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Electricity: Magnetic and Heating Effects

NCERT Class 8 · Science Based on NCERT Class 8 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter explores two fundamental effects of electric current: the magnetic effect and the heating effect. It explains how a current-carrying wire behaves like a magnet (electromagnetism) and how resistance in a wire generates heat. The one big idea a student must walk away with is that electric current is not just a flow of charge—it can produce both magnetism and heat, which are harnessed in countless devices from electromagnets to electric irons.
Key Concepts (13)
Magnetic effect of electric current
When electric current flows through a conductor (like a wire), it produces a magnetic field around it. This phenomenon is known as the magnetic effect of electric current.
e.g. When you switch on a current in a wire placed near a magnetic compass, the compass needle deflects, showing that a magnetic field is produced.
Magnetic field
The region around a magnet or a current-carrying wire where its magnetic effect can be felt, such as by the deflection of a compass needle.
e.g. The space around a bar magnet or a wire carrying current where a compass needle moves is called its magnetic field.
Electromagnet
A current-carrying coil that behaves as a magnet is called an electromagnet. For practical applications, most electromagnets have an iron core to make them stronger.
e.g. A nail wrapped with insulated wire and connected to a cell becomes an electromagnet that can pick up iron paper clips.
Heating effect of electric current
When an electric current passes through a conductor, it gets heated. This warming is known as the heating effect of electric current.
e.g. A nichrome wire becomes warm when current flows through it, as seen in an electric heater or a hair dryer.
Resistance
The opposition to the flow of electric current offered by a conductor. Different conductors offer different levels of resistance.
e.g. A nichrome wire offers higher resistance compared to a copper wire of the same size and length, so it heats up more.
Heating element
A rod or coil of wire in an electrical appliance that converts electrical energy into heat due to the heating effect of electric current.
e.g. The coil inside an electric iron or the filament in an incandescent lamp is a heating element.
Voltaic cell (Galvanic cell)
A cell containing two metal rods (electrodes) made of different materials and a liquid called an electrolyte, placed in a glass or plastic container. A chemical reaction between the rods and the electrolyte produces electricity.
e.g. A lemon cell made with a copper wire and an iron nail in lemon juice is a simple Voltaic cell.
Electrolyte
A liquid (usually a weak acid or salt solution) in a Voltaic cell that helps conduct electricity through chemical reactions.
e.g. Lemon juice or salt solution acts as an electrolyte in a homemade cell.
Electrodes
The metal rods in a Voltaic cell that are partly dipped in the electrolyte. They are made of different materials and act as positive and negative terminals.
e.g. In a lemon cell, the copper wire and iron nail are the electrodes.
Dry cell
A widely used electric cell where the electrolyte is a thick moist paste, not a liquid. It consists of a zinc container (negative terminal) and a carbon rod (positive terminal).
e.g. The common cylindrical battery used in a torch or remote control is a dry cell.
Rechargeable battery
A battery that can be recharged and reused multiple times, preventing wastage and saving money.
e.g. The battery in a mobile phone, laptop, or electric vehicle is a rechargeable battery.
Lifting electromagnet
A strong electromagnet hung from a crane that can lift and release iron/steel objects by switching the current ON and OFF.
e.g. In a scrap yard, a crane uses a lifting electromagnet to move heavy metal items.
Nichrome wire
A special kind of wire that offers high resistance and is commonly used in electrical heating devices.
e.g. The wire that glows red hot in an electric heater is made of nichrome.
How It Works
Demonstrating the magnetic effect of electric current (Oersted's experiment)
  1. Set up a circuit with a cell, a switch, and a long wire stretched between two nails on a cardboard.
  2. Place a magnetic compass beneath the wire between the nails.
  3. Close the switch (ON position) to allow current to flow through the wire.
  4. Observe the compass needle: it deflects from its original direction.
  5. Open the switch (OFF position) to stop the current.
  6. Observe the compass needle: it returns to its original direction.
  7. Repeat to confirm that deflection occurs only when current flows.
💡 The compass needle deflects only when current is flowing; it returns to its original position when current stops. This proves the magnetic effect is temporary and linked to current.
Making an electromagnet
  1. Take an iron nail and tightly wrap about 50 cm of insulated wire around it in the form of a coil.
  2. Secure the coil with adhesive tape.
  3. Connect the ends of the wire to a cell (for a few seconds only).
  4. Bring the nail near iron paper clips; the clips will cling to the nail.
  5. Disconnect the wire from the cell to stop the current.
  6. The clips will fall off, showing the magnetic effect disappears.
💡 The electromagnet works only when current flows. Using more cells or more turns of wire makes it stronger. The iron core (nail) increases the strength significantly.
Identifying poles of an electromagnet
  1. Label the two ends of the coil as A and B.
  2. Place a magnetic compass near end A and connect the coil to a cell.
  3. Note which pole of the compass (North or South) is attracted to end A.
  4. Since unlike poles attract, if North pole is attracted, end A is South pole.
  5. Repeat for end B; its polarity will be opposite to end A.
💡 An electromagnet has two poles (North and South) just like a bar magnet. Reversing the direction of current reverses the poles.
Demonstrating the heating effect of electric current
  1. Take a cardboard piece, mount two nails about 5 cm apart.
  2. Tie a nichrome wire between the nails.
  3. Connect the circuit with a cell and a switch in OFF position.
  4. Touch the nichrome wire initially (it feels cool).
  5. Move the switch to ON position for about 30 seconds, then OFF.
  6. Momentarily touch the nichrome wire again; it feels warm.
  7. Repeat to confirm that current causes heating.
💡 The wire heats up because of resistance. More cells (higher current) produce more heat. Do not hold the wire for long to avoid burns.
Making a Voltaic cell using a lemon
  1. Take a lemon and insert a copper wire/strip and an iron nail, keeping them apart.
  2. Repeat with 5-6 lemons.
  3. Join the copper wire of one lemon to the iron nail of the next lemon in series.
  4. Connect an LED between the copper wire of the first lemon and the iron nail of the last lemon.
