Thermal Physics CSEC

Define heat.

A form of energy transferred from areas of higher temperature to lower temperature until the objects reach thermal equilibrium.

What was the Caloric Theory, and how was it disproven?

Heat was believed to be a weightless fluid called "caloric." It was disproven by Count Rumford's cannon-boring experiments, which showed heat is generated from friction (mechanical energy converting to heat) — later confirmed by James Joule.

State the Kinetic Theory of Matter.

Molecules in a gas move freely and rapidly in straight lines; this random molecular bombardment can be observed as Brownian motion.

How do bond strength and density compare across solids, liquids, and gases?

Bond strength and density are highest in solids and lowest in gases. Solids have a fixed shape; liquids and gases both take the shape of their container.

What is the SI unit of temperature, and how do you convert Celsius to Kelvin?

Kelvin (K); add 273 to the Celsius value.

What is absolute zero?

The coldest possible temperature (0 K), at which a substance has no internal/thermal energy.

Why can't there be negative Kelvin temperatures?

Because 0 K (absolute zero) is defined as the coldest possible temperature.

Why is mercury preferred over alcohol in liquid-in-glass thermometers?

Mercury has a much higher boiling point (357°C) than alcohol (78°C).

What is the temperature range of a clinical thermometer, and what is its key design feature?

35-45°C; it has a large, thin bulb (conducts heat quickly) and a constriction that prevents mercury from returning to the bulb too fast.

What is the temperature range of a thermocouple, and why is it useful?

-200 to 1250°C; it gives quick, accurate readings from voltage and can measure the temperature of small masses.

What factors make a liquid-in-glass thermometer more sensitive?

A less expansive liquid, a larger bulb, and thinner glass around the bulb.

What causes thermal expansion in solids?

Heating adds kinetic energy, making molecules vibrate faster and move further apart, increasing the solid's volume.

How does a bimetallic strip work?

It consists of two metals (e.g. steel/iron and copper/brass) that expand at different rates when heated, causing the strip to bend.

Why does the Greenhouse Effect occur?

Short-wavelength radiation penetrates glass easily, but converts to longer wavelengths on reflection inside, which can't escape as easily — trapping heat.

Why does evaporation have a cooling effect?

The most energetic molecules escape from the liquid's surface, lowering the average kinetic energy (and temperature) of the remaining liquid.

Does depth affect the rate of evaporation? Why or why not?

No — evaporation only occurs at the surface of a liquid.

List factors that affect the rate of evaporation.

Humidity, air movement, pressure, temperature, and surface area.

Distinguish evaporation from boiling.

Evaporation occurs at any temperature with no bubbles present; boiling occurs only at the boiling point, with bubbles present.

Describe the three methods of heat transfer.

Conduction: transfer via vibrations passed to adjacent molecules. Convection: transfer via movement of the medium itself. Radiation: transfer via electromagnetic waves (can occur in a vacuum).

Why are metals good thermal conductors?

They have free electrons that allow efficient transfer of heat.

Why is air a poor conductor of heat?

It lacks free electrons and often has structural gaps/air spaces that don't transfer heat efficiently.

How does a vacuum flask keep a liquid's temperature stable?

Shiny inner walls reflect radiated heat back in; the vacuum layer between the double walls limits conduction; the cover prevents convection heat loss at the top.

Which surfaces are the best absorbers and emitters of thermal radiation?

Black, dull/matte surfaces with a large surface area (good absorbers are also good emitters).

How does a solar water heater work?

Black copper pipes absorb radiated heat from the Sun and conduct it into flowing water, while a glass pane traps heat inside via the Greenhouse effect.

Define temperature.

A quantitative measure of hotness or coldness; a scalar index of the average kinetic energy of the particles of a substance. SI unit: kelvin (K). It is an intensive property (independent of mass or volume).

Define thermal energy (heat).

The net energy transferred from a region of high temperature to a region of low temperature down a thermal gradient. SI unit: joule (J). It is an extensive property (depends on mass, material and state).

State the kinetic theory postulate about matter.

All matter is composed of tiny particles in continuous random motion. Macroscopic temperature is directly proportional to the average translational kinetic energy of these particles.

Compare temperature and heat.

Temperature: average particle kinetic energy; intensive; measured in K. / Heat: total energy transferred due to a temperature difference; extensive; measured in J.

What happens to molecular motion when temperature increases?

Particles move faster; their average kinetic energy increases. As a substance is heated, particle velocity, collision frequency and intermolecular separation all increase.

List the physical effects of heat on matter.

