Class 10 : Science (In English) – Lesson 12. Magnetic Effects of Electric Current

EXPLANATION & SUMMARY

🔵 Detailed Explanation

🌟 Introduction

🔵 Electric current produces magnetic effects, first discovered by Hans Christian Oersted in 1820.
🟢 This chapter explains how current and magnetism are interlinked, the magnetic field around conductors, right-hand rules, force on conductors, electric motor, electromagnetic induction, and electric generator.

🌍 Magnetic Field and Field Lines

🌿 Magnetic field: Region around a magnet/current-carrying conductor where its influence can be felt.

🟡 Magnetic field lines: Imaginary lines drawn to represent field.
Start from North pole, end at South pole.
Inside magnet, they go from South → North.
Crowded lines = strong field.

💡 Compass needle aligns along field lines.

⚡ Oersted’s Experiment
🔵 Passing current through a conductor deflects nearby compass needle.
🟢 Concluded: current produces magnetic field.

🧠 Right-Hand Thumb Rule
🌿 If you hold current-carrying conductor with right hand, thumb along current, curled fingers show direction of magnetic field.
🟡 Helps in predicting circular field around straight conductor.

🔴 Magnetic Field of a Solenoid
🌿 Solenoid = coil of many circular turns of insulated copper wire.
🟢 When current flows: behaves like a bar magnet.
🟡 Inside solenoid: field is uniform, parallel, strong.
🔵 Polarity depends on direction of current (Right-hand rule again).
💡 Applications: used in electromagnets, induction coils.

🟣 Force on a Current-Carrying Conductor in Magnetic Field
Discovered by Fleming’s Left-Hand Rule:
Stretch thumb, forefinger, middle finger mutually perpendicular.
Forefinger → field, middle → current, thumb → force (motion).
Basis of working of electric motor.

🔋 Electric Motor
Device that converts electrical energy → mechanical energy.

Components:
🔵 Coil of insulated copper wire (armature).
🟢 Permanent magnet or electromagnet.
🟡 Split-ring commutator (reverses current).
🔴 Brushes.

Principle: Force on current-carrying conductor in magnetic field.
Application: fans, mixers, machines.

🌞 Electromagnetic Induction
🔵 Discovered by Michael Faraday.
🟢 When magnetic field around conductor changes, induced current flows.
🟡 Factors: relative motion of magnet and coil, strength of magnet, number of turns.

✨ Right-Hand Rule (Fleming’s):
Forefinger → field, thumb → motion, middle finger → induced current.

⚙️ Electric Generator
Converts mechanical energy → electrical energy.
Principle: electromagnetic induction.
Components: armature coil, magnets, slip rings/commutator, brushes.
Generates AC or DC depending on arrangement.

💡 Domestic Electric Circuits
🔵 Electric power supplied at 220 V (in India).
🟢 Circuits have two wires: live and neutral.
🟡 Earth wire: safety measure.
🔴 Appliances connected in parallel for same voltage and independent operation.
🌿 Safety devices: fuse, MCB.

🟢 Summary
Current produces magnetic field (Oersted).
Direction: Right-hand thumb rule.
Solenoid acts as bar magnet with uniform field inside.
Force on conductor explained by Fleming’s Left-hand rule → basis of motor.
Electromagnetic induction → Faraday’s law, Fleming’s Right-hand rule → generator.
Household circuits: parallel connection, fuse, earth wire for safety.

📝 Quick Recap
⚡ Oersted: current produces magnetism.
👆 Right-hand thumb → direction of magnetic field.
🔄 Solenoid → bar magnet.
🖐️ Left-hand rule → motor principle.
🖖 Right-hand rule → generator principle.
🏠 Household wiring → parallel, fuse, earth wire.

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QUESTIONS FROM TEXTBOOK

Question 1
Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.
Answer
🔵 By right-hand thumb rule, magnetic field lines around straight current-carrying conductor form concentric circles.
✔️ Correct option: (d)

Question 2
At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
Answer
🟢 Short circuit → resistance drops → current becomes extremely high.
✔️ Correct option: (c)

Question 3
State whether the following statements are true or false:
(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.
(b) A wire with a green insulation is usually the live wire of an electric supply.
Answer
(a) 🔵 True → inside a solenoid or circular coil, field is nearly uniform parallel lines.
(b) 🔴 False → green insulation is earth wire, live wire is red/brown.

