Electric Current and its Effects Class 7 Science Notes - Chapter 14

According to the CBSE Syllabus 2023-24, this chapter has been renumbered as Chapter 10.

Introduction to Electric Current and Its Effects

Introduction to charge

• Electric charge is described as the basic physical properties of matter which allows it to experience a special kind of force when kept under the influence of electromagnetic field.
• There are two types of charges → positive and negative.
• Like charges repel each other and unlike charges attract each other
• The SI unit for the electric charge is Coulomb and is equal to around  6.242×10¹⁸ e.  The value of the charge of an electron is  −1.602×10^-19 C.

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Heating Effect of Electric Current

Conductors and Insulators

• Substances or materials that offer comparatively less opposition to electric current through them are called conductors. For example, copper, iron, water, etc.
• Substances which offer larger opposition to electric current through them are called insulators. For example, rubber, wood, sand, etc.

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Heating Effects of Electric Current

• A current-carrying wire gets hot whenever electricity passes through it. This is the heating effect of electric current.
• The amount of heat produced in the wire depends on the material, thickness and length of the conductor.
• If a large amount of electric current passes through the wire, the wire may become so hot that it may melt and break.
• The heating effects of electricity are applied to many household appliances such as electric heaters, toasters, and water heaters.
• They are also used in bulbs and used as safety devices(fuses).

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Electric Bulb and Electric Fuse

• Coils of wire that utilise the heating effects of electric current are called elements. These coils can be found in an electric room heater or electric iron box, or any other electric appliance.
• Two important appliances based on the heating effects of electric current, which are widely used, are the electric bulb and electric fuse.
• The filament of an electric bulb becomes so hot when an electric current passes through that it starts glowing.
• When wires are made of a material that melts on the passage of electric current is called a fuse. Electrical fuses are used as safety device that breaks a circuit in times of overloading. They prevent the cause of fire and damage to electrical circuits.

Magnetic Effect of Electric Current

Magnetic Effects of Electric Current

• When electric current flows through a current-carrying conductor, it produces a magnetic field around it. This can be seen with the help of a magnetic needle, which shows deflection. The more the current, the higher the deflection.
• If the direction of the current in the circuit is reversed, the direction of deflection of the magnetic needle is also reversed.

The magnetic needle experiences deflection only if there is an electric current flowing through the wire.

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Lightning and magnetism

• During thunderstorms, air currents move upwards, and water droplets move downwards. This causes the separation of charges between clouds and between clouds and the Earth.
• When the magnitude of charges increases, air (normally a bad conductor) starts conducting and allows the flow of electricity. This is called lightning, as this flow of charge is accompanied by bright streaks of light and sound.
•  Lightning also makes naturally occurring magnets called lodestones.

Electro Magnet

Electromagnets

• An electromagnet is an artificial magnet which produces a magnetic field around a conductor due to the passage of electric current through it.
• This magnetic field disappears when there is no current flowing through the conductor.
• There are differences between electromagnets and permanent magnets.

For more information on Electromagnetism, watch the below video

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Difference Between Electromagnets and Permanent Magnets

• Permanent magnets maintain their magnetism for long periods of time
• Electromagnets have their own magnetic field as long as current is flowing through them. Once the circuit is broken, they lose their magnetism.

Electric Bell

Hazards of Electricity

• Electricity can be hazardous as it can cause a fire due to heating or loss of life.
• Electric circuits must be properly insulated. If the insulation breaks and wires come in contact with each other, they can cause a short circuit, which may give electric shocks when touched.

Electric bell

• An electric bell consists of an electromagnet. The switch we press is used to make the circuit complete.
• The working of the bell is illustrated below. When the switch (K) is pressed, current flows through the circuit.
• The electromagnet (E) is powered and generates a magnetic field that attracts the iron strip towards it.
• The striker strikes the gong or bell (B).
• When the striking arm (A) strikes the gong, the contact at (T) is broken, and the current stops flowing through the circuit.
• This causes the electromagnet to lose its magnetic field
• The connected spring arm returns the striker to its original rest position
• The contact is restored, and current flows through the circuit (provided the main switch is still pressed)
• The process is repeated again until the switch (K) is made open.

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