1. Dimensional expansion (linear, superficial or volumetric). / 2. Phase transformations (melting, boiling, sublimation). / 3. Electrical resistance fluctuations (increases in metals; decreases in semiconductors). / 4. Thermoelectromotive force generation across bimetallic junctions.

State the two fixed points of the Celsius scale.

Lower fixed point (0 °C): pure melting ice at standard atmospheric pressure (101.3 kPa). / Upper fixed point (100 °C): pure steam point (boiling water) at standard atmospheric pressure.

Write the Celsius-Kelvin conversion formulas.

T (K) = θ (°C) + 273 / θ (°C) = T (K) - 273

Convert 37 °C (normal body temperature) to kelvin.

T = 37 + 273 = 310 K

Liquid oxygen boils at 90 K. Convert this to °C.

θ = 90 - 273 = -183 °C

Why is -300 °C physically impossible?

It is below absolute zero (0 K = -273 °C), the lowest possible temperature in the universe.

Explain thermal expansion.

As internal kinetic energy increases, molecular separation widens, so the material expands macroscopically (linear, superficial or volumetric). Examples: sagging overhead lines, expansion gaps in railway tracks.

What is a phase transformation?

An isothermal change of state (melting, boiling, sublimation) in which absorbed energy overcomes latent intermolecular binding forces rather than accelerating particles.

What is a thermometer?

An instrument used to measure temperature.

What property does a liquid-in-glass thermometer use?

The volume expansion of a fluid (mercury or alcohol) trapped in a uniform-bore capillary tube. Engineering advantage: high linearity and easy visual reading.

State the typical range of a mercury-in-glass thermometer.

About -39 °C to 357 °C. Mercury expands fairly uniformly; laboratory models can be read to about 0.1 °C.

List the advantages of mercury thermometers.

Wide temperature range; fairly uniform expansion; easy-to-see column; does not wet the glass (clear boundary).

List the disadvantages of mercury thermometers.

Mercury is toxic; requires careful handling/disposal; freezes at about -39 °C (not suitable for very low temperatures); largely replaced by safer alternatives.

State the typical range of an alcohol-in-glass thermometer.

Approximately -80 °C to 70 °C (depends on alcohol and design). The coloured liquid is easy to see and useful for low temperatures.

List the advantages of alcohol thermometers.

Safer than mercury if broken; measures much lower temperatures; inexpensive and simple; brightly coloured liquid for easy reading.

List the disadvantages of alcohol thermometers.

Cannot measure very high temperatures (alcohol boils at relatively low temperatures); expands less uniformly than mercury over some ranges; liquid may stick to glass; less suitable for high-temperature laboratory work.

What property does a thermocouple thermometer use?

Generation of an electromotive force (e.m.f. / voltage) across bimetallic junctions subject to a temperature difference (Seebeck effect). Advantages: extremely rapid response and point-measurement capability.

What property does a platinum resistance thermometer (PRT) use?

Variation of electrical resistance in a calibrated platinum wire. Advantage: ultra-precise over wide ranges with high stability. Range: -200 °C to 800 °C.

What property does a gas thermometer use?

Changes in pressure or volume of an enclosed gas. Used as a standard reference instrument to calibrate other thermometers; highly accurate but slow response.

Define a thermocouple thermometer.

An electrical temperature-measuring device of two wires of different metals joined at two junctions. A temperature difference between the junctions generates an e.m.f. (voltage) that increases with the temperature difference.

State three advantages of a thermocouple thermometer.

1. Wide temperature range (very low to very high). / 2. Fast response (detects rapidly changing temperatures). / 3. Durable and small (strong, lightweight, usable in small/difficult places).

State three disadvantages of a thermocouple thermometer.

1. Less accurate than some other devices. / 2. Requires calibration for reliable readings. / 3. Small electrical output (tiny voltage; sensitive equipment needed).

State three thermometric properties of a thermocouple.

1. High responsiveness (low heat capacity) — thin wires reach equilibrium quickly. / 2. Wide temperature range (solid-metal junctions, no liquid phase-change limits): below -200 °C to over +1700 °C. / 3. Direct electrical voltage output (e.m.f.) (Seebeck effect) — proportional to ΔT; feeds digital displays/data loggers.

Define thermocouple sensitivity.

The change in output voltage per unit change in temperature, calculated from the slope of the voltage-temperature curve: / S = ΔV / ΔT

State the units of thermocouple sensitivity.

Microvolts per degree Celsius (μV/°C) or millivolts per kelvin (mV/K).

