Physics

medium importance~1 Q in Tier 127 formulas⚡ 12 shortcuts6 subtopics

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Heat, temperature and sound

⏱ 6 min read🧩 6 question types🎯 20 practice Q

The idea in one minute

Heat is energy that flows from a hot body to a cold one. Temperature tells how hot a body is. Sound is a wave that needs a medium, and it travels fastest in solids. Exams ask temperature conversions, everyday heat effects and sound facts.

Heat and temperature are different

Heat is energy that flows from a hotter body to a colder one. Its unit is the joule. The older unit is the calorie.

Temperature tells how hot a body is. It is the average energy of its particles. Its unit is the kelvin.

A bucket of warm water holds more heat than a cup of boiling water. The cup has the higher temperature.

Temperature scales

Scale	Ice melts	Water boils
Celsius	0 °C	100 °C
Fahrenheit	32 °F	212 °F
Kelvin	273 K (273.15)	373 K

The conversions are $F = \dfrac{9}{5}C + 32$ and $K = C + 273$ .

Celsius and Fahrenheit read the same at −40°.
Normal body temperature is 37 °C = 98.6 °F = 310 K.
The lowest possible temperature is 0 K, called absolute zero. It is −273.15 °C.
A clinical thermometer reads about 35 to 42 °C. A pyrometer measures very high temperatures.

Example: Convert 25 °C to °F. $\dfrac{9}{5} \times 25 + 32 = 45 + 32 = 77$ °F.

Specific heat and latent heat

Specific heat is the heat needed to raise 1 kg of a substance by 1 °C. Water has a very high specific heat. It heats and cools slowly. So coastal places have mild climates, and water is used as a coolant.
Latent heat is heat taken or given during a change of state, with no change in temperature. Steam at 100 °C burns more than water at 100 °C. Steam also gives out its latent heat of vaporisation when it condenses on skin.
Evaporation causes cooling. A matka (earthen pot) keeps water cool. Sweat cools the body.

Tip: Evaporation is faster with more surface area, more heat, wind and dry air.

Boiling, melting and pressure

Higher pressure raises the boiling point. A pressure cooker cooks faster because water inside boils above 100 °C.

On mountains the air pressure is low. Water boils below 100 °C and food cooks slowly.

Pressure lowers the melting point of ice. This is called regelation. Two ice blocks pressed together join.

Water expands the wrong way

Most things expand when heated. Water shrinks from 0 °C to 4 °C and then expands. So water is densest at 4 °C.

In winter the top of a lake freezes first. The floating ice insulates the water below, which stays near 4 °C. Fish survive.

Pipes burst in frost because water expands when it freezes. Gaps in railway tracks and sagging wires in summer show ordinary thermal expansion.

Three ways heat travels

Conduction moves heat through solids, particle to particle. A spoon handle gets hot this way. Metals conduct well. Wood, plastic and air are insulators.
Convection moves heat when the liquid or gas itself moves. Examples are the sea breeze (day), the land breeze (night), boiling water and room heaters.
Radiation needs no medium. The Sun's heat reaches us this way.

Dark, rough surfaces absorb and emit heat well. Light, shiny surfaces reflect it. So white clothes keep us cool in summer.

Note: A thermos flask blocks all three. The vacuum stops conduction and convection. The silvered walls stop radiation.

Sound is a mechanical wave

Sound comes from vibration. It is a longitudinal wave and it needs a medium. It cannot travel in a vacuum. Astronauts use radio.

Medium	Speed of sound
Air at 0 °C	about 332 m/s
Air at 20 °C	about 343 m/s
Water	about 1500 m/s
Steel	about 5000 m/s or more

The order is solids > liquids > gases. Speed rises with temperature and humidity. Pressure alone does not change it.

Frequency, pitch and loudness

Humans hear 20 Hz to 20,000 Hz.
Below 20 Hz is infrasonic: earthquakes, elephants, whales.
Above 20 kHz is ultrasonic: bats, dolphins, SONAR, scans, cleaning.
Pitch depends on frequency. Women and children have higher-pitched voices.
Loudness depends on amplitude. It is measured in decibels (dB).
Quality (timbre) depends on the wave shape. It lets us tell a sitar from a flute on the same note.

Echo, Doppler effect and resonance

Echo: a reflected sound is heard separately if it returns after at least 0.1 s. The reflector must be about 17 m away. Distance = speed × time ÷ 2.
Doppler effect: an approaching horn sounds higher. A receding one sounds lower.
Resonance: a body vibrates strongly when pushed at its natural frequency. Soldiers break step on a bridge.
Sonic boom: made by objects that move faster than sound. Mach number is object speed ÷ speed of sound. Above 1 is supersonic.

Question types you will see

Each type: how to recognise it, the method step by step, and one question to try.

Type 1very common3 practice Q

Temperature scale conversion

How to spot it:

A temperature must be converted, or the question asks when two scales read the same or one is double the other.

\frac{C}{5}=\frac{F-32}{9}=\frac{K-273}{5}

Method

Write $\dfrac{C}{5}=\dfrac{F-32}{9}$ .
Put in the given value or the condition, such as F = C or F = 2C.
Solve for the unknown. For kelvin, just add 273.

Why it works:

100 Celsius steps cover the same range as 180 Fahrenheit steps, so the ratio is 5 to 9 with a 32° shift.

Try this

Convert 25 °C to Fahrenheit.

Show solution

$F = \dfrac{9}{5}\times 25 + 32$ .
That is 45 + 32.

Answer

77 °F

Type 2very common4 practice Q

Everyday heat effect and its reason

How to spot it:

The question says 'Why does steam burn more / pressure cooker cook faster / earthen pot cool water / fish survive frozen lakes / coast have mild climate?'

