1. Heat vs Temperature (Very Important)
Heat:
- A form of energy.
- Flows from a body at higher temperature to a body at lower temperature.
- SI unit: Joule (J) (older: calorie).
- Indicates how hot or cold a body is.
- Related to the average kinetic energy of particles.
- Measured in °C, °F, K.
हिंदी:
Heat = ऊर्जा जो गर्म से ठंडी वस्तु की ओर बहती है।
Temperature = गर्म–ठंडा होने की माप (कितनी गर्मी है, इसका पैमाना)।
दोनों अलग concepts हैं, अक्सर exam में confusion बनता है।
2. Units & Scales of Temperature
Common temperature scales:
- Celsius (°C)
- Fahrenheit (°F)
- Kelvin (K) – SI unit of temperature
- 0 °C – ice point (melting point of ice)
- 100 °C – steam point (boiling point of water at normal pressure)
हिंदी: NDA basic level पर सिर्फ इतना याद रखें –
दैनिक जीवन में Celsius, science में Kelvin ज्यादा use होता है।
3. Thermal Expansion (Expansion by Heat)
When substances are heated, their size generally increases – this is called
thermal expansion.
Solids: expand least
Liquids: expand more than solids
Gases: expand the most
Everyday uses:
Solids: expand least
Liquids: expand more than solids
Gases: expand the most
Everyday uses:
- Gaps left in railway tracks (to prevent bending in summer).
- Overhead electric wires hung loose (tight at low temperature).
- Bimetallic strips in switches/thermostats.
हिंदी: गर्म करने पर ज़्यादातर पदार्थ फैलते हैं –
इसलिए रेल की पटरी और पुल में joints छोड़े जाते हैं, वरना गर्मी में मुड़ सकते हैं।
4. Change of State & Latent Heat
Matter can change from one state to another on heating or cooling:
- Solid → Liquid (melting)
- Liquid → Gas (boiling/vaporisation)
- Gas → Liquid (condensation)
- Liquid → Solid (freezing)
- Latent heat of fusion: solid ↔ liquid
- Latent heat of vaporisation: liquid ↔ gas
हिंदी:
Ice जब 0 °C से पानी बनता है, temperature 0 °C पर ही रहता है –
लेकिन heat लगती है → यही latent heat है (छुपी हुई गर्मी)।
5. Modes of Heat Transfer
Heat can be transferred in three main ways:
- Conduction
- Convection
- Radiation
5.1 Conduction (Mainly in Solids)
Conduction: Heat transfer through a solid without the actual movement of particles.
Example: one end of metal rod heated → other end becomes hot.
Good conductors: metals (copper, aluminium, iron) Bad conductors (insulators): wood, plastic, rubber, glass, air
Good conductors: metals (copper, aluminium, iron) Bad conductors (insulators): wood, plastic, rubber, glass, air
हिंदी: Conduction = heat का एक कण से दूसरे कण तक pass होना,
लेकिन पदार्थ खुद नहीं चलता (जैसे गरम तवे से चम्मच तक गर्मी पहुँचना)।
5.2 Convection (Mainly in Liquids & Gases)
Convection: Heat transfer by actual movement of particles of liquid or gas.
Examples:
Examples:
- Water heating in a vessel (hot water rises, cold water sinks).
- Sea breeze & land breeze – important NDA concept.
हिंदी: Convection = liquid या gas के कण खुद चलकर heat लेकर जाते हैं।
दूध उबलते समय ऊपर–नीचे घूमता दिखाई देता है – यही convection है।
5.3 Radiation (No Medium Needed)
Radiation: Heat transfer in the form of waves; does not need any medium.
Examples:
Examples:
- Heat coming from the Sun to Earth.
- Feeling warm near a fire.
हिंदी: Radiation में heat wave के रूप में travel करती है –
Space में हवा नहीं, फिर भी सूरज की गर्मी हमें मिलती है → Radiation।
6. Everyday Examples (High-Value for NDA)
6.1 Sea Breeze & Land Breeze
Sea Breeze (Day):
- Land heats up faster than sea.