  5. If the LED does not glow, reverse its connections.
  6. A glowing LED indicates the cell is working.
💡 The lemon juice acts as the electrolyte. The copper and iron are the electrodes. The cell produces electricity through chemical reactions.
How a dry cell works
  1. The dry cell has a zinc container (negative terminal) and a carbon rod (positive terminal).
  2. The space between them is filled with a thick moist paste (electrolyte).
  3. Chemical reactions between the zinc, carbon, and electrolyte produce electricity.
  4. When connected in a circuit, current flows from the positive terminal through the circuit to the negative terminal.
  5. Over time, chemicals get used up and the cell becomes 'dead'.
💡 Dry cells are single-use and cannot be recharged. Rechargeable batteries (like lithium-ion) can be reused multiple times.
Exam Traps — Don't Make These Mistakes
⚠ Confusing magnetic effect with heating effect
✗ When current flows through a wire, it only produces heat.
→ The student forgets that current also produces a magnetic field around the wire, which is a separate effect. Both effects occur simultaneously.
✓ When electric current flows through a conductor, it produces both a magnetic field (magnetic effect) and heat (heating effect). The magnetic effect is used in electromagnets, and the heating effect is used in electric irons.
⚠ Thinking electromagnet is a permanent magnet
✗ An electromagnet is a permanent magnet that always attracts iron.
→ An electromagnet is a temporary magnet; it behaves as a magnet only when electric current flows through it. When current stops, the magnetic effect disappears.
✓ An electromagnet is a current-carrying coil that behaves as a magnet only when current flows. It loses its magnetism when the current is switched off.
⚠ Believing all cells and batteries are rechargeable
✗ All cells and batteries can be recharged and reused.
→ Only rechargeable batteries (like lithium-ion batteries in phones) can be recharged. Dry cells (like in a torch) are single-use and cannot be recharged.
✓ Dry cells are single-use and cannot be recharged. Rechargeable batteries, such as those in mobile phones and laptops, can be recharged and reused multiple times.
⚠ Mixing up the roles of electrodes in a Voltaic cell
✗ Both electrodes in a Voltaic cell are made of the same metal.
→ A Voltaic cell requires two different metals as electrodes (e.g., copper and iron, or zinc and copper) to produce electricity through chemical reactions.
✓ A Voltaic cell contains two electrodes made of different materials, such as copper and iron, dipped in an electrolyte. The difference in their chemical properties generates electric current.
⚠ Assuming the compass needle deflects due to heat
✗ The compass needle deflects because the wire becomes hot.
→ The deflection of the compass needle is due to the magnetic field produced by the current, not due to heat. Heat does not affect a compass needle.
✓ The compass needle deflects because the current-carrying wire produces a magnetic field around it. This magnetic field interacts with the needle, causing deflection.
⚠ Thinking more turns always mean stronger electromagnet regardless of current
✗ Increasing the number of turns always makes an electromagnet stronger, even with the same cell.
→ While increasing turns does increase strength, the current also matters. With a single cell, there is a limit. Using more cells (higher current) also increases strength. Both factors work together.
✓ The strength of an electromagnet can be increased by increasing the number of turns of the coil or by increasing the electric current (using more cells), or both.
Compare & Contrast
Nature of magnetism
Electromagnet
Permanent bar magnet
An electromagnet is a temporary magnet that works only when electric current flows through it, and its strength and poles can be changed. A permanent bar magnet always has magnetism and cannot be switched off or have its poles reversed easily.
Reusability
Dry cell
Rechargeable battery
A dry cell is a single-use cell; once its chemicals are used up, it becomes 'dead' and must be disposed of. A rechargeable battery can be recharged and reused multiple times, saving money and reducing waste.
Resistance and heating
Nichrome wire
Copper wire
Nichrome wire offers higher resistance than copper wire of the same size and length, so it heats up more when current flows. Copper wire is a better conductor and is used for connections, not for heating elements.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks)
Most asked: Magnetic effect of electric current and Oersted's experiment · Electromagnet: construction, working, and factors affecting strength · Heating effect of electric current and its applications · Voltaic cell and dry cell: structure and working · Difference between electromagnet and permanent magnet · Rechargeable batteries and their importance
Don't skip: Describe an activity to show the magnetic effect of electric current (Oersted's experiment) – 5 marks; Explain how an electromagnet is made and list two ways to increase its strength – 3 marks; Differentiate between an electromagnet and a permanent magnet – 3 marks; Explain the heating effect of electric current with an activity – 3 marks; Describe the structure and working of a dry cell – 3 marks
One-Line Revision (read aloud before the exam)
  1. When electric current flows through a wire, it produces a magnetic field around it—this is the magnetic effect of electric current.
  2. A current-carrying coil that behaves like a magnet is called an electromagnet.
  3. An electromagnet is a temporary magnet; it works only when current flows and loses its magnetism when current stops.
  4. The strength of an electromagnet increases with more turns of wire or more electric current (more cells).
  5. An electromagnet has two poles (North and South), and its poles can be reversed by reversing the direction of current.
  6. When electric current passes through a conductor, it gets heated—this is the heating effect of electric current.
  7. Nichrome wire has high resistance and is used in heating elements like electric irons and heaters.
  8. A Voltaic cell produces electricity through chemical reactions between two different metal electrodes and an electrolyte.
  9. A dry cell has a zinc container (negative terminal) and a carbon rod (positive terminal) with a moist paste electrolyte.
  10. Dry cells are single-use; rechargeable batteries (like lithium-ion) can be recharged and reused.
  11. Lifting electromagnets are used in cranes to lift and move heavy iron/steel objects in factories and scrap yards.
  12. The heating effect can cause problems like overheating in wires; safety devices are used to prevent fires.
  13. Earth's magnetic field is generated by movement of liquid iron in its core and protects life from harmful space particles.
  14. A magnetic compass needle deflects when brought near a current-carrying wire because of the magnetic field produced.

