Key Concepts & Formulas

10 Practice MCQs

Q1. In a simple pendulum experiment, if the length is increased by 4 times, the time period will: A) Remain same B) Double C) Halve D) Become 4 times

Answer: B) Double

Solution: Using T = 2π√(L/g) If L becomes 4L, then T’ = 2π√(4L/g) = 2 × 2π√(L/g) = 2T

Shortcut: T ∝ √L, so if L becomes 4 times, T becomes √4 = 2 times

Concept: Experiments - Pendulum time period relationship

Q2. A train’s headlight uses a concave mirror with focal length 20 cm. Where should the bulb be placed for parallel beam? A) At focus B) At center of curvature C) Beyond center D) Between focus and pole

Answer: A) At focus

Solution: For concave mirror, light source placed at focus (20 cm from mirror) produces parallel reflected rays

Shortcut: Remember “Focus gives parallel, parallel gives focus”

Concept: Experiments - Mirror reflection properties

Q3. In Ohm’s law experiment, a voltmeter should be connected: A) In series with resistor B) Parallel to resistor C) Anywhere in circuit D) Not needed

Answer: B) Parallel to resistor

Solution: Voltmeter has high resistance and measures potential difference, so must be parallel to measure voltage across component

Concept: Experiments - Circuit connection principles

Q4. A railway signal uses a convex lens of focal length 15 cm. If object is at 30 cm, image forms at: A) 30 cm B) 15 cm C) 10 cm D) 60 cm

Answer: A) 30 cm

Solution: Using lens formula: 1/f = 1/v - 1/u 1/15 = 1/v - 1/(-30) [u = -30 cm for real object] 1/v = 1/15 - 1/30 = (2-1)/30 = 1/30 Therefore, v = 30 cm

Shortcut: When u = 2f, v = 2f (real, same size, inverted)

Concept: Experiments - Lens formula application

Q5. In a calorimetry experiment, 200g water at 30°C mixed with 100g water at 90°C. Final temperature is: A) 40°C B) 50°C C) 60°C D) 45°C

Answer: B) 50°C

Solution: Heat lost = Heat gained 100 × 1 × (90 - T) = 200 × 1 × (T - 30) 9000 - 100T = 200T - 6000 15000 = 300T T = 50°C

Shortcut: Final temp = (m₁T₁ + m₂T₂)/(m₁ + m₂) = (200×30 + 100×90)/300 = 15000/300 = 50°C

Concept: Experiments - Calorimetry and heat exchange

Q6. A train’s speedometer cable has resistance 4Ω and carries 3A current. Potential difference across it is: A) 0.75V B) 1.33V C) 12V D) 7V

Answer: C) 12V

Solution: Using Ohm’s law: V = IR = 3 × 4 = 12V

Shortcut: V = IR (direct multiplication)

Concept: Experiments - Ohm’s law practical application

Q7. In a density experiment, a 54g metal piece displaces 6ml water. Its density is: A) 9 g/cm³ B) 0.11 g/cm³ C) 324 g/cm³ D) 48 g/cm³

Answer: A) 9 g/cm³

Solution: Density = Mass/Volume = 54g/6ml = 9 g/cm³

Shortcut: Direct division of mass by displaced volume

Concept: Experiments - Archimedes principle

Q8. Two resistors 3Ω and 6Ω in parallel give equivalent resistance: A) 9Ω B) 2Ω C) 4.5Ω D) 18Ω

Answer: B) 2Ω

Solution: 1/R = 1/3 + 1/6 = (2+1)/6 = 3/6 = 1/2 Therefore, R = 2Ω

Shortcut: For two parallel resistors: R = (R₁ × R₂)/(R₁ + R₂) = (3×6)/(3+6) = 18/9 = 2Ω

Concept: Experiments - Parallel resistance calculation

Q9. A pendulum of length 1m has time period 2s. If taken to a station where g = 9.8 m/s² to another where g = 4.9 m/s², new time period is: A) 1s B) 2√2 s C) 4s D) √2 s

Answer: B) 2√2 s

Solution: T₁/T₂ = √(g₂/g₁) 2/T₂ = √(4.9/9.8) = √(1/2) = 1/√2 T₂ = 2√2 s

Shortcut: T ∝ 1/√g, so if g halves, T increases by √2 times

Concept: Experiments - Pendulum and gravity relationship

Q10. In a lens experiment, if magnification is -2 and object distance is 15 cm, image distance is: A) 7.5 cm B) -30 cm C) 30 cm D) -7.5 cm

Answer: C) 30 cm

Solution: Magnification m = v/u -2 = v/(-15) [u = -15 cm for real object] v = 30 cm

Shortcut: |m| = |v/u|, sign indicates real/virtual and inverted/erect

Concept: Experiments - Magnification formula

5 Previous Year Questions

PYQ 1. In a simple pendulum experiment, the bob is replaced by a heavier one. The time period will: [RRB NTPC 2021 CBT-1]

Answer: C) Remain same

Solution: Time period T = 2π√(L/g) is independent of mass. Only depends on length and gravity.

Exam Tip: Remember pendulum time period formula by heart - mass never affects it

PYQ 2. A voltmeter has resistance 1000Ω and reads up to 2V. To extend range to 10V, required series resistance is: [RRB Group D 2022]

Answer: B) 4000Ω

Solution: Original current: I = V/R = 2/1000 = 0.002A For 10V range: R_total = 10/0.002 = 5000Ω Series resistance = 5000 - 1000 = 4000Ω

Exam Tip: Remember: R_series = R_original × (V_new/V_original - 1)

PYQ 3. In Ohm’s law experiment, a wire is stretched to double its length. Its resistance becomes: [RRB ALP 2018]

Answer: D) 4 times

Solution: R ∝ L² when volume constant (A ∝ 1/L) If L doubles, A halves, so R becomes 2 × 2 = 4 times

Exam Tip: R = ρL/A and volume V = AL remains constant

PYQ 4. A convex lens forms real image of same size when object is at 40 cm. Focal length is: [RRB JE 2019]

Answer: B) 20 cm

Solution: For same size real image: u = 2f Given u = 40 cm, so 2f = 40, therefore f = 20 cm

Exam Tip: Same size real image means u = 2f, always

PYQ 5. In calorimetry, 10g ice at 0°C mixed with 100g water at 50°C. Final temperature is: [RPF SI 2019]

Answer: C) 38.2°C

Solution: Heat to melt ice: Q₁ = 10 × 80 = 800 cal Heat from water cooling: Q₂ = 100 × 1 × (50 - T) Heat to warm melted ice: Q₃ = 10 × 1 × (T - 0) 800 + 10T = 100(50 - T) 800 + 10T = 5000 - 100T 110T = 4200 T = 38.2°C

Exam Tip: Don’t forget latent heat of fusion (80 cal/g for ice)

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Topic Connections

• Direct Link: Experiments directly relate to Physics Practical - all concepts tested through hands-on verification
• Combined Questions: Often paired with Units & Measurements (error analysis, significant figures) and Numerical Ability (calculation-based problems)
• Foundation For: Advanced topics like Electronics (circuit analysis), Instrumentation (measurement devices), and Engineering Physics (practical applications in railway systems)