- Air above land becomes hot & rises → low pressure over land.
- Cool air from sea moves towards land → sea breeze.
- Land cools faster than sea.
- Now air above sea is warmer → low pressure over sea.
- Cool air from land moves towards sea → land breeze.
हिंदी:
दिन में हवा समुद्र से ज़मीन की ओर → Sea breeze
रात में ज़मीन से समुद्र की ओर → Land breeze
ये सब convection की वजह से होता है।
6.2 Black vs White Clothes
- Black objects absorb more heat radiation.
- White/light-coloured objects reflect most heat.
- We prefer light-coloured clothes in summer.
- Dark-coloured clothes may feel warmer in sunlight.
6.3 Thermos Flask (Heat Loss Control)
Thermos flask is designed to reduce heat loss by:
- Double-walled glass with vacuum in between → reduces conduction & convection.
- Silvery surface → reflects radiation.
- Airtight stopper → reduces convection.
6.4 Pressure Cooker & Boiling Point (linked with Heat)
In a pressure cooker:
- Pressure inside increases when steam is trapped.
- Higher pressure → higher boiling point of water.
- Food can be cooked faster at higher temperature.
हिंदी: Cooker में दबाव बढ़ने से पानी का उबलने का तापमान बढ़ जाता है →
ज़्यादा temperature → खाना जल्दी पकता है।
📘 Practice Questions (NDA Style)
1. Heat is a form of:
a) Force
b) Energy
c) Matter
d) Pressure
2. The SI unit of temperature is:
a) °C
b) °F
c) K (Kelvin)
d) J
3. The heat required to change the state of a substance without change in temperature is called:
a) Specific heat
b) Latent heat
c) Sensible heat
d) Conduction heat
4. Sea breeze blows:
a) From land to sea during day
b) From sea to land during day
c) From land to sea during night
d) Only in winter
5. Which mode of heat transfer does not require a medium?
a) Conduction
b) Convection
c) Radiation
d) All of these
6. Thermos flask reduces heat loss mainly by:
a) Only conduction
b) Only convection
c) Only radiation
d) Reducing conduction, convection and radiation
a) Force
b) Energy
c) Matter
d) Pressure
2. The SI unit of temperature is:
a) °C
b) °F
c) K (Kelvin)
d) J
3. The heat required to change the state of a substance without change in temperature is called:
a) Specific heat
b) Latent heat
c) Sensible heat
d) Conduction heat
4. Sea breeze blows:
a) From land to sea during day
b) From sea to land during day
c) From land to sea during night
d) Only in winter
5. Which mode of heat transfer does not require a medium?
a) Conduction
b) Convection
c) Radiation
d) All of these
6. Thermos flask reduces heat loss mainly by:
a) Only conduction
b) Only convection
c) Only radiation
d) Reducing conduction, convection and radiation
Answers:
1 – b) Energy
2 – c) K (Kelvin)
3 – b) Latent heat
4 – b) From sea to land during day
5 – c) Radiation
6 – d) Reducing conduction, convection and radiation
1 – b) Energy
2 – c) K (Kelvin)
3 – b) Latent heat
4 – b) From sea to land during day
5 – c) Radiation
6 – d) Reducing conduction, convection and radiation
Quick Recap | सार
Now you should be able to:
- Recall the most important ideas of this topic.
- Solve typical NDA-style questions from this chapter.
- Spot and avoid common traps used in competitive exams.
Self-check: 3–4 random questions उठा कर देखिए – अगर आसानी से हो रहे हैं, तो chapter strong है।
Common Exam Mistakes | आम गलतियाँ
- Concept ठीक से clear न करना और सीधे रटने की कोशिश करना।
- Question पूरी तरह पढ़े बिना answer mark कर देना।
- Figures, dates, या formulas को last step में verify न करना।
NDA में smart strategy यह है कि आसान वाले सवाल पहले, doubtful बाद में – और silly mistakes से बचना सबसे ज़रूरी है।