- When electric current flows through a conductor (a wire), it produces a magnetic field around it — this is the magnetic effect of electric current (discovered by Hans Christian Oersted, 1820).
- A current-carrying coil that behaves like a magnet is an electromagnet; wrapping the coil around an iron core makes it much stronger. It has two poles (N and S) and works only while the current flows.
- Lifting electromagnets on cranes lift, move and sort heavy iron/steel in factories and scrap yards — switched ON/OFF to grab and release.
- When current passes through a conductor that offers resistance, electrical energy turns into heat — the heating effect of electric current. This runs heaters, irons, kettles, immersion rods, hair dryers and bulb filaments.
- A cell/battery (Voltaic cell, dry cell, rechargeable battery) makes current from a chemical reaction. A used-up cell is "dead"; rechargeable batteries can be recharged and reused.
- Exam weightage: ~4–5 marks — usually one electromagnet/heating-effect reasoning question plus MCQs on Oersted, nichrome and cells.
1. The big idea — three things current can do
You already know an electric circuit can light a bulb. This chapter shows current does three useful things, and asks where the current itself comes from:
- Magnetic effect — current makes a magnet (electromagnets, bells, motors, fans, loudspeakers).
- Heating effect — current makes heat (heaters, irons, kettles, bulbs).
- Chemical source — cells and batteries make current from chemical reactions.
The chapter opens with a science-exhibition model: an iron nail wrapped with wire and joined to a battery picks up paper clips when the circuit is closed, and drops them when it is opened — a magnet with no magnet inside, only current. That puzzle is what we now solve.
2. Does a current have a magnetic effect? (Oersted's discovery)
Place a magnetic compass (a tiny pivoted magnet) right under a straight wire that is part of a circuit. Watch the needle as you switch the current ON and OFF:
- Current ON → the needle gets deflected from its usual North–South rest direction.
- Current OFF → the needle returns to its original direction.
A compass needle only deflects when a magnetic field acts on it. So a current-carrying wire must be producing a magnetic field around itself. The region around a magnet or a current-carrying wire where its magnetic effect can be felt is called its magnetic field.
Danish professor Oersted noticed that whenever a nearby circuit was closed or opened, a magnetic compass beside it deflected. He confirmed that an electric current really does produce a magnetic field and published it. This was the first proof that electricity and magnetism are linked — the seed of motors, generators and much of modern technology.
3. Electromagnets — making a magnet with current
Wrap an insulated wire tightly into a coil (many turns) and pass current through it. The coil itself behaves like a magnet — it deflects compass needles at both ends and attracts iron paper clips. Stop the current and the magnetism vanishes.
Slide an iron nail (iron core) inside the coil and the magnet becomes much stronger: the compass deflects more and the coil grabs more clips. That is why, for real uses, most electromagnets have an iron core.
- Two poles: like a bar magnet, an electromagnet has a North and a South pole. Test with a compass — the end that attracts the compass's North pole is the South pole (unlike poles attract).
- Reversing current swaps the poles (N ↔ S).
This solves the opening puzzle: the exhibition model was an electromagnet — current through the coiled wire magnetised the iron nail, so it picked up clips; opening the circuit removed the magnetism, so the clips fell.
4. How to make an electromagnet stronger
The strength of an electromagnet can be increased (or its poles reversed) by changing what feeds the coil:
- More current — a battery of more cells gives a larger current → stronger field → more clips lifted. A single cell gives only a weak field.
- More turns — more turns of wire in the coil → stronger magnet.
- Iron core — an iron nail inside the coil greatly boosts the strength.
- Direction of current — reversing the current does not change the strength but reverses the poles (North and South swap).
Deep inside the Earth, moving liquid iron in the core creates electric currents, and these currents generate a magnetic field — the same electricity-makes-magnetism idea on a giant scale. Migratory birds, fish and animals use this field to navigate, and it also shields life from harmful particles from space.
5. Lifting electromagnets and other applications
Lifting electromagnets are strong electromagnets hung from cranes. The operator simply switches the current:
- Current ON → the electromagnet lifts iron/steel objects.
- Current OFF → magnetism disappears and the objects are released.
They are widely used in factories and scrap yards to move, lift and sort heavy metal items quickly and safely. The magnetic effect also powers electric bells, electric motors, fans and loudspeakers.
An electric bell uses an electromagnet. When you press the switch, current flows and the electromagnet pulls an iron strip (armature) so the hammer strikes the gong. That movement breaks the circuit, the magnetism dies, the strip springs back and the circuit closes again — so the hammer strikes repeatedly and the bell keeps ringing. A neat everyday use of a temporary, switchable magnet.
