What is the difference between accuracy and precision?

Accuracy is the closeness of a measured value to the true value. Precision is the closeness of two or more measured values to each other.

How many base quantities are there in the SI system?

Seven — length, mass, time, temperature, electric current, amount of substance, and luminous intensity.

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I. Choose the Best Answer (MCQs)

Q1. Which system of units is the British System?

(a) CGS (b) MKS (c) FPS (d) SI

Answer(c) FPS The FPS (Foot-Pound-Second) system was used in Britain and is called the British System.

Q2. Electric current belongs to ___ quantities.

(a) Base (b) Supplementary (c) Derived (d) Professional

Answer(a) Base Electric current is one of the seven base quantities in the SI system. Its SI unit is the ampere (A).

Q3. SI unit of temperature is

(a) Celsius (b) Fahrenheit (c) kelvin (d) Ampere

Answer(c) kelvin Kelvin (K) is the SI base unit for thermodynamic temperature. Celsius and Fahrenheit are commonly used but not SI base units.

Q4. Amount of substance is

(a) Directly proportional to the number of atoms
(b) Inversely proportional to the number of atoms
(c) Directly proportional to the square of number of atoms
(d) Inversely proportional to the square of number of atoms

Answer(a) Directly proportional to the number of atoms The amount of substance increases as the number of atoms increases, maintaining a direct proportionality.

Q5. Luminous intensity is the intensity of

(a) Laser light (b) UV light (c) Visible light (d) IR light

Answer(c) Visible light Luminous intensity measures the power emitted by a light source in a particular direction per unit solid angle, considering only visible light.

Q6. Which device measures electric current?

AnswerAmmeter An ammeter is connected in series in a circuit to measure the current flowing through it.

Q7. SI unit stands for

(a) International system of units (b) Integrated System of units
(c) International symbol of units (d) Integrated symbol of units

Answer(a) International system of units SI stands for Systeme Internationale d'Unites (French), meaning International System of Units.

Q8. Closeness of two or more measured values is called

(a) Accuracy (b) Precision (c) Error (d) Approximation

Answer(b) Precision Precision refers to the agreement among repeated measurements. Accuracy refers to closeness to the true value.

Q9. Quantities other than base quantities are called

(a) Supplementary quantities (b) Derived quantities
(c) Professional quantities (d) Energy quantities

Answer(b) Derived quantities Derived quantities are obtained by combining base quantities. Examples: area, volume, speed, force.

Q10. Which statement about approximation is wrong?

(a) Approximation gives accurate value
(b) Approximation simplifies the calculation
(c) Approximation is very useful when little information is available
(d) Approximation gives the nearest value only

Answer(a) Approximation gives accurate value Approximation provides a value close to the actual value, not an exact or accurate one. It gives the nearest value, which makes option (a) the incorrect statement.

II. Fill in the Blanks

1. The solid angle is measured in Steradian. 2. Scientists recognized the need of 'Standard Units' for physical quantities. 3. The coldness or hotness of a substance is expressed by Temperature. 4. Ammeter is used to measure electric current. 5. One mole of substance contains 6.023 x 10^23 atoms or molecules. 6. Luminous Intensity is the amount of visible light emitted in unit area per unit Solid angle. 7. Quartz clock uses Electronic oscillations. 8. The uncertainty in measurement is called Error. 9. Accuracy is the closeness of the measured value to the original value. 10. The intersection of two straight lines gives us Plane angle.

III. True or False

Q1. SI units are metric system of units.

AnswerTrue — The SI system is a modern, complete, and coherent metric system.

Q2. Temperature is a measure of total kinetic energy of the particles in a system.

AnswerFalse — Temperature measures the average kinetic energy of particles, not the total kinetic energy. Total kinetic energy depends on the number of particles too.

Q3. In thermometers, freezing point of water is taken as the Upper Fixed Point.

AnswerFalse — The boiling point of water is the Upper Fixed Point. The freezing point is the Lower Fixed Point.

Q4. One coulomb of charge flowing per minute is called 'ampere'.

AnswerFalse — Ampere is defined as one coulomb of charge flowing per second, not per minute.

Q5. Amount of substance gives the number of particles present in the substance.

AnswerTrue — Amount of substance is proportional to the number of entities (atoms, molecules, ions, etc.) present.

Q6. Intensity of light from a candle is approximately equal to one 'candela'.

AnswerTrue — The candela was historically defined based on the light output of a standard candle.

Q7. Angle formed at the top of a cone is an example of 'Plane Angle'.

AnswerTrue — The angle at the apex of a cone is a plane angle measured in radians.

Q8. Quartz clocks are used in GPS Devices.

