Class 8 Science NCERT Notes – Chapter 4: Electricity-Magnetic and Heating Effects (PDF, MindMap, Q&A, Quizzes) | EKADEMY | Online Courses, Live Classes, Lessons & Quizzes

Here are detailed study notes based on the NCERT (Curiosity) Class 8 Science Chapter 4: Electricity: Magnetic and Heating Effects, along with some practice questions and answers for exam preparation.

Magnetic Effect of Electric Current

When current passes through a wire, it creates a magnetic field around it.
This magnetic field can deflect a nearby magnetic compass needle.
The deflection disappears once the current stops.
This proves that electric current produces magnetism.
Discovery credited to Hans Christian Oersted in 1820, linking electricity and magnetism.
Magnetic field exists even if non-magnetic materials are between wire and compass.

Electromagnets

A current-carrying coil of wire wrapped around an iron core acts like a magnet.
This setup is called an electromagnet.
The electromagnet is strong only when current flows.
Increasing either current or coil turns increases the magnet’s strength.
Poles of an electromagnet can be reversed by reversing the current direction.
Used in many practical devices: electric bells, motors, and lifting electromagnets.

Experiment with Electromagnets

Wrap insulated wire around an iron nail to form a coil.
Connect coil to a battery; nail attracts iron clips.
Disconnect battery; nail loses magnetism, clips fall.
Adding an iron core (nail) makes coil magnet stronger.
Magnetic compasses near the coil deflect indicating magnetic poles.

Polarity of Electromagnets

Electromagnets have two poles (North and South).
When a compass’ north pole points toward one end of coil, that end acts as the south pole.
Opposite ends have opposite poles.
The number of battery cells and coil turns affect strength and compass needle deflection.

Lifting Electromagnets

Strong electromagnets used in cranes to lift heavy iron and steel.
Operator controls magnet by switching current ON and OFF.
When ON, magnet lifts objects; when OFF, objects are released.
Widely used in scrap yards and factories.

Heating Effect of Electric Current

Electric current flowing through a conductor produces heat.
This happens due to the resistance that conductor offers to current flow.
Example: Nichrome wire heats up and glows when current flows.
More current or longer duration means more heat.
Heating effect used in household heating devices like irons, heaters, kettles, and stoves.

Safety with Heating Effects

Too much heating can cause damage to wires, plugs, sockets, and may cause fires.
Use wires and switches rated for specific current ratings.
Safety devices (fuses, circuit breakers) are installed to prevent hazards.
Overheating in transmission lines causes energy loss.

Batteries and Cells: How Electricity is Generated

Voltaic Cell: Consists of two metal plates in a liquid electrolyte producing electric current by chemical reaction.
Over time, chemicals get used up; the cell becomes “dead.”
Dry Cell: Contains paste-like electrolyte; commonly used in household devices.
Rechargeable Batteries: Can be recharged and reused multiple times. Common types include lithium-ion batteries.
Eco-friendly disposal and recycling of batteries are essential to avoid environmental hazards.

Making a Voltaic Cell

Can be made using fruit (lemons), copper strips, and iron nails.
The electrolyte is lemon juice.
Multiple cells connected in series can power LED bulbs or small devices.

These notes simplify key concepts and experimental observations from the textbook, making revision smooth and effective.

Important Points to Remember

The magnetic effect of electric current and thermal effect are fundamental phenomena.
Electromagnets work only when current flows and can be switched ON and OFF.
Heating effect of current is harnessed for useful appliances but requires safety.
Batteries generate electricity through chemical reactions and have finite life.
Rechargeable batteries are important for sustainable energy.

Practice Questions and Answers

Q1. What happens to a compass needle when placed near a current-carrying wire? Why?
A1: The compass needle deflects because the electric current in the wire produces a magnetic field around the wire, exerting a force on the magnetic compass needle.

Q2. Define an electromagnet. How can the strength of an electromagnet be increased?
A2: An electromagnet is a magnet created by passing electric current through a coil of wire around a soft iron core. Its strength can be increased by increasing the current, increasing the number of turns in the coil, or inserting a soft iron core in the coil.

Q3. Explain the heating effect of electric current with an example.
A3: When electric current passes through a conductor, its resistance converts some electrical energy into heat. For example, the filament in an electric bulb heats up and glows due to this heating effect.

Q4. What are lifting electromagnets and where are they used?
A4: Lifting electromagnets are strong electromagnets used to lift heavy metal objects in factories and scrap yards. They are switched ON to lift metal and OFF to release it.

Q5. Why is nichrome wire preferred in electrical heating appliances?
A5: Nichrome wire is preferred because it has high resistance, generates more heat for a given current, and does not oxidize easily at high temperatures.

Q6. What precautions should be taken when using electrical heating appliances?
A6: Use appropriate-rated wires, plugs, and sockets to prevent overheating; avoid prolonged use without supervision; and ensure safety devices like fuses are in place to prevent fire hazards.

