Electric Current and Electric Circuits - Class 7 Notes & Olympiad Questions

Electricity

Electricity is a type of energy that powers many things we use every day, like lights, appliances, and electronic devices. It is essential for our homes, industries, and transportation. We can see electricity in natural events like lightning and feel its effects through things like heaters in our homes.

Conductors and Insulators

Conductors

a) Conductors are materials that allow the electric current to flow through them easily. Think of them as good "conductors" or pathways for electricity.
b) Some examples of conductors are metals like copper, silver, and aluminium.
c) These materials have special properties that make it easy for electric charges to move through them.

Insulators

a) On the other hand, we have insulators. Insulators are materials that do not allow the electric current to flow through them easily.
b) They act as barriers to electricity. Some examples of insulators are wood, plastic, rubber, and glass.
c) These materials don't let electric charges move around freely.
d) Insulators are important because they help keep us safe from electric shocks. Electrical wires are covered with insulating materials, such as rubber or plastic, to prevent the electric current from escaping and causing harm to people or other objects.

Electric Current

When we plug in a device, like a phone or a lamp, electricity flows from the power source (like a power station) through the wires to the device. This flow of electricity is called an electric current. It's like a stream of tiny particles called electrons moving through the wires.

Positive and Negative Charges

a) Electric charges come in two types: positive charges and negative charges.
b) Positive charges are usually found in protons, which are part of the atomic nucleus, while negative charges are carried by electrons. In electricity, electrons, which carry negative charges, play an important role. They move from the negative side of a power source to the positive side, creating the electric current.
c) Similar charges repel each other, and opposite charges attract each other.

Measurement of Electric Current

a) We measure electric current using a unit called Ampere (A).
b) The Ampere is a base unit of the International System of Units (SI), which is the modern metric system used worldwide for scientific and everyday measurements.
c) Ampere tells us how many charges are flowing through a conductor in a given amount of time. The more charges flowing, the higher the current.

Electric Circuit

a) An electric circuit is like a path that allows electric current to flow in a loop.
b) It consists of components like bulbs, batteries, switches, and wires.
c) When we connect these components properly, the current can move from one point to another continuously.
d) To make the electric current flow, the circuit needs to be closed. This means all the components, like the wires and switches, need to be connected in a complete loop. If any part is disconnected or open, the current cannot flow.

Direction of Current

a) When we talk about the flow of electric current, it can be a bit confusing because there are two different ways to describe it.
b) First, we have the actual movement of electrons, which are tiny particles carrying an electric charge. In most cases, electrons flow from the negative terminal of a battery to the positive terminal. It's like a stream of little particles moving from one end to the other.
c) However, the current flows from the positive terminal of the battery to the negative terminal. It's like they're following the opposite path of the electrons. It's just a convention that scientists and engineers use to make things easier to understand and communicate.

Voltage (Potential Difference)

a) Voltage is the driving force that pushes electric charges through a circuit. It is measured in volts (V) and can be provided by a battery, generator, or any other power source.
b) Current flows from a higher voltage point (positive terminal) to a lower voltage point (negative terminal) in a circuit.

Electric Symbols

Electric symbols are graphical representations used to represent electrical components and devices in circuit diagrams. These symbols are designed to be simple and easy to understand.

Electric Components

Electric Cell

An electric cell is a device that converts chemical energy into electrical energy. It has two terminals: the positive and negative terminals. Chemicals inside the cell create a chemical reaction that generates electrical energy and allows the flow of electric charges, creating an electric current.

Classification of Cells

Primary Cells: These cells are designed for one-time use. Once the chemicals inside are used up, they stop producing electricity. Examples include batteries used in remote controls or flashlights.

Secondary Cells: These cells are rechargeable and can be used multiple times. They can be recharged by sending electrical energy back into them. Examples include batteries used in smartphones, laptops, and electric vehicles.

Types of Cells

Dry Cells: These cells are commonly used in everyday devices and don't contain liquid. They use a chemical paste or gel as a source of electrical energy.

Car Battery: Car batteries are larger and more powerful than dry cells. They are rechargeable and provide energy to start a car's engine and power its electrical systems.

Solar Cell: Solar cells convert sunlight directly into electricity and are used in solar panels to generate clean energy.

Button Cell: Button cells are small, round-shaped cells used in devices like watches and calculators. They are often non-rechargeable and provide stable and long-lasting power.

Battery

When multiple cells are connected together, they form a battery. Batteries come in different sizes and shapes depending on the number and type of cells they contain.

Electric Switch

An electric switch is a device used to control the flow of electric current in a circuit. It can be ON or OFF. When the switch is ON, it allows the current to flow, and when it's OFF, it blocks the current.

Electric Fuse

An electric fuse is a device that protects electrical circuits from overloading. It has a rating and is designed to break or melt when too much current flows through it. This helps prevent damage to the circuit and connected devices.

Electric Bulb

An electric bulb, or light bulb, produces light when an electric current passes through a filament inside it. The filament gets heated and starts glowing. The bulb's outer case is made of glass to protect the filament and allow the light to shine through.

Simple Electrical Circuits

Series Circuit

In a series circuit, the components (such as bulbs or resistors) are connected one after the other, forming a single pathway for the electric current to flow. It's like a chain where each component is linked together.

In a series circuit

a) The same current flows through each component.
b) The total voltage is divided among the components.
c) If one component stops working or is removed, the entire circuit is broken, and all the components stop working. It's like a chain breaking; if one link breaks, the whole chain falls apart.

Example: Series circuits are commonly used in holiday lights, where all the bulbs are connected in a series. If one bulb burns out, the entire string goes dark.

Parallel Circuit

In a parallel circuit, the components are connected side by side, creating multiple pathways for the electric current to flow. It's like a group of roads that run parallel to each other.

In a parallel circuit

a) The current splits and flows through each component.
b) The voltage across each component is the same.
c) If one component stops working or is removed, the other components continue to function independently.

Example: Parallel circuits are used in homes, where different electrical devices can operate independently, even if one device is turned off or stops working.

Table Comparing Series and Parallel Circuits