6. Does a current-carrying wire get hot? (Heating effect)
Connect a thin nichrome wire between two nails in a circuit with a switch. Touch the wire when the current is OFF — cool. Switch ON for about 30 seconds, switch OFF and touch again (briefly!) — it feels warm.
Why? Every conductor offers some resistance (opposition) to the flow of current. This resistance turns part of the electrical energy into heat energy, so the wire heats up.
Why nichrome and not copper? Nichrome offers far higher resistance than a copper wire of the same size and length, so it produces a lot more heat for the same current — ideal for heating elements.
7. What the amount of heat depends on
Repeat the activity with a battery of 2 cells: the wire gets hotter. So the heat produced depends on several factors:
- Magnitude of current — more current (more cells) → more heat.
- Material of the wire — high-resistance materials (nichrome) heat more than low-resistance ones (copper).
- Thickness and length of the wire.
- Duration for which the current flows.
Safety first: never hold a current-carrying heating wire — touch only momentarily, and do such activities under a teacher's supervision.
8. Everyday and industrial uses of the heating effect
Many appliances contain a heating element — a rod or coil of high-resistance wire that gets hot (sometimes glowing red) when current passes:
- Electric room heater, electric stove, electric kettle, electric iron, water-heating immersion rod, hair dryer.
- An incandescent bulb glows because its filament is heated white-hot by the current (Grade 7 idea).
- Industry: steel plants use a high-temperature electric furnace (heat from current) to melt and recycle scrap steel into usable steel.
The heating effect can also cause harm: heat is lost in transmission wires, and overheating in appliances can melt plastic in plugs and sockets or even start fires. To prevent this, household circuits contain safety devices, and we must use wires, plugs and sockets that are rated for the correct current. A common safety device is the electric fuse — a thin wire of low melting point placed in the circuit. If the current grows dangerously large, the fuse heats up and melts (blows), breaking the circuit before the wiring or appliance is damaged. Same heating effect — here used to protect us.
9. Where does the current come from? — the Voltaic cell
Cells and batteries are portable sources of current. What is inside them? Start with the earliest cell.
A Voltaic cell (also called a Galvanic cell) has two metal rods (electrodes) of different materials, partly dipped in a liquid (electrolyte) — usually a weak acid or salt solution — held in a glass or plastic container. A chemical reaction between the electrodes and the electrolyte produces electricity.
- In the outside circuit, current flows from the positive (+) terminal to the negative (−) terminal.
- Over time the chemicals get used up; the cell stops working and is called "dead" — it cannot supply current any more.
The names come from Italian scientists Alessandro Volta and Luigi Galvani. Galvani saw a dead frog's leg twitch when touched with two different metals and thought the electricity came from the frog. Volta disagreed — using saltwater-soaked paper instead of the frog he still got current, proving it was the combination of metals and liquid that made electricity. That led to the first battery.
Push a copper wire and an iron nail (kept apart) into each of 5–6 juicy lemons, join several lemons in a chain, and connect an LED across the ends. The LED glows — proof your cell works. Here the electrodes are copper and iron and the electrolyte is the lemon juice (salt solution works too). The LED lights only if its longer (+) leg goes to the + terminal; if it doesn't glow, reverse the connections.
10. Dry cells and rechargeable batteries
Voltaic cells are bulky and spill, so for everyday use we use dry cells.
- Dry cell: "dry" because the electrolyte is a thick moist paste, not a liquid. A zinc container acts as the negative terminal; a central carbon rod (with a metal cap) acts as the positive terminal, surrounded by the paste electrolyte.
- A dry cell is a single-use cell — once used up it must be disposed of.
- Rechargeable batteries can be recharged and reused many times — less waste, cheaper over time. Found in phones, laptops, cameras, inverters and electric vehicles. They still wear out slowly with use (why an old phone needs charging more often).
Most rechargeable devices today use lithium-ion (Li-ion) batteries, which rely on metals like lithium and cobalt that are scarce, so countries race to secure supplies and recycle old batteries. The next leap is solid-state batteries (solid electrolyte) — safer, faster-charging and longer-lasting — part of the move toward environment-friendly power.
11. Quick comparison — magnetic vs heating effect
- Magnetic effect: current → magnetic field around the wire; needs a coil/core to be strong; used in electromagnets, bells, motors, fans, loudspeakers; the effect disappears when current stops.
- Heating effect: current + resistance → heat in the wire; stronger with high-resistance materials like nichrome; used in heaters, irons, kettles, bulbs; the wire cools down after the current stops.
- Common thread: both depend on the amount of current — more current gives a stronger magnet and more heat.
12. NCERT "Keep the curiosity alive" — solved
Q1. Fill in the blanks. (i) The solution used in a Voltaic cell is called the electrolyte. (ii) A current-carrying coil behaves like a magnet (electromagnet).
