Electricity: Magnetic and Heating Effects
Complete, concept-first notes covering every main idea in the new NCERT book—magnetic effect, electromagnets, heating effect, and the science of cells and batteries.
Chapter at a Glance
Electric current can do more than light a bulb. It produces a magnetic field around a wire, can turn a coil into a powerful temporary magnet, and produces heat in a conductor. Cells and batteries supply this current through chemical reactions.
Mind Map: One Look Revision
Magnetic &
Heating Effects
Sticky Notes: Must Remember
⚡ Current + wire
A current-carrying wire produces a magnetic field around it. A nearby compass needle deflects.
🧲 Temporary magnet
An electromagnet works only while current flows. Switch current OFF, and its magnetic effect disappears.
🔥 Heat is due to resistance
Nichrome has more resistance than copper of the same size and length, so it gets hotter.
🔋 A cell is chemical
A cell/battery generates current because chemical reactions occur inside it.
↕ More turns / more current
Both make an electromagnet stronger. Reversing current reverses its poles.
♻ Recharge, reuse
Rechargeable batteries can be used many times, but they also gradually wear out.
1. Magnetic Effect of Electric Current
When electric current flows through a conducting wire, it produces a magnetic field around the wire. This is called the magnetic effect of electric current.
Current-carrying wire → magnetic field around it → compass needle deflects
Place a magnetic compass below a straight wire in a circuit. When the switch is ON, current flows and the compass needle moves away from its original north–south direction. When the switch is OFF, the needle returns. This shows that the wire has produced a magnetic effect.
2. Electromagnets and Lifting Cranes
A current-carrying coil behaves like a magnet. Such a coil is called an electromagnet. In practical electromagnets, a soft iron core is placed inside the coil to make the magnet much stronger.
An electromagnet has two poles—North and South—just as a bar magnet does. Unlike a permanent magnet, it is a controllable and temporary magnet.
Insulated wire coil wrapped around an iron nail/core
How can we make an electromagnet stronger?
If the direction of current through the coil is changed, the North and South poles of the electromagnet get reversed.
Lifting Electromagnets
Strong electromagnets are suspended from cranes in factories and scrap yards. When current is switched ON, the electromagnet attracts and lifts iron or steel objects. When current is switched OFF, its magnetic field disappears and the objects are released. This makes sorting and moving heavy metal safe and efficient.
3. Heating Effect of Electric Current
When current flows through a conductor, it faces opposition to its flow. This opposition is called resistance. Because of resistance, electrical energy is converted into heat. This production of heat in a conductor due to current is the heating effect of electric current.
Nichrome gets hot easily
Nichrome offers higher resistance than a copper wire of the same length and thickness. Therefore, it becomes hotter when current passes through it.
Where heat is used
Electric room heater, stove, kettle, iron, immersion rod and hair dryer use the heating effect of current.
The heating effect is also used in high-temperature electric furnaces to melt and recycle scrap steel. However, unwanted heating in wires causes energy loss. Overheating can melt plastic parts of plugs and sockets and may cause fires.
4. How Do Batteries Generate Electricity?
A cell or battery is a device that generates electric current because of chemical reactions inside it. It is a portable source of electricity: it can light an LED, heat a wire or operate an electromagnet.
4.1 Voltaic (Galvanic) Cell
A Voltaic cell contains two different metal rods, called electrodes, partly dipped in an electrolyte—usually a weak acid or salt solution. A chemical reaction between the electrodes and electrolyte produces electricity. When its chemicals are used up, the cell becomes dead and cannot supply current.
4.2 Lemon Cell
A lemon can act as an electrolyte. Place a copper wire and an iron nail in a lemon without letting them touch. Connect several such lemon cells in series to light an LED. The copper wire and iron nail are electrodes; lemon juice is the electrolyte. An LED glows only when it is connected with the correct polarity: its longer wire to the positive terminal and shorter wire to the negative terminal.
4.3 Dry Cells
In a dry cell, the electrolyte is not a liquid; it is a thick moist paste. The zinc container forms the negative terminal. A carbon rod at the centre, covered by a metal cap, forms the positive terminal. A dry cell is generally a single-use cell.
4.4 Rechargeable Batteries
Rechargeable batteries can be charged and reused many times. They are used in watches, phones, cameras, laptops, tablets, inverters and electric vehicles. They reduce waste and save money, although their ability to hold charge slowly decreases after repeated use. Lithium-ion batteries are common rechargeable batteries; recycling used batteries is important.
Important Definitions
Magnetic effect of current
Production of a magnetic field around a conductor when electric current flows through it.
Electromagnet
A current-carrying coil that behaves like a magnet, usually made stronger using an iron core.
Heating effect of current
Generation of heat in a conductor due to the flow of electric current.
Resistance
The opposition offered by a conductor to the flow of electric current.
Voltaic / Galvanic cell
A cell in which chemical reaction between electrodes and an electrolyte produces electricity.
Electrolyte
A liquid or paste-like material in a cell that helps the chemical reaction and conduction of electricity.