AnswerFalse — Atomic clocks are used in GPS devices because of their extremely high accuracy (1 second in 10^11 seconds). Quartz clocks are less accurate (1 second in 10^9 seconds).

Q9. Candela is used to express electric field intensity.

AnswerFalse — Candela is the SI unit of luminous intensity, not electric field intensity.

Q10. The number 4.582 can be rounded off as 4.58.

AnswerTrue — The digit after the second decimal place is 2, which is less than 5, so we drop it without rounding up.

IV. Match the Following

#	Column A	Column B	Answer
1	Temperature	kelvin	1 — b
2	Luminous intensity	candela	2 — e
3	Amount of substance	mole	3 — d
4	Electric current	ampere	4 — a
5	Mass	kilogram	5 — c

V. Assertion and Reason

Mark: (a) Both true, reason explains assertion; (b) Both true, reason does not explain; (c) Assertion true, reason false; (d) Assertion false, reason true.

Q1. Assertion: SI system is suitable for measurements. Reason: SI unit of temperature is kelvin.

Answer(b) Both are true but the reason does not explain the assertion. SI is suitable because its units are precisely defined and have the same value everywhere, not merely because temperature is measured in kelvin.

Q2. Assertion: Electric current, amount of substance, luminous intensity are fundamental quantities. Reason: They are independent of each other.

Answer(a) Both true and reason correctly explains the assertion. Fundamental (base) quantities are independent of each other.

Q3. Assertion: The seconds hand of a clock has a least count of one second. Reason: Least count is the maximum measurement that can be measured accurately.

Answer(c) Assertion is true but reason is false. Least count is the minimum measurement that can be measured accurately, not the maximum.

Q4. Assertion: Avogadro's number is the number of atoms in one mole. Reason: Avogadro's number is a constant.

Answer(a) Both true and reason correctly explains the assertion. Avogadro's number (6.023 x 10^23) is indeed a constant representing the number of particles in one mole.

Q5. Assertion: Radian is the unit of solid angle. Reason: One radian is the angle subtended at the centre of a circle by an arc equal to its radius.

Answer(d) Assertion is false (radian is the unit of plane angle; steradian is the unit of solid angle), but reason is true.

VI. Very Short Answers

Q1. What is the unit of mass in FPS system? Answer: Pound.

Q2. How many base quantities are in the SI system? Answer: Seven.

Q3. Which instrument is used for measuring temperature? Answer: Thermometer.

Q4. What is the Lower Fixed Point of the Fahrenheit scale? Answer: 32 degree F (freezing point of water).

Q5. What is the SI unit of Luminous Intensity? Answer: Candela (cd).

Q6. What is the value of Avogadro's number? Answer: 6.023 x 10^23.

Q7. What type of oscillations are used in atomic clocks? Answer: Periodic vibrations within atoms (vibrations of caesium-133 atoms).

Q8. What are the types of clocks based on display? Answer: Analog clock and digital clock.

Q9. How many times does the minute hand rotate in one hour? Answer: One time.

Q10. How many hours are there in a minute? Answer: 1/60 = 0.0167 hours.

VII. Short Answers

Q1. What is measurement? Answer: Measurement is the process of finding an unknown physical quantity by using a standard quantity called a unit.

Q2. Name the common systems of measurement. Answer: 1. FPS (Foot-Pound-Second) — British System 2. CGS (Centimetre-Gram-Second) — Gaussian System 3. MKS (Metre-Kilogram-Second) — Metric System 4. SI (International System of Units) — Modern, complete system

Q3. Define Temperature. Answer: Temperature is a measure of the average kinetic energy of the particles in a substance. It indicates how hot or cold a body is.

Q4. Define ampere. Answer: One ampere is defined as one coulomb of charge moving through a conductor in one second. (1 A = 1 C/s)

Q5. What is electric current? Answer: Electric current is the amount of electric charge flowing through a conductor per unit time. I = Q/t, where Q is charge in coulombs and t is time in seconds.

Q6. What is luminous intensity? Answer: Luminous intensity is the measure of the power of the visible light emitted by a source in a particular direction, per unit solid angle. Its SI unit is candela (cd).

Q7. Define mole. Answer: Mole is the SI unit of amount of substance. One mole of any substance contains 6.023 x 10^23 elementary entities (atoms, molecules, or ions). This number is called Avogadro's number.

Q8. Differentiate between Plane Angle and Solid Angle.

Feature	Plane Angle	Solid Angle
Definition	Angle between intersection of two lines/planes	Angle formed at the intersection of 3 or more planes at a common point
Dimensions	Two-dimensional	Three-dimensional
Unit	Radian (rad)	Steradian (sr)

Q9. What are errors? Answer: The uncertainties in measurement values are called errors. An error is the difference between the true value and the measured value of a physical quantity.

