Notes: Electric Current & Circuits | Science & Pedagogy Paper 2 for CTET & TET Exams - CTET & State TET PDF Download

Electric Current

It is the rate at which an electric charge flows in a conductor. It is associated with charge in motion. Thus, it is classified under Dynamic Electricity.
Current is represented by the symbol I.
Mathematically      I = q/t
where t is time and q is a charge.
S.I. unit of current is ampere(A). But ampere is a very large unit so milliampere(mA) is often used.
Electric current is measured by a device known as an ammeter.

Types of Current

• Direct Current: the current in which the flow of current is always in the same direction and frequency remains zero, then it is called direct current. It is also called DC.
• Alternating Current: The current in which direction of current changes after a fixed interval of time is called alternating current. It is also called AC. Electricity is supplied in households in AC form with a frequency of 50 Hz.

Electric Circuit

• A complete path in which electricity can flow (or current can pass) is known as an electric circuit. Conventionally the direction of electric current in a circuit is from positive to the negative terminal.  
• If the terminals of a bulb are connected with terminals of an electric cell, the current passes through the filament of the bulb. This makes the bulb glow.

Electricity

• It is a form of energy which results due to the existence of charged particles, either at excess in rest (Static Electricity) or in motion (Dynamic Electricity).

Electric Charges and Its properties

William Gilbert discovered that when some substances are rubbed with some particular substances they gain the property of attracting light bodies. The substance after acquiring this property is said to be charged.
Charges are of two types

• Positive
• Negative

Charges have a property of attracting opposites i.e. opposite charges attract each other. On the other hand, they show repulsive properties if both the charges are of the same type.
The attractive or repulsive force between two charged particles is governed by Coulomb’s Law

Coulomb’s Law 

It states that Electric forces are directly proportional to the product of charges q1 and q2 and inversely proportional to the square of the distance between them.
Mathematically - F = k(q₁)(q₂)/r²
where k is the proportionality constant.
This force F is known as the Electrostatic Force.
The S.I. unit of charge is coulomb (C).
1 Coulomb = 1.6 *1018 electrons.

Electric Conductors and Insulators 

Conductors are materials which allow the electric current to pass through them. These materials are electron-rich material and have very little resistance to electricity.

• Examples of a conductor are Iron rod, copper wire, etc.

Insulators are materials which do not allow the passage of electric current through them. These materials are electron deficit materials and have very high resistance towards electricity.

• Examples of insulators are plastic, wooden stick, etc.

Semiconductors  -Substances that show intermediate or average resistance towards the flow of electrons are known as semiconductors. In these substances flow of electrons is intermediate between conductors and insulators.

• Examples of semiconductors are silicon, Germanium, etc.

Electric Cell

Electric cells are used in various electrical appliances like torches, watches, radios, cameras etc. Electricity in these devices is provided by electric cells. A cell converts chemical energy into electrical energy.
It has two terminals i.e. positive (+) and negative (-).
When the chemicals inside the cell are completely used up the cell stops functioning.

Batteries
A combination of two or more than two electric cells is known as a battery. These are used in TV Remotes, inverters, clocks, etc

Electric Potential and Potential Difference

• A potential difference is necessary to have a steady flow of current through a conductor. This potential difference can be achieved through the use of a battery.
• Electric Potential difference is defined as the work done needed to move a charge from one point to another in an electric field.
• Electric potential is defined as the amount of work done needed in order to move a unit positive charge from infinity to that point.
• The S.I. unit of electric potential and the electric potential difference is volt(V).

Ohm’s Law and Resistance

Ohm’s law states that the current flowing in a conductor is directly proportional to the potential difference applied across the end of the conductor.
Mathematically, it becomes
V= IR
where V is the potential difference
I is current and R is proportionality constant known as the resistance of the conductor.

Ohm’s law gives a direct relation between potential difference across the end and the current flowing through the conductor.

Resistance

• It is the property of a substance by virtue of which it resists the flow of current through it.
• Conductors like Copper and silver have very low resistance whereas Insulators like wood and glass have very high resistance.
• S.I. unit for resistance is ohm(Ω)

The resistance of a conductor depends on three factors:

• Length of the conductor: resistance is directly proportional to the length of the conductor.
• Area of the cross-section of the conductor: resistance of a conductor is inversely proportional to the area of cross-section.
• On the nature of its material.

So mathematically resistance of a conductor is given by R = ρL/A
where R is resistance, L is the length of the conductor, A is the area of cross-section of the conductor and ρ is the proportionality constant pronounced as rho.