Q2. True/False. (i) Dry cells are less portable than Voltaic cells — False (dry cells are more portable). (ii) A coil becomes an electromagnet only when current flows through it — True. (iii) A single-cell electromagnet attracts more clips than the same one with a 2-cell battery — False (2 cells → more current → more clips).
Q3. Current through a nichrome wire: (i) the wire becomes warm (heating effect) and (ii) a compass below it deflects (magnetic effect) — so (c) Both (i) and (ii) are correct.
Q4. Match. (i) Voltaic cell → (d) generates electricity by chemical reactions; (ii) Electric iron → (a) best suited as / works as a heater (heating effect); (iii) Nichrome wire → (c) works on heating effect of current; (iv) Electromagnet → (b) works on magnetic effect of current.
Q5. Nichrome is used in heating devices because it (ii) generates more heat for a given current (high resistance).
Q6. Electric heaters/stoves are more convenient than firewood or charcoal because they are cleaner (no smoke or harmful gases indoors → better health), easy to switch ON/OFF and control, do not need fuel collection, and save trees — better for society and the environment.
Q7. (Fig. 4.4a.) (i) Draw the current arrow from + terminal through the coil to − terminal. (ii) The current produces a magnetic field, and the compass needle (a tiny magnet) lines up with this field, so it deflects. (iii) Reversing the battery terminals reverses the current, which reverses the poles, so the needle deflects the opposite way.
Q8. Sumana's electromagnet stopped lifting clips but the wire is still warm. The current is still flowing (heating effect continues), but the magnetic strength dropped because the cell has weakened (going dead) after being left ON — the current is now too small to lift clips. (Connections may also have loosened or the iron core slipped.)
Q9. (Fig. 4.12.) The LED glows in (a) — lemon juice is an electrolyte, so the cell produces current. In (b) the liquid is pure water, which is a poor conductor (no electrolyte), so no current and the LED stays off.
Q10. Neha removes the iron nail, leaving only the coil. Yes, it still deflects the compass (the coil alone is an electromagnet), but the deflection is less than before, because the iron core was making it stronger.
Q11. (Fig. 4.13 — iron, copper, aluminium, nichrome coils.) As an electromagnet, the one with an iron core (a) is by far the strongest (iron is magnetic). For producing the most heat for a given current, the nichrome coil (d) wins (highest resistance).
13. Common mistakes to avoid
- Saying the electromagnet stays magnetic after switching off — it is temporary; magnetism vanishes with the current.
- Thinking reversing the current makes a stronger magnet — it only reverses the poles.
- Confusing the two effects: magnetic effect needs a coil, heating effect needs resistance.
- Believing copper is best for heaters — it has low resistance; nichrome (high resistance) makes the heat.
- Calling pure water a good conductor — it needs a dissolved electrolyte (acid/salt) to conduct.
- Mixing up dry-cell terminals: zinc container = negative, carbon rod = positive.
14. Quick revision checklist
- Current → magnetic field around the wire (Oersted, 1820).
- Coil = electromagnet; + iron core, more turns, more current → stronger; reversing current → poles swap.
- Lifting electromagnets on cranes (ON = lift, OFF = release); bells, motors, fans, loudspeakers.
- Resistance turns electrical energy into heat = heating effect; nichrome > copper for heat.
- Heat depends on current, material, thickness, length, time.
- Heater, iron, kettle, immersion rod, hair dryer, bulb filament; fuses/correct-rated wiring for safety.
- Cell = electrodes + electrolyte → chemical reaction → current. Voltaic, dry (single-use), rechargeable.
- Alessandro Volta
- Hans Christian Oersted
- Luigi Galvani
- Michael Faraday
- permanent magnet
- electromagnet
- electrolyte
- compass
- reducing the number of turns
- using fewer cells
- increasing the current and the number of turns
- removing the iron core
- becomes stronger
- stays the same
- disappears
- reverses
- makes it stronger
- reverses its North and South poles
- destroys it permanently
- has no effect at all
- heat metal
- generate electricity
- lift, move and sort heavy iron/steel
- recharge batteries
- magnetic field
- resistance
- colour
- length only
- is a better conductor
- has higher resistance and produces more heat
- does not conduct electricity
- is magnetic
- Electric iron
- Electric kettle
- Electric bell
- Immersion rod
- electrode
- electrolyte
- conductor
- insulator
- carbon rod
- metal cap
- zinc container
- paste electrolyte
- it is too hot
- its chemicals are used up so it cannot supply current
- it has reversed its terminals
- it has become magnetic
- the magnitude of the current
- the material of the wire
- the colour of the insulation
- the time the current flows
- electromagnet
- electrolyte
- fuse
- compass
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