Lesson 10: TS Class 10 Physical Science Lesson 10: Electromagnetism - Important Q&A, Solved Numericals & Objective Bits

🧲 Telangana Class 10 Physical Science — Lesson 10

Electromagnetism

Complete Chapter Guide: Oersted's Experiment, Magnetic Force on Current Conductors, Faraday's Law, Fleming's Left & Right Hand Rules, Transformer Equations & Part-B MCQs

📌 Essential Formulas & Electromagnetic Principles

1. Magnetic Force on Moving Charge

Force (F) on a charge q moving with velocity v at angle θ in magnetic field B:

F = q × v × B × sin(θ)

2. Faraday's Law of Induction

Induced EMF (Ε) in a coil with N turns due to rate of change of magnetic flux (ΔΦ/Δt):

Ε = -N × (ΔΦ / Δt)

3. Transformer Turns & Voltage Ratio

Relates Primary Voltage (V_p), Secondary Voltage (V_s), and Turn numbers (N_p, N_s):

V_p / V_s = N_p / N_s = I_s / I_p

🧭 Fundamental Rules of Electromagnetism

Core direction rules that govern magnetic fields, forces, and induced currents in electrical machines:

Electromagnetic Direction Rules At A Glance

1. Right Hand Thumb Rule

Determines: Direction of magnetic field lines around a straight current conductor.
Thumb: Direction of Current.
Curled Fingers: Direction of Magnetic Field.

2. Fleming's Left Hand Rule

Application: Electric Motors (Force on conductor).
Thumb: Motion / Force (F).
Forefinger: Magnetic Field (B).
Middle Finger: Current (I).

3. Fleming's Right Hand Rule

Application: Electric Generators (Induced current).
Thumb: Motion of Conductor.
Forefinger: Magnetic Field (B).
Middle Finger: Induced Current (I).

4. Lenz's Law

Statement: Induced current flows in a direction that opposes the magnetic flux change that produced it.
Principle: Based on Conservation of Energy.

Key Units: Magnetic Flux (Φ) = Weber (Wb) | Magnetic Field Induction (B) = Tesla (T) or Wb/m².

Comparison: AC Generator vs. DC Generator

Feature	AC Generator (Alternator)	DC Generator
Output Current Type	Produces Alternating Current (AC) which periodically reverses direction.	Produces Direct Current (DC) which flows continuously in a single direction.
Commutator Used	Uses two full continuous Slip Rings.	Uses a Split Ring Commutator.
Grid Transport	Ideal for long-distance power transmission with low power loss using transformers.	Not suitable for long-distance power transmission due to high transmission loss.
Working Principle	Electromagnetic Induction (Faraday's Law).	Electromagnetic Induction (Faraday's Law).

📝 Short Answer Questions (2 Marks & 4 Marks)

QUESTION 1 - 2 MARKS

Q1. State Faraday's Law of Electromagnetic Induction.

Answer:

• Faraday's Law: Whenever there is a continuous change in the magnetic flux linked with a closed circuit or coil, an electromotive force (EMF) is induced in the circuit.
• The magnitude of the induced EMF is directly proportional to the rate of change of magnetic flux linked with the coil:
  Ε = -N × (ΔΦ / Δt)

QUESTION 2 - 2 MARKS

Q2. State Fleming's Left Hand Rule and name the electrical device that works on this rule.

Answer:

• Statement: Stretch the thumb, forefinger, and middle finger of your left hand mutually perpendicular to each other.
  • If the Forefinger points in the direction of the Magnetic Field,
  • And the Middle finger points in the direction of Current,
  • Then the Thumb gives the direction of Force / Motion acting on the conductor.
• Device: The Electric Motor works on Fleming's Left Hand Rule.

QUESTION 3 - 4 MARKS

Q3. Differentiate between a Step-up Transformer and a Step-down Transformer.

Answer:

Feature	Step-up Transformer	Step-down Transformer
Voltage Conversion	Increases low AC voltage to high AC voltage (Vs > Vp).	Decreases high AC voltage to low AC voltage (Vs < Vp).
Number of Turns	Secondary turns greater than primary (Ns > Np).	Secondary turns smaller than primary (Ns < Np).
Output Current	Secondary current is lower (Is < Ip).	Secondary current is higher (Is > Ip).
Application	Power generation stations before long-distance transmission.	Substations & mobile chargers for safe home usage.

