Thermal Practicals

Overview

This page focuses on experimental determination of thermal quantities in H2 Physics.

Main practical themes:

• specific heat capacity of solids
• specific heat capacity of liquids
• continuous-flow calorimetry
• specific latent heat of fusion
• specific latent heat of vaporisation
• electrical heating methods
• heat-loss correction methods
• apparatus interpretation
• common sources of error

Most practicals use electrical energy:

$$E=IVt$$

Where:

• (I) = current
• (V) = potential difference
• (t) = heating time

This page supports:

• Thermal Physics A

Definition

Thermal practicals are experiments where electrical energy is converted into thermal energy and related to temperature rise or change of state.

Why It Matters

This topic develops practical skills in setting up apparatus safely, identifying heat losses, interpreting apparatus, choosing the right thermal model, and explaining how correction methods improve accuracy.

Key Representations

Core Experimental Principle

Electrical energy supplied is converted into thermal energy.

If losses are negligible:

$$IVt=Q$$

Then combine with:

$$Q=mc\Delta T$$

or

$$Q=ml$$

depending on the process.

Determination of Specific Heat Capacity of a Solid

Standard Heated Metal Block Method

A metal block contains:

• electric heater
• thermometer or temperature probe
• insulation around block

Measure:

• mass (m)
• current (I)
• voltage (V)
• heating time (t)
• temperature rise (\Delta T)

Formula

Assuming negligible heat loss:

$$IVt=mc\Delta T$$

Hence:

$$c=\frac{IVt}{m\Delta T}$$

Why Use a Metal Block?

Metals:

• conduct heat well
• become nearly uniform in temperature
• easy to machine holes for heater and probe

Experimental Precautions

• insulate block well
• ensure heater fits tightly
• thermometer inserted deeply
• record stable initial temperature
• avoid drafts
• measure mass accurately

Worked Example 1

Given:

• (I=2.0\ \mathrm{A})
• (V=12\ \mathrm{V})
• (t=300\ \mathrm{s})
• (m=1.5\ \mathrm{kg})
• (\Delta T=12\ \mathrm{K})

Find (c).

$$c=\frac{IVt}{m\Delta T}$$ $$c=\frac{(2.0)(12)(300)}{(1.5)(12)}$$ $$c=400\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$$

Determination of Specific Heat Capacity of a Liquid

Calorimeter Method

Liquid placed in insulated calorimeter with:

• immersion heater
• thermometer
• stirrer

Measure:

• mass of liquid
• electrical input
• temperature rise

Formula (Ignoring Container)

$$IVt=mc\Delta T$$

More Accurate Formula

If calorimeter heat capacity (C):

$$IVt=mc\Delta T+C\Delta T$$

So:

$$IVt=(mc+C)\Delta T$$

Importance of Stirring

Stirring helps:

• uniform temperature
• more accurate thermometer reading
• faster equilibrium

Continuous-Flow Method for Liquids

Principle

Liquid flows continuously through a heated tube.

Measure:

• mass flow rate (\dot m)
• inlet temperature
• outlet temperature
• electrical power

Formula

If heat loss is negligible:

$$IV=\dot{m}c\Delta T$$

Hence:

$$c=\frac{IV}{\dot{m}\Delta T}$$

Advantages

Compared with static calorimeter:

• less heat stored in apparatus
• easier steady-state measurement
• more suitable for liquids

Two-Trial Heat-Loss Correction

Real Situation

Some power is lost to surroundings:

$$IV=\dot{m}c\Delta T+h$$

Where:

• (h) = heat loss rate

Repeat experiment with different flow rate but same temperature rise.

Then:

$$I_1{V}_1={\dot{m}}_{1}c\Delta T+h$$ $$I_2{V}_2={\dot{m}}_{2}c\Delta T+h$$

Subtract equations to eliminate (h).

This gives more accurate (c).

Why Keep Same Temperature Difference?

Because heat loss depends strongly on temperature difference between apparatus and surroundings.

Keeping same (\Delta T) helps keep (h) approximately constant.

Determination of Specific Latent Heat of Fusion

Typical Ice Method

Crushed melting ice in funnel.

Heater inserted into ice.

Water produced is collected.

Use second identical setup without heater to estimate environmental melting.

Why Must Ice Be Melting?

Melting ice is at:

$$0^{\circ }\mathrm{C}$$

So no energy is needed first to raise temperature.