Electrodes
The two conducting materials in a cell through which current enters or leaves the cell.
Dry cell
A single-use cell with a thick moist paste as electrolyte instead of a liquid electrolyte.
Rechargeable battery
A battery that can be charged and used repeatedly, though it wears out gradually.
Important Tables
Electromagnet vs Permanent Magnet
| Feature | Electromagnet | Permanent magnet |
|---|---|---|
| Magnetism | Present only while current flows | Present without electric current |
| Strength | Can be changed by current and number of coil turns | Usually fixed |
| Poles | Can be reversed by reversing current | Cannot be easily reversed |
| Example use | Lifting crane, electric bell, motor | Compass, refrigerator magnet |
Types of Cells and Batteries
| Type | Main parts / electrolyte | Key point | Example |
|---|---|---|---|
| Voltaic cell | Two different metal electrodes + liquid electrolyte | Chemical reaction produces current | Lemon cell |
| Dry cell | Zinc container, carbon rod, paste electrolyte | Portable and generally single-use | Clock / torch cell |
| Rechargeable battery | Special chemical materials | Can be recharged and reused many times | Phone, laptop, inverter, EV |
Quick Cause–Effect Table
| Change made | Effect observed |
|---|---|
| Switch ON near a compass | Compass needle deflects because current produces a magnetic field. |
| Iron core inserted in a current-carrying coil | Electromagnet becomes stronger. |
| More current or more coil turns | Electromagnet becomes stronger and attracts more clips. |
| Direction of current reversed | Poles of electromagnet reverse. |
| Current passed through nichrome | Wire becomes hot due to its relatively high resistance. |
Important Question Answers
1. How can you show that a current-carrying wire has a magnetic effect?
Place a magnetic compass near a wire connected in a circuit. On switching ON, the compass needle deflects. When the current is switched OFF, it returns to its original position. The deflection proves that the current-carrying wire produces a magnetic field.
2. What is an electromagnet? Why is an iron core used in it?
An electromagnet is a current-carrying coil that behaves like a magnet. An iron core is inserted because it makes the electromagnet much stronger, allowing it to attract more magnetic material.
3. State two ways to increase the strength of an electromagnet.
(i) Increase the electric current through the coil. (ii) Increase the number of turns of wire in the coil. Inserting an iron core also makes it stronger.
4. How can the poles of an electromagnet be reversed?
Reverse the direction of electric current flowing through the coil. The North pole becomes South and the South pole becomes North.
5. Explain the working of a lifting electromagnetic crane.
When current is switched ON through its coil, the iron core becomes a strong electromagnet and lifts iron or steel objects. When the current is switched OFF, the magnetic field disappears and the objects are released.
6. Why does a nichrome wire become hotter than a copper wire of the same length and thickness?
Nichrome offers greater resistance to the flow of current than copper. Therefore, more electrical energy is converted into heat in nichrome.
7. Give four appliances that use the heating effect of current.
Electric iron, room heater, electric kettle and immersion rod. An electric stove and hair dryer are other examples.
8. Why can overheating in a household circuit be dangerous?
Overheating wastes energy and can damage plugs and sockets by melting plastic parts. It may also create a fire risk. Correctly rated wires, plugs and sockets help prevent such problems.
9. What is a Voltaic cell? Name its essential parts.
A Voltaic (Galvanic) cell generates electricity by a chemical reaction. It has two different metal electrodes and an electrolyte, usually a weak acid or salt solution.
10. Explain a lemon cell.
Insert a copper wire and an iron nail separately into a lemon. They act as electrodes and lemon juice acts as electrolyte. Joining several lemon cells can provide enough current to glow an LED when connected with the correct polarity.
11. Write the structure of a dry cell.
A dry cell has a zinc container as the negative terminal, a carbon rod in the centre with a metal cap as the positive terminal, and a thick moist paste electrolyte around the carbon rod.
12. Why are rechargeable batteries preferred for many applications?
They can be charged and reused many times. This reduces waste and can save money over time. They are used in devices such as phones, laptops, cameras, inverters and electric vehicles.
13. HOTS: A lifting electromagnet has been left ON. After some time it cannot lift clips, but its wire is warm. Give possible reasons.
The cell/battery may have weakened because it has been supplying current for a long time. The remaining current may be enough to warm the wire but too small to produce a strong magnetic field. A loose connection or damaged coil may also reduce the electromagnet’s strength.
Mini Quiz
Attempt first. Then open the answer key.
Show answer key
60-Second Final Revision
- Current in a wire produces a magnetic field.
- A current-carrying coil is an electromagnet; an iron core makes it stronger.
- More current and more turns make an electromagnet stronger; reverse current to reverse poles.
- Resistance causes the heating effect. Nichrome has higher resistance than copper.
- Cells and batteries produce current through chemical reactions.
- Voltaic cell: two electrodes + electrolyte. Dry cell: paste electrolyte. Rechargeable battery: charge and reuse.
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