Practice Questions

What happens to the compass needle when brought near a current-carrying wire? Why?
Define electromagnet. How can the strength of an electromagnet be increased?
Mention two applications of electromagnets.
Why does a nichrome wire get heated when current flows through it?
What safety measures should be taken with electric heating appliances?
How does a Voltaic cell produce electricity?
What is the difference between a dry cell and a rechargeable battery?
Why do some batteries stop working after repeated charging and discharging?
Explain why lifting electromagnets are useful in scrap yards.
How does reversing the current direction affect an electromagnet?

Here are 20 multiple-choice questions (MCQs) based on the Chapter 4: Electricity – Magnetic and Heating Effects for Class 8 Science, along with an answer key.

Multiple Choice Questions (MCQs)

What happens to a compass needle when placed near a current-carrying wire?
(a) It remains unaffected
(b) It gets deflected
(c) It spins continuously
(d) It loses its magnetism
Who discovered the magnetic effect of electric current?
(a) Alessandro Volta
(b) Michael Faraday
(c) Hans Christian Oersted
(d) Thomas Edison
What is an electromagnet?
(a) A permanent magnet
(b) A magnet made by rubbing two metals
(c) A current-carrying coil of wire around an iron core
(d) A natural magnet found in rocks
Which factor increases the strength of an electromagnet?
(a) Reducing number of coil turns
(b) Increasing electric current
(c) Removing the iron core
(d) Using fewer coils
What will happen if the electric current in an electromagnet is switched OFF?
(a) It becomes stronger
(b) It behaves like a permanent magnet
(c) It loses its magnetic effect
(d) No change occurs
What is the shape of the coil used to make an electromagnet?
(a) Rectangular coil
(b) Cylindrical coil
(c) Spiral with no turns
(d) Square coil
What is the result when the direction of current in an electromagnet is reversed?
(a) It stops working
(b) Poles of the electromagnet reverse
(c) It catches fire
(d) It becomes a permanent magnet
Nichrome wire is preferred in electric heaters because it:
(a) Conducts electricity with no resistance
(b) Has high resistance and produces heat
(c) Is a good insulator
(d) Is cheaper than copper wire
The heating effect in a wire depends on:
(a) Colour of wire
(b) Length and thickness of wire
(c) Shape of wire
(d) Temperature outside the wire
Which of the following appliances does NOT work on the heating effect of electric current?
(a) Electric kettle
(b) Electric bell
(c) Electric stove
(d) Electric iron
In a Voltaic cell, electricity is generated due to:
(a) Friction between electrodes
(b) Chemical reactions between electrodes and electrolyte
(c) Magnetic field of earth
(d) Heat produced in the cell
Dry cells have:
(a) Liquid electrolyte
(b) Thick moist paste as electrolyte
(c) No electrolyte
(d) Electrolyte in solid form
Rechargeable batteries can be:
(a) Used once only
(b) Recharged and reused many times
(c) Damaged by charging
(d) Used without charging
Which metal pairs are commonly used in Voltaic cells?
(a) Copper and iron
(b) Zinc and copper
(c) Aluminum and glass
(d) Silver and wood
What does a lifting electromagnet do when the current is ON?
(a) Releases heavy objects
(b) Lifts iron/steel objects
(c) Acts as a heater
(d) Releases heat only
The magnetic field produced by a current-carrying wire exists:
(a) Only on one side of the wire
(b) Around the wire in all directions
(c) Only at the ends of the wire
(d) None of the above
What happens when an iron nail is inserted inside a current-carrying coil?
(a) The coil becomes weaker
(b) The coil becomes a stronger magnet
(c) The coil stops working
(d) The nail becomes an electromagnet
Which wire would heat up more for the same current applied?
(a) Thick wire
(b) Thin wire
(c) Wire with low resistance
(d) Wire with no resistance
Why are rechargeable Li-ion batteries important?
(a) They can be disposed of freely
(b) They are expensive but last only once
(c) They can be recharged, reused, and reduce waste
(d) They do not store energy
What is the main safety device used in electrical household circuits to prevent overheating and fire?
(a) Fuse
(b) Resistor
(c) Capacitor
(d) Transformer

MCQs Answer Key

(b) It gets deflected
(c) Hans Christian Oersted
(c) A current-carrying coil of wire around an iron core
(b) Increasing electric current
(c) It loses its magnetic effect
(b) Cylindrical coil
(b) Poles of the electromagnet reverse
(b) Has high resistance and produces heat
(b) Length and thickness of wire
(b) Electric bell
(b) Chemical reactions between electrodes and electrolyte
(b) Thick moist paste as electrolyte
(b) Recharged and reused many times
(b) Zinc and copper
(b) Lifts iron/steel objects
(b) Around the wire in all directions
(b) The coil becomes a stronger magnet
(b) Thin wire
(c) They can be recharged, reused, and reduce waste
(a) Fuse

These MCQs cover key concepts, applications, and experiments from the chapter to aid revision and self-assessment.