Method

A state change with no temperature change points to latent heat.
Slow heating or cooling of water points to high specific heat.
Boiling point rises with pressure and falls with height.
Cooling without a fridge points to evaporation. Lakes freezing from the top point to 4 °C density.

Why it works:

Each everyday effect has one standard reason.

Try this

Why does food take longer to cook on high mountains?

Show solution

Air pressure is low there.
Water boils below 100 °C.
The food gets less heat.

Answer

Water boils at a lower temperature, so food cooks slowly

Type 3common2 practice Q

Mode of heat transfer

How to spot it:

A situation (sea breeze, Sun's heat, hot spoon handle, thermos flask, white clothes) with conduction, convection and radiation as options.

Method

If heat moves through a solid and the solid stays still, choose conduction.
If the warm fluid itself moves (breezes, boiling), choose convection.
If no medium is needed, or colour matters, choose radiation.

Why it works:

The three modes differ in how the particles take part.

Try this

A land breeze at night is an example of:

Show solution

At night the sea is warmer.
Air over the sea rises.
Cooler air from the land moves in.

Answer

Convection

Type 4common2 practice Q

Speed of sound and the medium

How to spot it:

The question says 'Sound travels fastest in ...', 'cannot travel through ...' or 'speed of sound increases with ...'.

Method

Recall the order: solids > liquids > gases. A vacuum has no sound.
Temperature and humidity raise the speed in air. Pressure alone does not.
Recall the values: air 343 m/s, water 1500 m/s, steel 5000 m/s.

Why it works:

Particles in a solid are closer, so the vibration passes faster.

Try this

In which medium is sound fastest: air, water, steel or vacuum?

Show solution

A vacuum has no sound.
Solids carry sound fastest.
Steel is a solid.

Answer

Steel

Type 5very common3 practice Q

Frequency ranges, pitch, loudness and quality

How to spot it:

The question gives the audible range, users of infrasonic or ultrasonic sound (bats, dolphins, elephants), or asks which property decides pitch, loudness or timbre.

Method

Audible range is 20 Hz to 20 kHz. Below it is infrasonic. Above it is ultrasonic.
Pitch goes with frequency. Loudness goes with amplitude (dB). Quality goes with the wave shape.
Ultrasound is used in SONAR, medical scans and bat echolocation.

Why it works:

Each property of the note is set by a different property of the wave.

Try this

Sound waves of frequency 10 Hz are called:

Show solution

The lower limit of hearing is 20 Hz.
10 Hz is below it.

Answer

Infrasonic

Type 6occasional3 practice Q

Echo, Doppler effect and resonance

How to spot it:

An echo-distance sum, a horn whose pitch rises as it approaches, or soldiers breaking step on a bridge.

d=\frac{v\times t}{2}

Method

For echo, sound goes to the reflector and back. So distance = v × t ÷ 2.
If the heard pitch changes because source or listener moves, it is the Doppler effect.
If a body vibrates strongly at its natural frequency, it is resonance.

Why it works:

Echo time covers a round trip, so you halve it.

Try this

An echo returns in 3 s. The speed of sound is 330 m/s. How far is the wall?

Show solution

Round trip $= 330 \times 3 = 990$ m.
One way is 990 ÷ 2.

Answer

495 m

Formula sheet

Temperature scales

\frac{C}{100} = \frac{F-32}{180} = \frac{K-273}{100}

−40 °C = −40 °F

Heat absorbed

Q = mc\,\Delta T

c = specific heat

Latent heat

Q = mL

no temperature change during phase change

Wave speed

v = f\lambda

speed = frequency × wavelength

Echo distance

d = \frac{v\,t}{2}

sound travels to the wall and back

Shortcuts that save time

⚡ Sound speed order

Solids are fastest, then liquids, then gases. A vacuum has no sound at all.

Example

Why is an approaching train heard earlier through the rail?

Show solution

The rail is a solid.
Sound is faster in solids than in air.

Answer

Sound travels much faster in steel than in air

⚡ Pitch and loudness

Pitch goes with Frequency. Loudness goes with Amplitude.

Example

The pitch of a sound depends on its:

Show solution

Pitch is how high or low a note is.
It follows the frequency.

Answer

Frequency

Mistakes to avoid

Where most students lose marks on this subtopic.

Mistake 01

Thinking sound speed rises with pressure.

It rises with temperature and humidity, not pressure alone.

Mistake 02

Calling the sound of bats infrasonic.

Bats use ultrasonic sound, above 20 kHz.

Mistake 03

Saying the boiling point rises at hill stations.

It falls, because the air pressure is lower.

Mistake 04

Using 'degree kelvin' or forgetting to add 273 for kelvin.

K = C + 273.

Mistake 05

Thinking water is densest at 0 °C.

Water is densest at 4 °C.

Quick revision

Read this the night before the exam.

F = 9C/5 + 32 and K = C + 273. −40 °C = −40 °F. Body temperature 37 °C = 98.6 °F.
Water has the highest specific heat and is densest at 4 °C.
Steam burns worse because of latent heat. A pressure cooker raises the boiling point.
Conduction: solids. Convection: fluids and breezes. Radiation: Sun, no medium needed.
Sound: solids fastest, no sound in vacuum, audible range 20 Hz to 20 kHz.
Pitch goes with frequency. Loudness goes with amplitude. Echo distance = vt/2.

Practice: 20 questions

Sets of 10, mixed across the question types above. Every answer has a step-by-step explanation.

Topic test · 10 questions

Suggested time 3 min · wrong answers go to your mistake notebook automatically.

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