1.2 Physical and Non-Physical Quantities

In our daily lives, we encounter a wide variety of things. Some we can measure, like the length of a room or the mass of a bag, while others we can only experience, like the taste of a fruit or the feeling of happiness. Physics, as a science, relies on precise measurements. This requires us to clearly distinguish between quantities that can be measured and those that cannot.

Definition

Physical Quantities: Physical quantities are those quantities which can be measured and defined in terms of a standard unit. They have a numerical magnitude and a unit. Non-Physical Quantities: Non-physical quantities are those quantities which cannot be measured or defined in terms of any standard unit.

Main Idea

The main idea is the fundamental requirement of science: measurement. For a quantity to be studied scientifically, it must be measurable. Physical quantities form the entire basis of physics because they allow us to observe, calculate, and predict natural phenomena. Non-physical quantities, while real and important to human experience, fall outside the scope of scientific measurement and thus outside the direct study of physics.

Core Concept

Measurement Measurement is the process of comparing an unknown physical quantity with a standard to see how large or small it is compared to that standard.

Unit A unit is the standard with which physical quantities are compared.

• Numerical Magnitude: The number that represents the size of the quantity.
• Unit: The standard in which it is measured.

• "1.65" is the numerical magnitude.
• "metre" is the unit.

To record a measurement, you choose an appropriate unit and then find the size of the quantity using an instrument with a proper scale, like a measuring tape for length.

Detailed Explanation

• Length
• Mass
• Time
• Density
• Temperature

Because we can assign a specific number and unit to these (e.g., 5 kilograms, 10 seconds, 25 °C), we can use them in calculations and scientific laws.

• Taste
• Feeling (like happiness, sadness, or pain)
• Color
• Beauty

While these are real, you cannot use an instrument to measure "5 units of happiness" or "10 units of sweetness." Therefore, they are not considered physical quantities in the context of science.

Intermediate Level

The distinction between physical and non-physical quantities is crucial for the scientific method. Science requires objective, repeatable data. A measurement of 5 kg of mass is objective; anyone using a correct scale will get the same result. A "feeling" is subjective; different people can have different experiences of the same thing. By defining physical quantities and their units, scientists create a universal language to describe the universe. A measurement is always a comparison to a standard unit, ensuring that when a physicist in Pakistan says an object has a mass of 1 kg, a physicist in Japan understands exactly what that means.

Important Points

• - Physical quantities can be measured and expressed with a numerical magnitude and a unit.
• - Non-physical quantities cannot be measured.
• - Measurement is a comparison between an unknown quantity and a standard unit.
• - Every measurement has two parts: a number and a unit.
• - Physics deals exclusively with physical quantities.

Remember

• - Physical = Measurable (has number + unit).
• - Non-Physical = Not Measurable (based on feeling/senses).
• - Measurement = Numerical Magnitude + Unit.

Common Mistakes

• - Thinking any quantity with a number is physical: "I have 2 brothers" is a count (an exact number), not a measurement of a physical quantity with a unit.
• - Forgetting the unit: Writing "The length of the table is 2" is meaningless in physics. It must be "2 metres."
• - Confusing non-physical with unreal: Taste and color are real things, but they are non-physical because they can't be objectively measured with a standard unit.

Quick Revision

Physical quantities are those that can be measured and expressed with a numerical magnitude and a unit, like length, mass, and time. Non-physical quantities, such as taste and feelings, cannot be measured with a standard unit. Measurement is the process of comparing a physical quantity to a standard unit, which is essential for all scientific study.

60-Second Revision

Physical quantities can be measured (e.g., length in metres). Non-physical quantities cannot be measured (e.g., taste). Every scientific measurement must include a numerical value and a unit. Physics is built entirely on measurable, physical quantities.

Frequently Asked Questions (FAQs)

Q1. What is the basic difference between a physical and a non-physical quantity? Answer: The basic difference is measurability. A physical quantity can be measured, defined, and expressed with a numerical value and a standard unit (like 5 kg). A non-physical quantity cannot be measured with any instrument or unit (like taste or feeling).