MCQs — 10 Questions with Answers

Q1. When electric current flows through a wire, what magnetic effect is observed using a compass?

  • A. The compass needle deflects from its original direction ✓
  • B. The compass needle does not move at all
  • C. The compass needle moves only if the wire is made of iron
  • D. The compass needle breaks

Answer: A — An electric current creates a magnetic field around the wire, which causes the compass needle to deflect because the compass needle is a tiny magnet sensitive to magnetic fields.

Q2. A coil of wire is wound tightly around an iron nail. When current flows through the coil, the nail can pick up iron filings. What is this device called?

  • A. A permanent magnet
  • B. An electromagnet ✓
  • C. A magnetic compass
  • D. A magnetic field

Answer: B — A current-carrying coil that behaves as a magnet is called an electromagnet, especially when it has an iron core to make it stronger.

Q3. In the activity where a compass was placed near the ends of an electromagnet, it was observed that the north pole of the compass was attracted to end A. What does this tell us about end A?

  • A. End A is the North pole of the electromagnet
  • B. End A is the South pole of the electromagnet ✓
  • C. End A has no magnetic effect
  • D. End A is made of iron

Answer: B — Unlike magnetic poles attract each other; since the north pole of the compass is attracted to end A, end A must be the South pole of the electromagnet.

Q4. What happens to an electromagnet made from a coil of wire wrapped around an iron nail when the electric current is switched off?

  • A. The nail becomes a permanent magnet
  • B. The electromagnet loses its magnetic effect immediately ✓
  • C. The electromagnet continues to attract iron objects for several hours
  • D. The nail breaks into pieces

Answer: B — Unlike a permanent magnet, an electromagnet only attracts objects while current flows through it; switching off the current stops the magnetic field and the electromagnet loses its attractive force.

Q5. A student made two electromagnets: Electromagnet X with 20 turns of wire and Electromagnet Y with 50 turns of wire. Both use the same iron nail and are connected to the same battery. Which electromagnet will be stronger?

