Basic Electrical Quantities – Charge, Current, Voltage, Power and Energy



In electrical and electronic circuits, there are five major electrical quantities that used to analyze circuits. These quantities are electric charge, electric current, voltage, electric power and electrical energy. In this article, we shall learn about these quantities in detail.

What is Electric Charge?

Electric charge or simply charge is the property of subatomic particles like protons and electrons. The electric charge is denoted by symbol Q or q and measured in Coulombs (C).

Technically, electric charge is the concept which explains the electrical behavior of materials. Therefore, it is the electric charge that forms the basis of existence of the electricity. The electric charge is the most elementary quantity in an electric circuit.

As per the electron theory of matter, we know that every material consists of tiny particles called molecules, and molecules in turn made up of atoms. An atoms consists of three fundamental particles namely electrons, protons and neutrons. Where, electrons bear a negative charge, protons carry a positive charge and neutron is neutral, i.e. there is no charge on the neutron. These three particles are called subatomic particles.

Therefore, electric charge can be of two types −

  • Positive charge − carried by a proton

  • Negative charge − carried by a electron

In the nature, the smallest amount of charge that exists is the charge carried by an electrons, denoted by e, where it is equal to $\mathrm{1.6 \times 10^{-19}}C$. Though, a proton also carries a charge of $\mathrm{1.6 \times 10^{-19}}C$, but it is positive.

Two Fundamental Principles of Electric Charge

There are two fundamental principles associated with the electric charge −

  • The Quantization of Charge − According to this theory, the electric charge can occur as the integral multiple of fundamental charge or electronic charge, i.e.

$$\mathrm{Q=ne;\:Where,n=0,1,2,..}$$

  • Law of Conservation of Charge − According to this law, the electric charge can neither be created nor destroyed. Thus, for a closed system, the electric charge remains constant.

What is Electric Current?

Electric current is defined as the directed flow of electric charge under the influence of an electric field. It is denoted by symbol I or i, where I is used to denoted a constant electric current and i is used to denote a time-varying current.

Mathematically, the time rate of flow of electric charge (electrons) is known as electric current, i.e.,

$$\mathrm{I=\frac{Q}{t}}$$

In differential form,

$$\mathrm{i(t)=\frac{dq(t)}{dt}}$$

Where, Q is electric charge measured in coulombs and t is time in seconds. Therefore, the unit of electric current is Coulomb per second (C/s). However, in practice, we use SI unit Ampere (A) to measure the electric current.

The conventional direction of the electric current is from the point of higher potential (positive terminal) to the point of lower potential (negative terminal). However, the actual direction of electric current is the direction of flow of electrons which is from the negative terminal to the positive terminal.

As from the definition of the electric current, it is clear that the electric current is due to the flow of electric charge or electrons. Therefore, it shows two fundamental effects viz. heating effect and magnetic effect. Which means, when the electric current flows through a conductor, it produced a magnetic field around the conductor and generates heat in the conductor.

Types of Electric Current

Depending on the direction of flow of charge, the electric current may be classified into two types −

  • Direct Current − A direct current (DC) is one which flow in only one direction.

  • Alternating Current − An alternating current (AC) is one whose magnitude changes continuously and direction changes periodically.

What is Voltage?

Voltage, also known as potential difference, is the electric pressure which makes the electric charges to flow in a conductor. The voltage is defined as the work or energy required to move a unit charge from one point to another in an electric circuit. It is denoted by the symbol V (constant voltage) and v (time-varying voltage).

$$\mathrm{Voltage,V=\frac{W}{Q}}$$

In differential form,

$$\mathrm{v(t)=\frac{dw}{dq}}$$

Where, W is the work done measured in joules (J) and Q is the charge in Coulombs (C). Thus, the voltage is measured in Joules per Coulomb (J/C). However, in practice, we measure the voltage in volts (V).

Most important fact about voltage is that it does not exists at a point by itself, which means it is always measured with respect to some other point. Because of this, voltage is also known as "potential difference". In any electric circuit, the voltage is the factor which entirely responsible for flow of current in the circuit.

Types of Voltage

Just like the electric current, the voltage is also classified into two types −

  • Direct Voltage − A direct voltage is one which has constant magnitude with respect to time.

  • Alternating Voltage − An alternating voltage is one which changes its polarity in alternate direction with respect to time.

What is Electric Power?

The time rate of expanding or absorbing energy in an electric circuit is known as electric power. In other words, the rate of doing work in an electric circuit is called electric powerelectric power. It is denoted by P or p. Where, P is used to represent constant or average power, while p is used to denoted instantaneous power.

Mathematically, the electric power is given by,

$$\mathrm{P=\frac{Work\:done (𝑊)}{Time (𝑡)}}$$

And the instantaneous power is given by,

$$\mathrm{p(t)=\frac{dw}{dt}}$$

Where, W is work done measured in Joules (J) and t in time in seconds (s). Thus, the unit of electric power is Joules per second (J/s). But, in practice, we use the SI unit of power that is Watt (W).

$$\mathrm{∵dw=v(t).dq}$$

and $$dt=\frac{dq}{i(t)}$$

$$\mathrm{∴p(t)=\frac{v(t).dq}{dq/i(t)}=v(t).i(t)}$$

Hence, the electric power is simply the product of the voltage across a circuit element and the current flowing through the element.

In any electric circuit, if the electric current enters the circuit element at the positive terminal and leaves the element at the negative terminal, then the power is said to be absorbed by the element. On the other hand, if the current enters the element through the negative terminal and exits through the positive terminal, the element delivers the electric power.

For any electric, the power balance equation must be satisfied, i.e.,

Power delivered = Power absorbed

What is Electrical Energy?

The ability to do work in the electric circuit is known as electrical energy. The electrical energy is defined as the total amount of work done or total energy expended over a certain period of time in an electric circuit.

Mathematically, it is obtained by integrating the electric power over a certain period time as,

$$\mathrm{w(t)=\int_{t_{1}}^{{t_{2}}}p(\tau)d\tau}$$

The power is measured in Watts and time in seconds, hence the unit of electrical energy is Wattsecond (W-s). The energy is also measured in Joule.

The electric utility companies measure the electrical energy consumed by the consumers in Watt-hours (Wh), where

$$\mathrm{1Wh = 3600\;Joules}$$

Conclusion

In this article, we discussed that electric charge is the most fundamental electrical quantity in an electric circuit. When this charge flows through a conducting material, it constitutes an electric current in the conductor. The factor which pressures the electric charge to flow is the voltage or potential difference. In addition to charge, current and voltage, we require two more quantities namely "power" and "energy" for practical calculations. Power is the rate at which energy expended in the circuit, while Energy is the total amount of work done over a certain period of time in the circuit.


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