Science Class 8 — Curiosity
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Magnetic Effect of Electric Current and Electromagnets

🎓 Class 8 Science CBSE Theory Ch 4 — Materials: Metals and Non-Metals ⏱ ~29 min
🌐 Language: [gtranslate]

This MCQ module is based on: Magnetic Effect of Electric Current and Electromagnets

[myaischool_lt_science_assessment grade_level="class_8" science_domain="chemistry" difficulty="basic"]

Probe and Ponder

At a science exhibition in a railway yard, a giant crane suddenly picks up an entire scrap wagon — without chains, hooks, or magnets you can see. Then, at the flick of a switch, the wagon drops. What magic is this?

  • How could you tell that current is flowing in a wire if you did not have a bulb?
  • A normal iron bar is not a magnet. Can we make it magnetic just by using a battery?
  • Why does the giant electromagnet drop its load as soon as the power is cut?
  • What connects electricity and magnetism — are they really the same thing in disguise?

By the end of this part, you will understand how one simple discovery by a Danish scientist in 1820 changed the world forever.

4.1 Magnetic Effect of Electric Current

For a long time, scientists thought electricity and magnetism were two totally separate things. That changed in 1820, when Hans Christian Oersted noticed that the needle of a nearby compass twitched every time he switched on an electric current.

Magnetic Effect of Current: When an electric current flows through a wire, a magnetic field is produced around the wire. This is called the magnetic effect of electric current.
🔬 Activity 4.1 — Oersted's Classic ExperimentL3 Apply
🤔 Predict first: If you bring a magnetic compass close to a wire and then pass current through the wire, what will the compass needle do?
  1. Lay a copper wire flat on a table, running north–south.
  2. Place a small magnetic compass just below the wire.
  3. Connect the wire to a cell through a switch. Keep the switch open (OFF).
  4. Note: the compass needle points north–south as usual.
  5. Close the switch (ON) — current flows. Watch the compass!
  6. Open the switch — what happens now?
Observation: The moment current flows, the needle deflects (turns sideways). When current stops, the needle returns to north–south. This proves that the current itself creates magnetism around the wire — exactly what Oersted discovered 200+ years ago.
Oersted's Experiment — Current Deflects a Compass CELL switch (ON) current N S needle deflected!
Fig 4.1 — When current flows, the compass needle turns away from north-south, showing a magnetic field exists around the wire.

The Hero of the Story — Hans Christian Oersted

In 1820, while preparing a physics lecture in Copenhagen, Oersted happened to place a compass near a wire carrying current. The needle jumped. That one accident connected two separate branches of science — electricity and magnetism — into one field we now call electromagnetism. Every motor, fan, loudspeaker, and MRI machine owes something to that moment.

4.2 Making an Electromagnet

If a straight wire with current produces a weak magnetic field, what if we coiled the wire into many loops? And what if we stuffed an iron nail inside the coil? We'd get a much stronger, switchable magnet — an electromagnet.

🔬 Activity 4.2 — Build Your Own ElectromagnetL3 Apply
🤔 Predict first: After wrapping wire around an iron nail and switching on the current, how many steel pins do you think the nail can pick up?

You need: a long iron nail, about 1 m of insulated copper wire, a 1.5 V cell, a switch, a few steel pins.

  1. Wind the wire tightly around the nail 20–30 times. Leave two free ends.
  2. Connect the free ends to the cell through a switch.
  3. Keep the switch OFF. Touch the nail to the pins — they don't stick.
  4. Close the switch (ON). Touch the nail to the pins again — they cling!
  5. Open the switch (OFF) — the pins drop off immediately.
What happened: Current in the coil turned the iron nail into a strong temporary magnet. Switch off the current → no more magnetic field → pins fall. This is the key feature of electromagnets: they can be turned ON and OFF at will.
Simple Electromagnet — Nail + Coil + Cell Iron nail (core) insulated copper coil switch 1.5 V pins
Fig 4.2 — A coil of insulated wire around an iron nail becomes a strong magnet when current flows.

How to Make the Electromagnet Stronger

🔁
More Turns of Wire
Doubling the number of coil loops roughly doubles the magnetic strength.
🔋
More Current
Adding more cells in series pushes more current through the coil — stronger field.
🧲
Soft Iron Core
An iron core concentrates the magnetic field and makes the magnet far stronger than an air-filled coil.

🧪 Interactive — Electromagnet Strength Simulator L3 Apply

Move the sliders and change the core. Watch the lifting power meter change.

Lifting strength: 100 units

4.3 Uses of Electromagnets

Because they switch ON and OFF at will, electromagnets are everywhere in daily life.

Lifting Electromagnet (Railway Yards & Scrap Yards)

A huge electromagnet hangs from a crane. Turn it ON → it picks up tons of iron scrap, wagons or plates. Move it over the dumping place, turn OFF → load drops. No chains, no hooks.

Lifting Electromagnet at a Scrap Yard ELECTROMAGNET Iron scrap lifted by magnetic force scrap pile
Fig 4.3 — Switch ON: load picked up. Switch OFF: load drops safely into the pile.