Question 4
List two methods of producing magnetic fields.
Answer
🌿 Passing electric current through a straight conductor.
🌍 Using a current-carrying coil/solenoid.

Question 5
When is the force experienced by a current-carrying conductor placed in a magnetic field largest?
Answer
🟢 Force = maximum when conductor is placed perpendicular to magnetic field.
🔵 Zero when conductor is parallel to field.

Question 6
Imagine you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?
Answer
🌿 Fleming’s Left-Hand Rule → electron current opposite to conventional.
Beam deflected to right → force to right, motion forward.
Magnetic field must be downward.

Question 7
State the rule to determine the direction of –
(i) Magnetic field produced around a straight conductor-carrying current.
(ii) Force experienced by a current-carrying straight conductor placed in a magnetic field perpendicular to it.
(iii) Current induced in a coil due to its rotation in a magnetic field.
Answer
(i) 🔵 Right-hand thumb rule → thumb = current, fingers curl = field.
(ii) 🟢 Fleming’s Left-Hand Rule → thumb = force, forefinger = field, middle = current.
(iii) 🟡 Fleming’s Right-Hand Rule → thumb = motion, forefinger = field, middle = induced current.

Question 8
When does an electric short circuit occur?
Answer
🔴 When live and neutral wires come into direct contact due to damaged insulation/overload.
🌍 Resistance drops, current increases abnormally → heating/fire.

Question 9
What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Answer
🔵 Function: provides path for leakage current to ground.
🟢 Necessity: prevents electric shock if metallic body becomes live.
🟡 Ensures safety of user.

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OTHER IMPORTANT QUESTIONS FOR EXAMS

Question 1
The direction of the magnetic field around a straight conductor is given by:
Fleming’s left-hand rule
Right-hand thumb rule
Fleming’s right-hand rule
Clock rule
Answer: 2 👍

Question 2
Which instrument is used to detect current in a circuit?
Voltmeter
Ammeter
Galvanometer
Rheostat
Answer: 3 🧭

Question 3
Magnetic field inside a long current-carrying solenoid is:
Zero
Strong and uniform
Weak and irregular
Same as Earth’s field
Answer: 2 🌿

Question 4
The function of brushes in an electric motor is to:
Provide insulation
Reverse current
Connect coil to supply
Reduce heating
Answer: 3 🔋

Question 5
Electromagnetic induction was discovered by:
Oersted
Fleming
Faraday
Ampere
Answer: 3 ⚡

Question 6
In domestic wiring, appliances are connected in:
Series
Parallel
Both
Either
Answer: 2 🏠

Question 7
The Earth’s magnetic field protects us from:
UV rays
Cosmic rays
Solar winds
Infrared radiation
Answer: 3 🌍

Question 8
A solenoid with more turns and higher current will produce:
Weaker field
Stronger field
No field
Same field
Answer: 2 🟢

Question 9
Magnetic field due to straight wire is in form of:
Straight lines
Radial lines
Concentric circles
Parallel lines
Answer: 3 🔵

Question 10
Which rule is used to find direction of induced current?
Right-hand thumb rule
Fleming’s left-hand rule
Fleming’s right-hand rule
Clockwise rule
Answer: 3 🖖

Question 11 (Assertion–Reason)
A: Magnetic field lines never intersect.
R: At intersection, field direction would be twofold, which is impossible.
Answer: 1 ✔️

Question 12
A: Current-carrying conductor experiences a force in magnetic field.
R: Charge in motion interacts with field.
Answer: 1 ⚡

Question 13
A: Field inside solenoid is uniform.
R: Field lines inside are parallel and equidistant.
Answer: 1 🌿

Question 14
A: Split-ring commutator in motor reverses current.
R: This ensures continuous rotation in one direction.
Answer: 1 🔄

Question 15
A: Generator works on electromagnetic induction.
R: It converts electrical to mechanical energy.
Answer: 3 ❌ (It converts mechanical to electrical)

Question 16
A: Fuse wire has low melting point.
R: It melts to break circuit under excess current.
Answer: 1 🔥

Question 17
A: Electric motor converts electrical energy to mechanical energy.
R: Fleming’s right-hand rule is applied.
Answer: 3 ❌ (Motor uses left-hand rule)