  • A. Electromagnet X will be stronger
  • B. Electromagnet Y will be stronger ✓
  • C. Both will have equal strength
  • D. Neither will have any magnetic effect

Answer: B — The strength of an electromagnet increases with the number of turns of the coil; therefore, Electromagnet Y with 50 turns will produce a stronger magnetic field than Electromagnet X with 20 turns.

Q6. In a scrap yard in Delhi, a large crane uses a lifting electromagnet to move steel plates. Why is the electromagnet more useful than a permanent magnet for this job?

  • A. Because the permanent magnet is more expensive
  • B. Because the electromagnet can be turned ON and OFF by controlling the electric current ✓
  • C. Because the electromagnet never loses its magnetic properties
  • D. Because the electromagnet does not need any wiring

Answer: B — The electromagnet can be controlled by switching the current ON to lift the steel plates and OFF to release them, making it practical for sorting and moving heavy metal items repeatedly.

Q7. A school science exhibition shows an electromagnet that can pick up 10 iron paper clips when connected to a single cell. If the same coil is connected to a battery with 3 cells instead, what change would you expect?

  • A. The electromagnet would pick up fewer clips
  • B. The electromagnet would pick up more clips ✓
  • C. The electromagnet would pick up the same number of clips
  • D. The electromagnet would not work at all

Answer: B — A battery with more cells provides a larger electric current; a larger current creates a stronger magnetic field, allowing the electromagnet to attract more iron paper clips.

Q8. Student A says that an electromagnet and a bar magnet are the same because both have North and South poles. What is the key difference that Student A has overlooked?

  • A. A bar magnet has two poles while an electromagnet has only one pole
  • B. An electromagnet loses its magnetic effect when current stops, but a bar magnet remains magnetic permanently ✓
  • C. A bar magnet cannot be made stronger, but an electromagnet can
  • D. An electromagnet is always weaker than a bar magnet

Answer: B — The critical difference is that an electromagnet is temporary and depends on electric current to function, whereas a bar magnet is permanent and retains its magnetic properties indefinitely.

Q9. In Activity 4.3, when an iron nail was inserted into the cylindrical coil of wire, the deflection of the magnetic compass needles increased significantly. Which of the following best explains why?

  • A. The iron nail reduced the amount of current flowing through the coil
  • B. The iron nail concentrated and amplified the magnetic field produced by the current-carrying coil ✓
  • C. The iron nail changed the direction of the electric current
  • D. The iron nail made the coil conduct electricity better

Answer: B — The iron core acts as a material that gets magnetized by the coil's magnetic field and amplifies it, creating a much stronger electromagnet than the coil alone.

Q10. A scientist wants to create an electromagnet that can reverse its poles without changing the coil or the battery. Based on your understanding of electromagnets, what must the scientist change?

  • A. The thickness of the wire
  • B. The material of the iron core
  • C. The direction of the electric current flowing through the coil ✓
  • D. The distance between the coil and the compass

Answer: C — Reversing the direction of electric current through the coil reverses the direction of the magnetic field, which causes the poles of the electromagnet to reverse.

Flashcards

What happens to a magnetic compass needle when electric current flows through a nearby wire?

The compass needle deflects from its original direction because the current-carrying wire produces a magnetic field.

Define magnetic field.

The region around a magnet or current-carrying wire where its magnetic effect can be felt, such as by deflection of a compass needle.

What is the name of the phenomenon where electric current produces a magnetic effect?

The phenomenon is called the magnetic effect of electric current.

What is an electromagnet?

A current-carrying coil that behaves as a magnet and can attract iron and steel objects while current flows.

How many poles does an electromagnet have?

An electromagnet has two poles — North and South — just like a bar magnet.

What happens to an electromagnet when the electric current is switched off?

The electromagnet loses its magnetic effect and no longer attracts iron or steel objects.

Name two factors that affect the strength of an electromagnet.

The strength depends on the amount of electric current flowing through the coil and the number of turns of the coil.

What is a lifting electromagnet used for?

Lifting electromagnets are used in factories and scrap yards to lift, move, and sort heavy metal items by switching current ON and OFF.

Who discovered the magnetic effect of electric current and in what year?

Hans Christian Oersted, a Danish scientist, discovered it in 1820.

How can the polarity of an electromagnet be reversed?

By changing the direction of the electric current flowing through the coil.

Important Board Questions

What is the magnetic effect of electric current? [1 mark]

Define: when current flows through conductor, magnetic field is produced around it. This phenomenon is called magnetic effect of electric current.

