point of installation back to the generating station.

Kilowatt-hour (kWh):

Kilowatt-hour is the energy consumed by 1000 watts in one hour. If 1KW (1000 watts) of electrical equipment is operated for 1 hour, it would consume 1 Kwh of energy (1 unit of electricity). For a company, it is the amount of electrical units in Kwh recorded in a plant over a month for billing purpose. The company is charged/billed based on Kwh consumption.

Electricity Tariff:

Calculation of electric bill for a company-

Electrical utility or power supplying companies charge industrial customers not only based on the amount of energy consumed (Kwh) but also on the peak demand (KVA) for each month.

Contract Demand

Contract demand is the amount of electrical power that a consumer demands from the utility in a specified interval. Unit used is KVA or KW. It is the amount of electric that the consumer agreed upon with the utility. This would mean that utility has to plan for the specified capacity.

Maximum Demand:

Maximum demand is the highest average KVA recorded during any one-demand interval within the month. The demand interval is normally 30 minutes. The demand is measured using tri-vector meter/digital energy meter.

Time of Day (TOD) Tariff:

Energy meter will record peak and non-peak consumption separately by timer control. Wherein, off peak hour tariff charged is quite low in comparison to peak hour tariff.

Thermal Energy Basics


It is the degree of hotness or coldness measured on a definite scale. Heat is a form of energy; temperature is a measure of its thermal effects. In other words, temperature is a means of determining sensible heat content of the substance.


It is the force per unit area applied to outside of a body. When we heat a gas in a confined space, we create more force; a pressure increase. For example, heating the air inside a balloon will cause the balloon to stretch as the pressure increases. Pressure, therefore, is also indicative of stored energy. Steam at high pressures contains much more energy than at low pressures.


Heat is a form of energy, a distinct and measurable property of all matter. The quantity of heat depends on the quantity and type of substance involved. Calorie is the unit for measuring the quantity of heat. It is the quantity of heat, which can raise the temperature of 1 g of water by 1 deg C. Calorie, is too small a unit for many purposes. Therefore, a bigger unit Kilocalorie (1 Kilocalorie = 1000 calories) is used to measure heat. 1 kilocalorie can raise the temperature of 1000g (i.e. 1kg) of water by 1 deg C. However, nowadays generally joule as the unit of heat energy is used. It is the internationally  accepted unit. Its relationship with calorie is as follows: 1 Calorie = 4.187 J

Specific Heat

Quantity of heat required to raise the temperature of 1 Kg of substance through 1 deg, Celsius.

Sensible heat

It is that heat which when added or subtracted results in a change of temperature.


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