Question 18
A: Earth wire prevents shock.
R: It provides path to leakage current.
Answer: 1 🟡

Question 19
A: Direction of field due to current in solenoid is given by right-hand thumb rule.
R: Curl of fingers shows field, thumb gives current.
Answer: 4 ❌ (That is straight conductor rule; solenoid uses clock/coil rule)

Question 20
A: Electric generator produces only AC.
R: Slip rings are used in AC generators.
Answer: 2 🟠 (Both true, but R not explanation)

🟢 Section B (Q21–26: Very Short Answers, 2 marks)
Question 21
State Right-hand thumb rule.
Answer:
Thumb → current.
Curled fingers → magnetic field.

Question 22
Name device converting mechanical energy into electrical energy.
Answer: Generator ⚡

Question 23
What is function of commutator in motor?
Answer:
Reverses current direction every half-turn.
Ensures continuous rotation in one direction.

Question 24
Why are appliances connected in parallel in household wiring?
Answer:
Each appliance gets same voltage.
Independent working, one failure doesn’t affect others.

Question 25
What is electromagnetic induction?
Answer:
Current induced in conductor when magnetic field linked with it changes.

Question 26
State one use of solenoid.
Answer:
Used to make electromagnets for cranes, motors, relays.

🟡 Section C (Q27–33: Short Answers, 3 marks)
Question 27
Draw field lines due to bar magnet.
Answer:
Lines from N → S outside, S → N inside.
Do not intersect.
Crowded at poles.

Question 28
State three characteristics of magnetic field lines.
Answer:
🔵 Outside magnet N → S.
🟢 Inside magnet S → N.
🔴 Do not intersect.

Question 29
Explain principle of electric motor.
Answer:
Force acts on current-carrying conductor in magnetic field.
Given by Fleming’s left-hand rule.

Question 30
Give differences between AC and DC.
Answer:
AC changes direction periodically, DC flows in one direction.
AC from generators, DC from cells.
AC transmitted long distance with less loss.

Question 31
What is role of earth wire?
Answer:
Provides low-resistance path to ground.
Protects users from shock.

Question 32
What is short circuit?
Answer:
Direct contact between live and neutral wires.
Very low resistance, high current.

Question 33
Why is fuse important in household wiring?
Answer:
Prevents overheating, fire.
Breaks circuit when current exceeds safe limit.

🔴 Section D (Q34–36: Long Answers, 5 marks)
Question 34
Explain with diagram the working of an electric motor.
Answer:
Coil placed between poles of magnet.
Current flows → force acts on arms → rotation.
Split-ring commutator reverses current → continuous motion.
Converts electrical → mechanical energy.

Question 35
Explain with diagram the working of AC generator.
Answer:
Coil rotates in magnetic field.
Flux changes → current induced (Faraday’s law).
Slip rings + brushes → current output.
Produces alternating current.

Question 36
What safety measures are used in household circuits?
Answer:
Parallel connections.
Fuse/MCB to break excess current.
Earth wire.
Proper insulation, earthing of appliances.

🟣 Section E (Q37–Q39: Case Based, 4 marks)
Question 37
Case: A student switches on electric iron without fuse; spark observed.
(a) Which phenomenon?
(b) Why did it happen?
(c) What device should be used?
(d) Why must fuse be on live wire?
Answer:
(a) Short circuit.
(b) Live and neutral touched.
(c) Fuse/MCB.
(d) To disconnect supply safely.

Question 38
Case: In physics lab, a coil connected to galvanometer is brought near bar magnet.
(a) What is observed?
(b) Why does needle deflect?
(c) What if magnet withdrawn?
(d) Which law explains this?
Answer:
(a) Needle deflects.
(b) Induced current flows.
(c) Deflection in opposite direction.
(d) Faraday’s law.

Question 39
Case: A washing machine motor runs on 220 V supply.
(a) Which energy conversion?
(b) Which rule explains direction of force?
(c) Why is commutator used?
(d) What would happen without commutator?
Answer:
(a) Electrical → mechanical.
(b) Fleming’s left-hand rule.
(c) To reverse current each half turn.
(d) Motor would oscillate instead of rotating.

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MIND MAPS

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