Units, Measurements & Instruments

🎯 Learning Objective

Know the SI units of common quantities, tell scalars from vectors, and match instruments to what they measure.

💡 Concept

• SI base units: length = metre (m), mass = kilogram (kg), time = second (s)
• Also base units: electric current = ampere (A), temperature = kelvin (K), amount = mole (mol), luminous intensity = candela (cd)
• Derived units: force = newton (N), work/energy = joule (J), power = watt (W), pressure = pascal (Pa), frequency = hertz (Hz)
• Scalar = magnitude only: mass, distance, speed, time, temperature, energy
• Vector = magnitude + direction: displacement, velocity, acceleration, force, momentum
• Barometer measures atmospheric pressure; thermometer measures temperature; hygrometer measures humidity
• Ammeter measures current; voltmeter measures voltage; seismograph records earthquakes
• Lactometer checks milk purity; anemometer measures wind speed; speedometer shows speed
• Least count = the smallest value an instrument can measure accurately

🧮 Key Formulas

Speed = Distance / Time (scalar)

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Velocity = Displacement / Time (vector)

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Max electrons in a shell = 2n²

✏️ Easy Example

Q. Which of the following is a VECTOR quantity? (a) Speed (b) Distance (c) Velocity (d) Mass

1. A vector needs both size AND direction
2. Velocity has a direction; speed, distance and mass do not

Answer: (c) Velocity

🇮🇳 Real-Life Example

The 'speedometer' on a bike shows speed (a scalar), but Google Maps giving you '2 km north-east' is a vector — it adds direction. Same journey, two different physics quantities.

📝 Exam-Level Example

Q. The SI unit of force is: (a) joule (b) newton (c) watt (d) pascal

1. Force = mass × acceleration
2. Its SI unit is the newton (N)

Answer: (b) newton

📝 Exam-Level Example

Q. Which instrument is used to measure atmospheric pressure?

1. 'Baro' relates to pressure/weight
2. Barometer measures atmospheric pressure

Answer: Barometer

🪄 Memory Trick

Scalars have 'no arrow' — Speed, Distance, Mass, Time, Energy. Vectors carry an arrow (direction) — Velocity, Force, Displacement, Acceleration, Momentum.

⚠️ Common Mistakes

• ❌ Calling speed a vector — speed is scalar; velocity is its vector version
• ❌ Mixing up joule (energy) and newton (force)
• ❌ Confusing hygrometer (humidity) with hydrometer (liquid density)

🏆 Exam Tips

• ✅ Learn instruments as pairs: barometer→pressure, thermometer→temperature, ammeter→current
• ✅ If a quantity answers 'which direction?', it's a vector

📌 Summary

• Base SI units: metre, kilogram, second, ampere, kelvin, mole, candela
• Force = newton, Energy = joule, Power = watt, Pressure = pascal
• Scalar = magnitude only; Vector = magnitude + direction
• Barometer→pressure, Ammeter→current, Lactometer→milk