🧮 Solved Board Exam Numericals & Experimental Essays

4 MARKS - SOLVED TRANSFORMER NUMERICAL

Q4. Solve the following transformer numericals:
(a) A transformer has 500 turns in its primary coil and 2500 turns in its secondary coil. If the primary voltage is 220 V AC, calculate the secondary output voltage.
(b) Is this a Step-up or Step-down transformer?

Solution for (a):

Given: Primary turns N_p = 500 | Secondary turns N_s = 2500

Primary Voltage V_p = 220 V

Formula: V_p / V_s = N_p / N_s ⇒ V_s = V_p × (N_s / N_p)

V_s = 220 × (2500 / 500) = 220 × 5 = 1100 V

Solution for (b):

Since output voltage V_s (1100 V) > input voltage V_p (220 V) and N_s > N_p, it is a Step-up Transformer.

✓ Final Answer:
1. Secondary Output Voltage V_s = 1100 V.
2. Transformer Type: Step-up Transformer.

6 MARKS - EXPERIMENTAL ESSAY

Q5. Describe Oersted's experiment to demonstrate that an electric current produces a magnetic field around a conductor.

Answer:

• Aim: To show that a magnetic field is generated around a current-carrying straight wire.
• Apparatus Required: Straight copper wire, magnetic compass needle, battery, plug key, and connecting wires.
• Procedure:
  1. Place a straight copper wire parallel to and directly above a magnetic compass needle aligned in the North-South direction.
  2. Connect the copper wire to a battery and a plug key.
  3. Close the key to pass current through the wire from South to North. Observe the compass needle.
  4. Reverse the battery terminals so that current flows from North to South. Observe the deflection again.
• Observation:
  • When current flows from South to North, the North pole of the compass needle deflects towards the West.
  • When the direction of current is reversed (North to South), the North pole deflects towards the East.
• Conclusion: An electric current produces a magnetic field around the conductor, proving the magnetic effect of electric current.

🎯 PART-B Objective Bits Bank (10 MCQs)

BIT 1

1. Who experimentally discovered the magnetic effect of electric current in 1820?

• Michael Faraday
• Hans Christian Oersted
• James Clerk Maxwell
• Heinrich Hertz

✓ Correct Option: B (Hans Christian Oersted)

BIT 2

2. What is the SI unit of Magnetic Flux (Φ)?

• Tesla
• Weber
• Gauss
• Ampere-meter

✓ Correct Option: B (Weber - Wb)

BIT 3

3. An Electric Generator works on the principle of:

• Heating effect of current
• Electromagnetic Induction
• Chemical effect of current
• Electrostatic repulsion

✓ Correct Option: B (Electromagnetic Induction)

BIT 4

4. The direction of induced current in a generator is determined using:

• Fleming's Left Hand Rule
• Fleming's Right Hand Rule
• Ampere's Swimming Rule
• Ohm's Law

✓ Correct Option: B (Fleming's Right Hand Rule)

BIT 5

5. Lenz's Law is a direct consequence of the Law of Conservation of:

• Charge
• Energy
• Mass
• Momentum

✓ Correct Option: B (Conservation of Energy)

BIT 6

6. The frequency of Alternating Current (AC) supplied for domestic usage in India is:

• 50 Hz
• 60 Hz
• 100 Hz
• 220 Hz

✓ Correct Option: A (50 Hz, 220 V)

BIT 7

7. What is the shape of magnetic field lines around a long straight current-carrying conductor?

• Straight parallel lines
• Concentric circles
• Parabolic curves
• Elliptical paths

✓ Correct Option: B (Concentric circles centered on the wire)

BIT 8

8. A device that converts electrical energy into mechanical energy is called an:

• Electric Generator
• Electric Motor
• Transformer
• Galvanometer

✓ Correct Option: B (Electric Motor)

BIT 9

9. In a Step-down transformer, the secondary voltage V_s compared to primary voltage V_p is:

• V_s > V_p
• V_s < V_p
• V_s = V_p
• V_s = 0

✓ Correct Option: B (V_s < V_p)

BIT 10

10. What is 1 Tesla equal to in terms of Weber per square meter?

• 1 Wb/m²
• 10 Wb/m²
• 100 Wb/m²
• 1000 Wb/m²

✓ Correct Option: A (1 Tesla = 1 Wb/m²)