Worked example: A thermocouple's voltage rises from 1.0 mV to 5.0 mV as temperature rises from 20 °C to 120 °C. Find its sensitivity.

ΔV = 5.0 - 1.0 = 4.0 mV / ΔT = 120 - 20 = 100 °C / S = 4.0 / 100 = 0.04 mV/°C

State the operating ranges of common thermocouple types.

Type K (Chromel-Alumel): -270 to 1372 °C / Type E (Chromel-Constantan): -270 to 1000 °C / Type R/S (Platinum-Rhodium): -50 to 1768 °C / Type B (Platinum-Rhodium): 0 to 1820 °C

Define responsiveness as it relates to a thermocouple.

How quickly the thermometer detects and adjusts to a temperature change to reach thermal equilibrium. Exceptionally high because of low heat capacity (thin wires need little heat) and rapid thermal equilibrium (junction tracks rapidly changing temperatures in real time).

State two advantages of a platinum resistance thermometer.

1. High accuracy: precise, repeatable readings, often ±0.1 °C or better. / 2. Excellent stability: platinum is chemically inert, resisting oxidation and drift over long use.

State two disadvantages of a platinum resistance thermometer.

1. Lead-wire resistance errors in 2-wire setups (need 3- or 4-wire compensation). / 2. High cost (platinum is a precious metal).

State two advantages of a gas thermometer.

1. Resistance to vibration (durable, less affected by shock). / 2. High accuracy and sensitivity (gases expand regularly with minute changes; precision up to 0.005 °C; used as a reference standard).

State two disadvantages of a gas thermometer.

1. Slow response time (gas in a large bulb takes long to reach equilibrium). / 2. Complex calibration and setup (tedious adjustments; needs specialised expertise).

Define sensitivity of a thermometer.

The change in the physical property per unit change in temperature (e.g. ΔL/Δθ in mm/°C). Maximized by a large bulb (more liquid to expand) plus an ultra-narrow capillary bore (forces greater linear displacement).

Define the range of a thermometer.

The operating interval between minimum and maximum detectable temperatures. Altered by changing capillary length or choosing a fluid with widely separated melting/boiling points (alcohol for cold, mercury for heat). High sensitivity restricts range for a fixed stem size.

Define responsiveness of a thermometer.

The speed with which the instrument reaches thermal equilibrium with its surroundings. Maximized by a thin-walled bulb (low thermal resistance) and small bulb volume (low thermal capacity). Thermocouples excel due to low wire mass.

How can sensitivity and range be traded off in a liquid thermometer?

Increasing bulb volume (more liquid) increases sensitivity but reduces the usable range for a fixed stem length, because the column travels further per degree.

State the linear calibration formula for a thermometer.

For a property X that varies linearly with temperature: / θ = [ (Xθ - X₀) / (X₁₀₀ - X₀) ] × 100 °C / where X₀, X₁₀₀ are readings at the ice and steam points.

Worked example: A mercury column reads 4.0 cm at 0 °C and 24.0 cm at 100 °C. At an unknown bath it reads 14.0 cm. Find the temperature.

θ = [(14.0 - 4.0)/(24.0 - 4.0)] × 100 = (10.0/20.0) × 100 = 50.0 °C

Worked example: A thermocouple gives 0.2 mV at 0 °C and 5.2 mV at 100 °C. Find °C and K when the e.m.f. is 3.2 mV.

θ = [(3.2 - 0.2)/(5.2 - 0.2)] × 100 = (3.0/5.0) × 100 = 60.0 °C / T = 60.0 + 273 = 333.0 K / CSEC tip: do the linear fraction in °C first, then convert to K.

What is a clinical thermometer?

A thermometer designed for measuring human body temperature, calibrated from 35 °C to 42 °C. Narrow glass bore with a mercury/alcohol bulb; contains a kink (constriction) that traps the liquid column after removal so the reading stays visible.

How does a clinical thermometer work?

Place under tongue or armpit; wait for the liquid to reach thermal equilibrium; read the trapped column; reset by shaking vigorously to force the liquid below 35 °C. Accuracy depends on reaching thermal equilibrium.

What is an infrared (IR) thermometer?

A non-contact thermometer that detects infrared energy naturally emitted from any surface above absolute zero. An internal detector converts IR energy into an electrical signal processed into a temperature reading.

State the blackbody principle used by IR thermometers.

All objects above absolute zero emit radiation proportional to their temperature (blackbody radiation). IR sensors must be calibrated for the target material's emissivity for accuracy.

Define emissivity.

The ratio of energy radiated by a real object to that radiated by a perfect blackbody (emissivity = 1.0). Most materials emit less than a blackbody.