Describe in brief what happens when the electric current is switched OFF in an electromagnet. How is this property useful in lifting electromagnets used in scrap yards? [2 marks]

When current stops, magnetic field disappears and electromagnet loses its magnetic effect. This allows the crane operator to release heavy metal objects at will by switching current off.

An electromagnet is made by wrapping insulated wire around an iron nail and connecting it to an electric cell. List three ways in which the strength of this electromagnet can be increased. Explain one of them briefly. [3 marks]

Three ways: (1) increase number of turns in coil, (2) increase electric current (use more cells), (3) use better iron core material. Pick one and explain how it increases magnetic field strength.

Draw and label a diagram of an electromagnet setup using a coil of wire, an iron nail, and a magnetic compass. Describe an activity (Activity 4.4) that proves an electromagnet has two poles like a bar magnet. How can you reverse the polarity of an electromagnet without changing anything else? [5 marks]

Diagram must show: coil wound on iron nail, battery connections, compass placement. Activity 4.4: bring compass to each end and observe which pole is attracted (opposite poles at each end). Polarity reversal: change direction of current flow through coil.

Assertion-Reason Questions for Electricity: Magnetic and Heating Effects

Q1. Assertion (A):

A compass needle deflects when placed near a current-carrying wire.

Reason (R):

Electric current produces a magnetic field around the conductor, and this magnetic field exerts a force on the compass needle.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both the assertion and reason are true, and the reason correctly explains why the compass needle deflects. The magnetic field created by current is the direct cause of the needle's deflection, making this a complete and correct explanation.

Q2. Assertion (A):

Electromagnets are temporary magnets that can be switched on and off.

Reason (R):

The magnetic properties of electromagnets depend on the continuous flow of electric current through the coil.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true—electromagnets are indeed temporary and can be controlled, and current flow is necessary for their operation. However, the reason doesn't fully explain why they're temporary (the reason just states what's needed, not why they're temporary when current stops). The assertion is explained by current-dependence, but the reason is more of a condition than a complete explanation.

Q3. Assertion (A):

Inserting an iron nail into a current-carrying coil increases the strength of the magnetic field.

Reason (R):

Iron is a non-magnetic material that reduces electrical resistance in the coil.

Answer: C — A is true but R is false — The assertion is true—iron does amplify the magnetic field. However, the reason is false because iron is a ferromagnetic (magnetic) material, not non-magnetic, and its effect on magnetic field strength is not due to resistance reduction but due to its ability to be magnetized and amplify the field. The assertion is correct but the reason is scientifically incorrect.

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Case-Based Questions (CBQ) for Electricity: Magnetic and Heating Effects

Case Study:

A school science teacher demonstrated the magnetic effect of electric current to Class 8 students using Activity 4.3. She used a cardboard cylinder with 50 turns of insulated wire wrapped around it, connected to an electric cell with a switch. Initially, with no iron nail inside, she placed two magnetic compasses near the ends of the coil and closed the circuit. The compass needles showed slight deflection. Then she inserted an iron nail into the cylinder and repeated the same experiment with the same current. This time, the compass needles showed much stronger deflection, and when she brought iron paper clips near the ends, they were strongly attracted and clung to the nail.

Q1. Why did the compass needles show deflection even before the iron nail was inserted?

  • A. Because the coil itself produced a magnetic field when current flowed through it ✓
  • B. Because the cardboard cylinder is magnetic
  • C. Because the compass needles were defective and would deflect to any stimulus
  • D. Because the electric cell produces a magnetic field directly

Answer: A

Q2. What was the most significant difference in observation between the coil without and with the iron nail?

  • A. The magnetic field strength increased dramatically when the iron nail was inserted ✓
  • B. The direction of current changed when the iron nail was inserted
  • C. The compass needles stopped deflecting when the iron nail was inserted
  • D. The paper clips were repelled instead of attracted when the iron nail was inserted

Answer: A

Q3. If the teacher had reversed the direction of current through the coil with the iron nail, what would happen to the paper clips?

  • A. They would fall off immediately because current reversal stops the magnetic effect
  • B. They would remain clung to the nail because the magnetic field strength stays the same
  • C. They would remain clung to the nail because reversing current doesn't stop the magnetic field, just reverses its direction ✓
  • D. They would be repelled away from the nail with greater force

Answer: C

Q4. Which factor demonstrated in this activity is most important for creating a strong electromagnet for practical applications like cranes?

  • A. Using only cardboard cylinders without any metal core
  • B. Combining multiple turns of wire with a ferromagnetic core like iron ✓
  • C. Increasing the thickness of the wire used for the coil
  • D. Using a permanent magnet instead of an electric cell

Answer: B

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