Q2. Why are taste and color considered non-physical quantities? Answer: They are considered non-physical because they are subjective sensations. There is no standard unit or instrument to objectively measure the exact "amount" of a taste or color. They depend on human perception, not an objective, reproducible standard.

• Q3. What are the two essential components of a measurement?
• Answer: A complete measurement consists of two parts:
• 1. A numerical magnitude (a number).
• 2. A unit (the standard of measurement).
• For example, in "10 seconds," 10 is the magnitude and seconds is the unit.

Q4. Is time a physical quantity? Answer: Yes, time is a physical quantity. It can be defined and measured using instruments like clocks and stopwatches, and it has standard units like seconds, minutes, and hours.

Q5. What is the definition of a unit? Answer: A unit is a standard of measurement with which other physical quantities of the same kind are compared to determine their size. For example, the metre is the standard unit for measuring length.

Q6. Can a quantity be physical if it has a number but no unit? Answer: No. In physics, a number alone is not a measurement. For example, saying "the distance is 5" is meaningless. It must be "the distance is 5 metres." The unit provides the context and standard for the numerical magnitude.

Multiple Choice Questions (MCQs)

• Q1. Which of the following is a physical quantity?
• A. The beauty of a painting
• B. The taste of an apple
• C. The mass of a book
• D. The feeling of happiness

• Correct Answer: C
• Explanation: Mass can be measured with an instrument (a balance) and has a standard unit (kilogram). The others are subjective experiences.

• Q2. Which of the following is an example of a non-physical quantity?
• A. Temperature
• B. Length
• C. Color
• D. Time

• Correct Answer: C
• Explanation: Color is a subjective perception that cannot be measured with a standard unit, unlike temperature, length, and time.

• Q3. A measurement of a physical quantity consists of:
• A. Only a number
• B. Only a unit
• C. A standard and an instrument
• D. A numerical magnitude and a unit

• Correct Answer: D
• Explanation: The text explicitly states that a measurement consists of a numerical magnitude and a unit.

• Q4. The process of comparing an unknown quantity with a standard is called:
• A. Calculation
• B. Measurement
• C. Definition
• D. Observation

• Correct Answer: B
• Explanation: The text defines measurement as a comparison between an unknown physical quantity and a standard.

• Q5. In the measurement "1.65 metres," what does "1.65" represent?
• A. The unit
• B. The standard
• C. The numerical magnitude
• D. The instrument

• Correct Answer: C
• Explanation: The text uses this example to show that 1.65 is the numerical magnitude and "metre" is the unit.

• Q6. Which of the following cannot be defined or measured?
• A. Density
• B. Temperature
• C. Feeling
• D. Mass

• Correct Answer: C
• Explanation: Feeling is a non-physical quantity and cannot be measured with a standard unit.

• Q7. A standard with which physical quantities are compared is called a:
• A. Number
• B. Magnitude
• C. Unit
• D. Scale

• Correct Answer: C
• Explanation: The text defines a unit as the standard with which physical quantities are compared.

• Q8. Which of these is NOT a physical quantity?
• A. The speed of a car
• B. The sweetness of honey
• C. The volume of a box
• D. The temperature of water

• Correct Answer: B
• Explanation: Sweetness is a taste and cannot be measured. Speed, volume, and temperature are all measurable physical quantities.

• Q9. To record a measurement, you need an appropriate unit and:
• A. An instrument with a proper scale
• B. A non-physical quantity
• C. A subjective feeling
• D. A random guess

• Correct Answer: A
• Explanation: The text says to record a measurement, you find the size of the quantity using an instrument with a proper scale.

• Q10. Density is an example of a:
• A. Non-physical quantity
• B. Physical quantity
• C. Feeling
• D. Unit

• Correct Answer: B
• Explanation: The textbook lists density as a physical quantity because it can be measured.

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