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Physical Quantities and Units

Introduction

The topic "Physical Quantities and Units" is fundamental in General Science and frequently appears in exams like SSC CGL, RRB NTPC, and IBPS PO. Understanding the basic physical quantities and their standard units is essential for solving numerical problems and conceptual questions in physics and chemistry sections of these exams.

Pattern: Physical Quantities and Units

Pattern

This pattern tests knowledge of fundamental physical quantities and their corresponding SI units, including derived units and commonly used non-SI units.

Key Concept:

Physical quantities are measurable properties of physical phenomena. Each physical quantity has a standard unit defined by the International System of Units (SI).

Important Points:

  • Fundamental Quantities = Length (meter, m), Mass (kilogram, kg), Time (second, s), Electric Current (ampere, A), Temperature (kelvin, K), Amount of Substance (mole, mol), Luminous Intensity (candela, cd)
  • Derived Quantities = Velocity (m/s), Force (newton, N), Energy (joule, J), Power (watt, W), Pressure (pascal, Pa)
  • Commonly Confused Units = Newton (unit of force) vs Joule (unit of energy), Watt (unit of power) vs Volt (unit of electric potential)

Related Topics:

  • Measurement and Errors
  • Units of Measurement
  • Dimensional Analysis

Step-by-Step Example

Question

The SI unit of force is:

Options:

  • A. Joule
  • B. Newton
  • C. Watt
  • D. Pascal

Solution

  1. Step 1: Understand the quantity

    Force is a physical quantity that causes an object to accelerate or change its motion.

  2. Step 2: Recall the SI unit

    The SI unit of force is defined as newton (N), which is equal to kg·m/s².

  3. Step 3: Differentiate from other units

    Joule is the unit of energy, watt is the unit of power, and pascal is the unit of pressure.

  4. Final Answer:

    Newton → Option B

  5. Quick Check:

    SI unit of force = Newton ✅

Quick Variations

This pattern may appear as questions asking for:

  • 1. SI units of derived quantities like pressure, power, or energy.
  • 2. Identification of fundamental physical quantities and their units.
  • 3. Distinguishing between units of similar quantities such as force and energy.

Trick to Always Use

  • Remember the mnemonic "M T L I T A" for fundamental quantities: Mass, Time, Length, Current, Temperature, Amount of substance.
  • Associate Newton with force by recalling Newton’s second law: Force = mass × acceleration (kg·m/s²).

Summary

Summary

  • Physical quantities are measured using standard SI units.
  • Fundamental quantities include length, mass, time, electric current, temperature, amount of substance, and luminous intensity.
  • Force is measured in newtons, energy in joules, power in watts, and pressure in pascals.

Remember:
Force unit is Newton, not Joule or Watt

Practice

(1/5)
1. The SI unit of electric current is:
easy
A. Ampere
B. Volt
C. Ohm
D. Watt

Solution

  1. Step 1: Identify the concept

    The question tests knowledge of fundamental physical quantities and their SI units, specifically electric current.

  2. Step 2: Apply the concept

    Electric current is measured in amperes (A), which is the SI unit. Volt is the unit of electric potential, ohm is the unit of resistance, and watt is the unit of power.

  3. Final Answer:

    Ampere → Option A

  4. Quick Check:

    SI unit of electric current = Ampere ✅
Hint: Remember 'A' for Ampere as current unit.
Common Mistakes: Confusing ampere with volt or ohm.
2. Which of the following is the SI unit of pressure?
easy
A. Newton
B. Pascal
C. Joule
D. Watt

Solution

  1. Step 1: Understand the quantity

    Pressure is defined as force applied per unit area.

  2. Step 2: Recall the SI unit

    The SI unit of pressure is pascal (Pa), which equals one newton per square meter (N/m²). Newton is unit of force, joule is energy, and watt is power.

  3. Final Answer:

    Pascal → Option B

  4. Quick Check:

    SI unit of pressure = Pascal ✅
Hint: Pressure = Force/Area → Pascal (N/m²).
Common Mistakes: Mistaking newton or joule as pressure unit.
3. The SI unit of energy is:
easy
A. Joule
B. Newton
C. Watt
D. Pascal

Solution

  1. Step 1: Identify the physical quantity

    Energy is the capacity to do work or produce heat.

  2. Step 2: Recall the SI unit

    The SI unit of energy is joule (J). Watt is unit of power, newton is force, and pascal is pressure.

  3. Final Answer:

    Joule → Option A

  4. Quick Check:

    SI unit of energy = Joule ✅
Hint: Energy unit = Joule (work done = force × distance).
Common Mistakes: Confusing joule with watt or newton.
4. Which of the following physical quantities has the SI unit 'candela'?
medium
A. Temperature
B. Electric current
C. Luminous intensity
D. Amount of substance

Solution

  1. Step 1: Understand the physical quantities

    The question asks about the SI unit 'candela' and which physical quantity it measures.

  2. Step 2: Apply knowledge of SI units

    Candela (cd) is the SI unit of luminous intensity. Electric current is measured in ampere, temperature in kelvin, and amount of substance in mole.

  3. Final Answer:

    Luminous intensity → Option C

  4. Quick Check:

    SI unit of luminous intensity = Candela ✅
Hint: Candela relates to brightness or light intensity.
Common Mistakes: Confusing candela with ampere or kelvin.
5. Which of the following is a derived physical quantity with SI unit watt?
medium
A. Pressure
B. Force
C. Energy
D. Power

Solution

  1. Step 1: Identify the physical quantities and their units

    The question tests knowledge of derived quantities and their SI units.

  2. Step 2: Match quantity with unit

    Power is the rate of doing work and its SI unit is watt (W). Force is measured in newton, energy in joule, and pressure in pascal.

  3. Final Answer:

    Power → Option D

  4. Quick Check:

    SI unit of power = Watt ✅
Hint: Power = Energy/time → Watt (Joule/second).
Common Mistakes: Confusing watt with joule or newton.

Mock Test

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