Formula

Let:

• (m_1) = water collected with heater
• (m_2) = water collected without heater

Mass melted by heater only:

$$m=m_1-m_2$$

Then:

$$IVt=m{l}_f$$

So:

$$l_f=\frac{IVt}{m_1-m_2}$$

Worked Example 2

Given:

• (I=3.0\ \mathrm{A})
• (V=10\ \mathrm{V})
• (t=600\ \mathrm{s})
• (m_1=0.070\ \mathrm{kg})
• (m_2=0.016\ \mathrm{kg})

$$m=0.054\mathrm{kg}$$ $$l_f=\frac{(3)(10)(600)}{0.054}=3.33\times 10^5\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}$$

Determination of Specific Latent Heat of Vaporisation

Boiling Method

Liquid is boiled electrically.

Steam or vapour produced is condensed and collected.

Measure:

• electrical input
• mass vaporised
• time

Formula (Idealised)

$$IVt=m{l}_v$$

Real Case With Heat Loss

$$IV=\dot{m}{l}_v+h$$

Repeat with second power setting:

$$I_1{V}_1={\dot{m}}_1{l}_v+h$$ $$I_2{V}_2={\dot{m}}_2{l}_v+h$$

Subtract to remove (h).

Apparatus Interpretation Skills

You may be asked:

• why insulation is used
• why stirring is needed
• why thermometer is placed centrally
• why crushed ice is used
• why second control setup is needed
• why steady state is required

These are common exam questions.

Common Sources of Error

Heat Loss to Surroundings

Causes measured values to be too high or too low depending on setup.

Poor Thermal Contact

Heater not fitted tightly into metal block.

Temperature Lag

Thermometer responds slowly.

Incomplete Stirring

Liquid temperature non-uniform.

Evaporation or Splashing

Mass measurement inaccurate.

Reading Errors

Stopwatch, ammeter, voltmeter, thermometer.

Improving Accuracy

• use insulation
• use digital sensors
• repeat and average
• wait for steady readings
• stir continuously
• reduce drafts
• use two-trial correction methods

Worked Example 3

Continuous flow heater:

• (I=2.5\ \mathrm{A})
• (V=12\ \mathrm{V})
• flow rate (=0.020\ \mathrm{kg,s^{-1}})
• temperature rise (=0.36\ \mathrm{K})

Find (c).

Power:

$$IV=30\mathrm{W}$$

Then:

$$30=(0.020)c(0.36)$$ $$c=\frac{30}{0.0072}=4.17\times 10^3\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$$

(close to water)

Data Analysis Tips

Graph Method

If experiment is repeated for different powers:

Plot:

• power (IV) vs (\dot m \Delta T)

Gradient may give (c).

Intercept may represent heat loss.

Links

• Thermal Physics A
• Thermal Measurement and Scales
• Heat Capacity and Latent Heat
• Thermal Physics A Common Exam Traps
• Work, Energy and Power

Summary

Thermal practicals are mainly about tracking where electrical energy goes, identifying losses, and justifying how the method improves accuracy.

$$so:$$

c=\frac{IVt}{m\Delta T}

$$Foraliquid,ifthecalorimeteralsoabsorbsenergy:$$

IVt=mc\Delta T+C\Delta T

$$Formixingcalorimetry:$$

\text{heat lost}=\text{heat gained}

$$Forlatentheatoffusionorvaporisation:$$

IVt=mL

$$Ifthesubstancestartsbelowthephase-changetemperature,warmingtermsmustbeincluded.Foracontinuousflowcalorimeter:$$

P=\dot{m}c\Delta T

where $\dot{m}$ is mass flow rate. Useful plots include $\Delta T$ against $t$, $Q$ against $\Delta T$, and temperature against time. A plateau indicates phase change. Real systems lose energy by conduction, convection, radiation, and evaporation. Reduce heat loss using insulation, a lid, short heating times, stirring, larger temperature rise, and repeated measurements. ## Common Exam Traps Do not assume $IVt=mc\Delta T$ is exact unless heat losses are negligible. Include container heat capacity if given. Stir liquids for uniform temperature. Avoid measuring temperature too late after heating stops because cooling begins immediately. For latent heat of fusion, ice should be dry before measuring mass; surface water causes error. Convert units carefully: g to kg, min to s, and kJ to J. Check zero error and calibration for thermometers and balances. ## Links - [Thermal Physics A](/content/topics/12_thermal_physics_A/thermal_physics_A) - [Temperature and Heat](/content/concepts/temperature_and_heat) - [Thermal Properties of Matter](/content/concepts/thermal_properties_of_matter) - [First Law of Thermodynamics](/content/concepts/first_law_thermodynamics) - [Measurement](/content/topics/01_measurement/measurement) - [Current Electricity Fundamentals](/content/topics/13_current_electricity_fundamentals/current_electricity_fundamentals)