The Electric Bell

An electric bell uses an electromagnet plus a clever "make-and-break" trick.

  1. Press the bell switch → current flows through the coil → electromagnet becomes active.
  2. The electromagnet pulls the iron armature (a bar with a hammer). The hammer strikes the metal gong → "TING!"
  3. But as the armature moves, it breaks the circuit → electromagnet switches off → spring pulls armature back.
  4. Circuit is remade → coil energised again → hammer strikes again.
  5. This happens so fast — many times per second — that you hear continuous ringing.
Electric Bell — Labelled Diagram Electromagnet Armature Hammer Gong Spring Contact Cell Switch
Fig 4.4 — Inside an electric bell: coil → armature → hammer → gong → break-and-make contact.

Loudspeaker & Earphone

A loudspeaker has a permanent magnet and an electromagnet (coil) attached to a paper cone. When varying audio current flows, the coil is alternately pushed and pulled — the cone vibrates — air vibrates — you hear music! A telephone earpiece works the same way on a smaller scale: coil vibrates a diaphragm.

Loudspeaker — Cross-section Paper cone Voice coil N S Permanent magnet sound out
Fig 4.5 — Current in the voice coil + magnet → cone vibrates → sound waves travel to your ear.

Other High-Tech Uses

  • MRI machines in hospitals use super-powerful electromagnets to take detailed pictures of the body.
  • Particle accelerators use huge electromagnets to bend beams of tiny particles travelling near the speed of light.
  • Maglev trains float on electromagnets and reach speeds above 400 km/h.

4.4 Electric Motor — A Quick Peek

An electric motor uses the magnetic effect to keep rotating. A coil carrying current, placed between magnets, feels a push on one side and a pull on the other → it spins. Every spinning device around you — fan, mixer, washing machine drum, electric toy, water pump — is powered by a motor inside.

Energy conversion in a motor: Electrical energy → Mechanical (rotational) energy. We'll study motors in much more detail in Class 10.

📋 Competency-Based Questions

The railway yard at Delhi uses a crane with a lifting electromagnet to sort iron scrap. One day, the operator notices that the electromagnet is picking up only half the usual weight. He checks the cells and finds two are very old. He also realises the coil on the iron core has loosened and some turns have come off.

Q1. L2 Understand Which scientist first demonstrated that an electric current produces a magnetic field?

  • A. Michael Faraday
  • B. Hans Christian Oersted
  • C. Thomas Edison
  • D. Alessandro Volta
Answer: B. Oersted's 1820 compass-needle deflection experiment began the science of electromagnetism.

Q2. L3 Apply Name any two practical devices in which an electromagnet is used. (Short Answer)

Answer: Any two of — electric bell, loudspeaker, telephone earpiece, lifting crane at scrap yard, MRI machine, maglev train, electric motor.

Q3. L4 Analyse Give two reasons why the crane above is now lifting less weight, and suggest one fix for each. (3 marks)

Reason 1: Old/weak cells → smaller current → weaker magnet. Fix: Replace with fresh cells. Reason 2: Fewer turns of coil → weaker field. Fix: Re-wind the coil tightly with the full number of turns.

Q4. L1 Remember State true or false: An electromagnet retains its magnetism even after current is switched off.

False. An electromagnet acts as a magnet only while current flows. That is its big advantage — it can be switched ON and OFF.

Q5. L6 Create Design an experiment to show that the strength of an electromagnet depends on the number of turns of wire, keeping current and core the same. (3 marks)

Hint: Use the same iron nail and same cell. Make Coil A with 20 turns, Coil B with 50 turns. Connect each in turn. Count how many steel pins each lifts. Coil B should lift more pins, proving strength ∝ number of turns.

🔗 Assertion–Reason Questions

Assertion (A): A compass needle gets deflected when placed near a wire carrying electric current.

Reason (R): An electric current produces a magnetic field around itself.

  • A. Both A and R are true, and R correctly explains A.
  • B. Both A and R are true, but R does not explain A.
  • C. A is true, R is false.
  • D. A is false, R is true.
Answer: A. The magnetic field produced by the current pushes the compass needle, causing the deflection.

Assertion (A): A soft iron core makes an electromagnet stronger than an air-filled coil.

Reason (R): Soft iron gets easily magnetised and concentrates the magnetic field lines.

  • A. Both A and R are true, and R correctly explains A.
  • B. Both A and R are true, but R does not explain A.
  • C. A is true, R is false.
  • D. A is false, R is true.
Answer: A. The ease with which soft iron magnetises concentrates the flux inside the coil, boosting strength.

Assertion (A): Electromagnets are preferred over permanent magnets in cranes used at scrap yards.

Reason (R): Electromagnets can be switched on and off, allowing loads to be picked up and released.

  • A. Both A and R are true, and R correctly explains A.
  • B. Both A and R are true, but R does not explain A.
  • C. A is true, R is false.
  • D. A is false, R is true.
Answer: A. The ON/OFF control is exactly what is needed to pick-up and drop scrap — permanent magnets cannot do this.

💡 Did You Know?
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