Electricity and Circuits Notes


  • The machinery devices that we used in our daily life run on electricity. For example, we watch our favorite shows on TV, wash and dry our clothes on the washing machine and enjoy playing video games on mobile or PC.
  • The devices that run on electricity are called electric appliances.
  • We can define “electricity” as the form of energy that converts into other types of energy. For example, a doorbell converts electrical energy into sound energy, and the geyser converts electrical energy into heat energy.


  • When you observe the torch, you will notice a small bulb in front of it and a switch located at the side of the torch that has two markings on it: ON and OFF: If you press the “ON” side of the switch, the bulb glows then press the “OFF” side of the switch, the bulb stops illuminating. When the switch is on, the “electric current” starts flowing inside the torch that provides energy to the bulb for glowing.
  • Ammeter: An instrument that measures the amount of electricity flowing in a circuit.


The main sources of electricity are based on the production or conversion of the energy:-

Large-scale production

1. Power stations:
  • Power stations are responsible for generating a big amount of electricity for industries, factories, houses and school etc.
  • They are located near the rivers, waterfalls or highly windy areas.
2. Windmills:
  • A group of aerogenerators placed in windy areas that convert wind energy into electrical energy.
  • They also produce electrical energy on a large scale.
3. Solar Panels or Plates:
  • An arrangement of photovoltaic cells on a framework of plates that converts solar energy into electrical energy.
  • There are varieties of solar panels that exist based on the requirement of the energy in a particular place.
4. Standby generators:
  • When the electricity cuts, the standby generators provide electricity to the place.
  • They work on fuels like petrol and diesel.
5. Inverters:
  • They are used when the electricity cuts.
  • They work on batteries that are charged by electricity.

Portable sources of electricity

 Electric cell
  • An Italian scientist named “Alessandro Volta” invented the electric cell.
  • A device that is considered a source of electricity(Figure 1).
  • A cell has two terminals: positive and negative.
  • The positive end of the cell has a metal cap on it, while the negative end has a metal disc attached to it.

Figure 1: An Electric Cell

  • The cells combined to form a battery. They are arranged by opposite terminals. For example, the negative terminal of a cell is connected with the positive terminal of another cell.
  • There are some chemicals inside a cell that react with each other to create an electric current.
  • When the chemicals are saturated, the cell stops producing electric current.
  • In an electric cell, the body is designed in a cylindrical shape, and its outer cover is covered with plastic or rubber. So, we prevent ourselves from the electric shock.
Types of electric cell
There are two types of electric cells:-
Primary electric cell:
  • The chemicals inside the cell convert the chemical energy into electrical energy. This process is completely irreversible.
  • They generate only a small amount of electric current.
  • They are used in the devices such as remote controls, wall clocks, toys flashlights and radios.
Secondary electric cell:
  • It is a reversible process which means the cell can be recharged and used repeatedly.
  • They are also called storage cells or accumulators.
  • These types of cells are used in mobile devices, camcorders, laptops, power tools and motor drives.
  • In vehicles, they are used for the highlights and air-conditioning system.

Electrical Devices

Electric Bulb

  • It is the first electrical device(Figure 2) used by a common man.
  • An electric bulb consists of a transparent glass covering and enclosed with a thin-coiled metal wire called the filament.
  • The filament is made up of tungsten and has two thick wires that are connected to the metal base of the bulb. These are called the terminals of the bulb.
  • A bulb converts electrical energy into heat and light energy.
  • The filament becomes hot when the bulb glows.
  • If the filament is broken then the bulb does not glow.

Figure 2: An Electric Bulb


  • It is a link between the supply of electric current and a device.
  • It manages the flow of the electric current in a circuit. When the switch is on, the circuit has electric flow. The circuit is called closed circuit. If the switch is off then the circuit has no current source. This type of circuit called open circuit.
  • The symbol of an electrical switch is represented as:



A torch is a device that works on two or more cells to light a bulb(Figure 3).

Figure 3: Parts of a torch
The different parts of an electric torch are:-
Wire and Spring:
  • The wire is connected with the bulb, whereas the spring is responsible for the connection of the cells and the switch.
  • The cells are placed between the spring and the terminals.
  • The position of the positive terminal of the battery points towards the bulb.
  • The one end of the wire is connected to a terminal of the bulb, while the other end of the wire is joined with the switch.
  • A  switch is the link between the wire and the spring.
Working of a torch
  • When the switch is on, the bulb glows because the wire and the spring connected by the metal strip under the switch. Thus, the parts of the torch form a closed circuit path.
  • When the switch is off, the bulb does not glow because the parts of the torch form an open circuit.
  • The closed path in which the electric current flows is called electric circuits.
  • The main elements of an electric circuit are cells, bulb and switch. They are connected with each other through wires.
  • If the wires are connected properly then the circuit forms a closed path, which makes the bulb glow. This type of circuit is called a closed circuit or complete circuit. While the connection between the parts of a circuit is loose then the bulb does not glow, and this type of circuit is called an open circuit

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Practice Question

Question 1 What is $\frac {1}{2} + \frac {3}{4}$ ?
A)$\frac {5}{4}$
B)$\frac {1}{4}$
D)$\frac {4}{5}$
Question 2 Pinhole camera produces an ?
A)An erect and small image
B)an Inverted and small image
C)An inverted and enlarged image
D)None of the above