State the measurement range of an IR thermometer.

About -50 °C to 1300 °C+ — far beyond clinical needs.

List four advantages of infrared thermometers.

1. Non-contact (no cross-contamination; hygienic). / 2. Rapid response (~1 second). / 3. Versatile (moving, hazardous or inaccessible objects). / 4. Hygienic (no cleaning between measurements).

Compare contact (clinical) and non-contact (IR) thermometers.

Speed: clinical slow (needs equilibrium) vs IR ~1 s. / Hygiene: clinical risks contamination vs IR no contact. / Accuracy: clinical high when used correctly vs IR depends on emissivity/line-of-sight. / Range: clinical 35-42 °C vs IR -50 to 1300+ °C. / Limitations: clinical fragile glass/mercury hazard vs IR dust/smoke reduces accuracy.

Compare sensitivity and responsiveness of IR vs contact thermometers.

IR thermometers have high sensitivity (detect small changes over small areas) and high responsiveness (far faster than contact thermometers, which rely on slow thermal conduction through glass and liquid).

Describe the arrangement and motion of particles in a solid.

Packed tightly together in fixed positions; the particles only vibrate about those positions.

Describe the arrangement and motion of particles in a liquid.

Close together but not fixed in position; the particles can slide past one another.

Describe the arrangement and motion of particles in a gas.

Far apart; the particles move rapidly and randomly in all directions, filling the whole container.

Why does a bimetallic strip bend when heated, and where is it used?

Two different metals are joined together and one expands more than the other, so the strip bends (the metal that expands more is on the outside of the curve). / Used as a thermostat/cut-off switch in kettles and irons: it bends away and breaks the circuit when too hot.

Who was James Joule, and why were his experiments important?

His experiments showed that work (mechanical energy) can be converted into heat. They helped establish the Principle of Conservation of Energy.

State the Principle of Conservation of Energy.

Energy cannot be created or destroyed, only changed from one form to another.

Which surfaces are best at absorbing and which are best at reflecting heat radiation?

Black, dull surfaces absorb heat best. / Shiny, silver surfaces reflect heat best.

Which method of heat transfer can travel through empty space (a vacuum)? Why?

Radiation — it travels as waves, so it needs no medium (e.g. the Sun warming the Earth). Conduction and convection need particles.

Give an everyday example of conduction, convection and radiation.

Conduction (solids): a metal spoon heating up in hot tea. / Convection (liquids/gases): hot air rises, cool air sinks. / Radiation (waves): the Sun warming the Earth.

Compare evaporation and boiling (temperature, where it happens, effect).

Evaporation: at any temperature; only at the surface; causes cooling (e.g. sweat cools you down). / Boiling: only at the boiling point; throughout the whole liquid; temperature stays constant.

Define specific heat capacity.

The amount of heat required to change the temperature of 1kg of a substance by 1K.

Define heat capacity, and how does it differ from specific heat capacity?

The amount of heat required to change the temperature of a whole BODY by 1K. Unlike specific heat capacity (a constant per kg of material), heat capacity depends on the mass of the body.

What is the specific heat capacity of water?

4200 J/(kg K)

In the Method of Mixtures, what principle is used to find an unknown specific heat capacity?

Heat gained by the water = Heat lost by the substance, i.e. mcΔT (water) = mcΔT (substance).

What is latent heat?

Thermal energy used to form or break intermolecular bonds; it is absorbed or released only during a change of state, without any temperature change.

Define the specific latent heat of fusion.

The amount of heat energy required to change 1kg of a solid to a liquid, without a temperature change.

Define the specific latent heat of vaporization.

The amount of heat energy required to change 1kg of a liquid to a gas, without a temperature change.

According to Kinetic Theory, what causes air (gas) pressure?

The random motion of gas molecules colliding with the surfaces of objects (not molecules colliding with each other).

State Boyle's Law.

For a fixed mass of gas at constant temperature, pressure and volume are inversely proportional (P₁V₁ = P₂V₂).

State Charles' Law.

For a fixed mass of gas at constant pressure, volume is directly proportional to temperature.

State the Pressure Law.

For a fixed mass of gas at constant volume, pressure and temperature are directly proportional.

What is the General (Combined) Gas Law used for?

It is used when none of pressure, volume, or temperature is held constant — it combines all three individual gas laws.

In Charles' Law and the Pressure Law, why must temperature be measured in Kelvin rather than Celsius?

Because the direct proportionality only holds true on an absolute temperature scale that starts at true zero (absolute zero).