What is electric current?

The term electric current is the flow of electrons or electric charges. The drift of electrons is proportional to the amount of voltage or potential difference applied. The more is the voltage, the more will be the electron drift. The amount of electric current a circuit possesses is proportional to the density of electrons or electric charge.

Voltage

The voltage applied across the two terminals of an electrical circuit initiates the flow of electric charge. The rate of charge flowing is proportional to the potential difference applied to the material. Electrical resistance governs the amount of charge flow, or it simply retards the flow of electrons in a circuit.

Electric Power

Electric power is the rate of transfer of electric charges through an electric circuit. Consumption of electrical energy in commercial and industrial applications is rated in terms of electric power.

Electric charge and static electricity

Electric charge is the fundamental property possessed by the atomic and subatomic particles of a matter. This property of electric charge governs the behavior of the particles in the influence of the electric and magnetic fields. Electric charge obeys the principle of energy conservation, it cannot be created nor destroyed.
Electrical charges can be positive charges and negative charges. For example, the electrons of an atom possess a negative charge, while the protons are of positive charge. Like charges induces a force of repulsion while unlike charges induce a force of attraction. Electrons and protons are experimentally found to have the same magnitudes of negative and positive charges respectively. The neutrons in the nucleus of an atom possess zero charges. The SI unit of electric charge is coulomb which is equal to 6.24×10¹⁸ electrons.
The imbalance of electric charge on the surface of the material leads to the development of static electricity. The excess of charge stays on the surface until it is moved through an electric discharge or passes through a bridge of an electric circuit. Static electricity can be created by rubbing a material with another, this results in the transfer of negative charges or electrons.

Ohm's law

Ohm's law establishes a relationship between voltage, resistance, and current in an electrical circuit. The Ohm's law states that, in a circuit, the voltage drop varies proportionally with the electric current.

Mathematically,

$V \alpha I$,

where V represents the voltage and I represents the current.

After interchanging the constant of proportionality with a constant, the equation reduces to,

$V=IR$,

where $R$ is the constant of proportionality known as electrical resistance. It unit is Ohm, $\Omega$.

The above relation signifies that voltage V and current I shares a linear relationship with resistance,$R$ as the slope. 

Hence, we can say that Ohm's law is only valid if voltage V and current I vary linearly.

Ampere is the SI unit of electric current accepted by scientists and engineers worldwide. One ampere signifies the flow of one coulomb of charge in one second.
The electric current in a circuit is measured by an ammeter or multimeter. The ammeter measures only electric current whereas, the amperage multimeter is a multipurpose instrument. It measures multiple parameters like current, voltage, potential difference, resistance, etc.

Electric power and heating effect of electric current

Electric power (P) is the rate at which an electric charge is transferred to complete a specific task. It is the rate of electrical energy transfer to be used for work. The work may be to light an electric bulb in a circuit or consumption electricity in households.
Electric power is the ratio between work done $W$ and time rate $t$. Mathematically it can be written as,

$P=\frac{W}{t}$

On simplifying we get,

$P=\frac{W}{Q}\times \frac{Q}{t}$

Here the rate of charge transfer, $\frac{Q}{t}$ is the electric current, I, and ratio of work done to charge transfer is the voltage drop, V. Therefore, the equation becomes

$P=VI$.

This expression for power is the fundamental relation in the current electricity concept. 

Many of the electrical components such as electric motors consume electric power, but at the same time undergo hysteresis losses in the form of heat energy. These hysteresis losses are due to internal resistances and imperfections. This loss is known as electrical hysteresis. Consumption of electric energy into these devices is achieved as mechanical work but at the expense of heat energy. The power of the heating generated can be calculated by

$P=I^{2}R$

This relationship is popularly known as Joule's law,or Joule's heating or Ohmic heating or resistance heating. 

The magnitude of heat generated is given by the expression,

$H=I^{2}Rt$

Where, $H$ represents the heating value and t is the time interval.

Every current-carrying conductor possesses resistance. This relation determines the magnitude of heat generated in different conductors having different cross-sections.

Context and Applications

This topic is taught in various undergraduate and postgraduate degree courses like:

• Bachelors of Technology (Electrical Engineering)
• Bachelors of Technology (Electronics Engineering)
• Bachelors of Technology (Instrumentation Engineering)
• Master of Technology (Electrical Engineering)

Practice Problems

Q 1. Which of the following parameter drives the electrons in an electric circuit?

1. Voltage from a voltage source
2. Resistance
3. Electric field
4. Magnetic field

Answer: Option a

Explanation: The voltage provided by any voltage source is the parameter that drives the electrons in an electric circuit.

Q 2. In which of the following conditions do we use Ohms law?

1. When current and voltage share a linear relationship
2. When current and voltage shares a nonlinear relationship
3. When current electricity parameters vary parabolically
4. None of these

Answer: Option a

Explanation: Ohms law can be applied, when the current and voltage varies linearly.

Q 3. What is the other name of Joule's heating?

1. Ohmic heating
2. Resistance heating
3. Both a and b
4. None of these

Answer: Option c

Explanation: The other name of Joule's heating is Ohmic heating or resistance heating.

Q 4. Which of the following electrical devices undergoes heating when operated?

1. Electric motor
2. Electric generator
3. Alternator
4. All of these

Answer: Option d

Explanation: An electric motor, electric generator, and alternator all undergo heating when operated.

Q 5. What is the SI unit of electric charge?

1. Candela
2. Farad
3. Coulomb
4. Ohm

Answer: Option c

Explanation: Coulomb is the SI unit of electric charge.