VIII. Detailed Answers

Q1. List the base quantities with their units and symbols.

Quantity	Unit	Symbol
Length	metre	m
Mass	kilogram	kg
Time	second	s
Temperature	kelvin	K
Electric Current	ampere	A
Amount of Substance	mole	mol
Luminous Intensity	candela	cd

Q2. Write a note on different types of clocks.

Analog clocks: Have three hands — hour hand (short and thick), minute hand (long and thin), second hand (long and very thin). They can be mechanical (powered by springs/pendulums) or electronic.

Digital clocks: Display time in numerals or symbols. They may use 12-hour or 24-hour format. Often called electronic clocks.

Quartz clocks: Activated by electronic oscillations controlled by a quartz crystal. The quartz crystal vibrates at a precise frequency when an electric field is applied. Accuracy: one second in every 10^9 seconds.

Atomic clocks: Use periodic vibrations within atoms (caesium-133). Accuracy: one second in every 10^11 seconds. Used in GPS and GLONASS navigation systems.

Q3. Explain accuracy and precision with examples.

Accuracy: The closeness of a measured value to the actual (true) value. Example: If the true temperature is 30 degree C and your thermometer reads 30.1 degree C, the measurement is accurate.
Precision: The closeness of two or more measured values to each other. Example: If repeated measurements read 30.1, 30.1, 30.2 degree C, the measurements are precise (close to each other) even if not perfectly accurate.

A measurement can be precise without being accurate, and vice versa. The difference between the true value and the measured value is called error.

Q4. Explain Greenwich Mean Time (GMT). Answer: GMT is the mean solar time at the Royal Observatory in Greenwich, London, measured at 0 degree longitude. The Earth is divided into 24 time zones, each spanning 15 degree of longitude, with a 1-hour time difference between adjacent zones. India is 5 hours 30 minutes ahead of GMT (IST = GMT + 5:30).

Q5. Write a note on approximation. Answer: Approximation is the process of finding a number that is acceptably close to the exact value. It involves rounding off numbers to the nearest place value. When data are inadequate or exact calculations are unnecessary, physicists use approximations based on scientific assumptions. Approximations can be refined when greater accuracy is needed. For example, rounding 4.582 to 4.58 is an approximation.

IX. Higher Order Thinking Questions

Q1. A friend claims to have had a 100 degree C fever. Is this possible? Answer: No. Normal human body temperature is about 98.4 degree F (37 degree C). The friend likely meant 100 degree F, not 100 degree C. A 100 degree C fever would be fatal since water boils at 100 degree C.

X. Problems for Practice

Q1. Convert 36 degree C to kelvin. Solution: K = C + 273 = 36 + 273 = 309 K

Q2. Convert 100 K to degree Celsius. Solution: C = K - 273 = 100 - 273 = -173 degree C

Q3. What is the current if 5 C of charge flows for 20 seconds? Solution: I = Q/t = 5/20 = 0.25 A

Q4. Convert 90 degree to radians. Solution: theta = (pi/180) x 90 = pi/2 radians

Q5. Round off 5.323 to two decimal places. Solution: The digit after the second decimal place is 3 (< 5), so we retain the last digit. Answer: 5.32

Common Mistakes

Confusing mass with weight: Mass (kg) is the amount of matter; weight (N) is the gravitational force on that mass. They are not the same.
Confusing accuracy with precision: Accuracy is closeness to the true value; precision is closeness among repeated measurements. A set of readings can be precise without being accurate.
Using Celsius or Fahrenheit as SI units: The SI unit of temperature is kelvin (K), not degree Celsius or degree Fahrenheit.
Confusing plane angle with solid angle: Plane angle is measured in radians (2D); solid angle is measured in steradians (3D).
Saying ampere is coulomb per minute: Ampere is coulomb per second, not per minute.
Mixing up atomic clocks with quartz clocks: Atomic clocks use caesium atom vibrations; quartz clocks use electronic oscillations in quartz crystals.

Exam Checklist

[ ] Can you list all 7 SI base quantities, units, and symbols?
[ ] Can you differentiate between FPS, CGS, MKS, and SI systems?
[ ] Can you define temperature, ampere, mole, candela?
[ ] Can you explain the difference between accuracy and precision?
[ ] Can you convert between Celsius and kelvin?
[ ] Can you calculate current from charge and time?
[ ] Can you convert degrees to radians?
[ ] Can you round off numbers correctly?
[ ] Can you distinguish between types of clocks (analog, digital, quartz, atomic)?
[ ] Can you explain GMT and time zones?

Frequently Asked Questions

The kilogram (kg).