Differences Between Joule and Calorie: Understanding Units of Heat
Heat is a fundamental concept in physics, especially in thermodynamics and everyday phenomena involving temperature change. While we often describe heat as warmth we feel, scientifically, heat refers to the transfer of energy from a higher temperature body to a lower temperature body until thermal equilibrium is reached. Every substance contains some internal energy, and heat flows between objects due to temperature differences.
Definitions and Basic Concepts
Heat is defined as the energy that is spontaneously transferred between bodies because of a temperature difference. This flow continues until both bodies reach equilibrium.
Temperature indicates the average kinetic energy of molecules in a body. It is a measure of how hot or cold an object is.
Internal energy is the total energy (kinetic + potential) stored in all the molecules within a substance.
Specific heat (also called heat capacity) is the amount of heat required to raise the temperature of one unit mass of a substance by one degree.
Formula for Heat and Units
The main formula connecting mass, specific heat, and temperature change is:
Where:
m = mass of the substance
c = specific heat capacity
ΔT = change in temperature
In the SI system, all forms of energy, including heat, are measured in Joules (J). Thus, the standard unit of heat is the Joule.
Units of Heat: SI, CGS, and Other Systems
Heat can be measured in different systems depending on context:
| Quantity | SI Unit | CGS Unit | Other Common Units |
|---|---|---|---|
| Heat | Joule (J) | Calorie (cal) | BTU (British Thermal Unit) |
| Specific Heat | J/kg·K | cal/g·°C | BTU/lb·°F |
| Heat Capacity | J/K | cal/°C | BTU/°F |
Here are standard conversions you should remember:
- 1 Calorie (cal) = 4.184 Joules (J)
- 1 BTU ≈ 1055.06 Joules (J)
- 1 Joule = 0.000239 Calorie = 0.000948 BTU
Key Concepts: Heat vs Temperature
Many students confuse heat and temperature. Temperature is a measure of the average kinetic energy of molecules in a substance, while heat is the energy transferred because of temperature difference. For example, a large iceberg at 0°C has more internal energy than a small cup of boiling water at 100°C, because of its much greater mass.
Types of Heat Transfer
- Conduction: Transfer of heat through direct contact, mostly in solids.
- Convection: Transfer via movement of fluids (liquids or gases), such as warm air rising.
- Radiation: Transfer through electromagnetic waves, for example, heat from the sun warming the Earth.
Temperature Scale Conversions
| Conversion | Formula |
|---|---|
| Celsius to Kelvin | K = °C + 273.15 |
| Kelvin to Celsius | °C = K – 273.15 |
| Celsius to Fahrenheit | °F = (°C × 9/5) + 32 |
| Fahrenheit to Celsius | °C = (°F - 32) × 5/9 |
| Fahrenheit to Kelvin | K = (°F - 32) × 5/9 + 273.15 |
Sample Problems and Solutions
Example 1: An electric kettle contains 1.5 kg of water. The specific heat capacity of water is 4180 J/kg·K. Calculate the energy required to raise the water's temperature from 15°C to 100°C.
- ΔT = (100 - 15) = 85°C = 85 K
- Q = m × c × ΔT = 1.5 × 4180 × 85 = 533,550 J = 533.6 kJ
Example 2: Calculate the energy needed to raise 0.7 kg of water from 20°C to 90°C. Specific heat = 4200 J/kg·K.
- ΔT = 90 - 20 = 70°C
- Q = 0.7 × 4200 × 70 = 205,800 J = 205.8 kJ
Summary Table: Key Formulas and Units
| Formula | Physical Meaning | Standard Units (SI) |
|---|---|---|
| Q = m × c × ΔT | Heat absorbed or released | Q in J; m in kg; c in J/kg·K; ΔT in K or °C |
| C = Q / (m × ΔT) | Specific heat capacity | J/kg·K |
| 1 cal = 4.184 J | Unit conversion | — |
Further Learning and Practice
- Heat: Introduction and Classification
- Heat Energy Concepts
- Thermal Properties of Matter
- Specific Heat at Constant Pressure and Volume
- Latent Heat: Explanation and Examples
Quick Tips for Students
- Always state SI units (Joules) when answering heat-related questions, unless otherwise instructed.
- Know how to convert between Joules, calories, and BTUs for calculation consistency.
- Remember heat and temperature are not the same; focus on what the question asks for.
- Practice numericals involving unit conversion and conceptual questions using above formulas and examples.
FAQs on Unit of Heat: SI and CGS Units Explained for Physics
1. What is the SI unit of heat?
The SI unit of heat is Joule (J). This standard unit is used for measuring all types of energy, including heat, in scientific calculations. Using Joules provides consistency with other forms of energy such as mechanical and electrical energy.
2. What are the other common units for measuring heat besides the Joule?
Besides Joule, heat can also be measured in:
- Calorie (cal): Commonly used in calorimetry and food energy.
- Kilocalorie (kcal): Equal to 1000 calories, often referred to as large calorie used in food energy.
- British Thermal Unit (BTU): Used in heating and cooling industries, mainly in imperial systems.
3. How is heat different from temperature?
Heat and temperature are related but distinct physical quantities:
- Heat is the transfer of energy due to temperature difference, measured in Joules (J).
- Temperature is the measure of the average kinetic energy of the molecules in a substance, commonly measured in Celsius, Kelvin, or Fahrenheit.
- Heat is energy in transit between objects, while temperature describes the hotness or coldness of an object itself.
4. What is the CGS unit of heat and how is it defined?
The CGS (Centimeter-Gram-Second) unit of heat is the calorie (cal). One calorie is defined as the amount of heat energy needed to raise the temperature of one gram of water by one degree Celsius (at standard atmospheric pressure).
5. How do you convert between the Celsius, Kelvin, and Fahrenheit temperature scales?
Temperature conversions use the following formulas:
- Celsius to Kelvin: K = °C + 273.15
- Kelvin to Celsius: °C = K - 273.15
- Celsius to Fahrenheit: °F = (°C × 9/5) + 32
- Fahrenheit to Celsius: °C = (°F - 32) × 5/9
6. Why is the Joule, a unit for energy, also used as the SI unit for heat?
Joule is the SI unit for both heat and energy because scientific experiments, like those by James Prescott Joule, established that heat is a form of energy. This allows all forms of energy—including thermal, mechanical, and electrical—to be measured consistently in Joules.
7. In the specific heat formula Q = mcΔT, what are the standard units for each component?
In the specific heat equation Q = mcΔT, standard SI units are:
- Q (heat transferred): Joules (J)
- m (mass): kilograms (kg)
- c (specific heat capacity): J/kg·K
- ΔT (change in temperature): Kelvin (K) or °C (as a 1 K change equals a 1°C change in difference)
8. Can an object at a lower temperature have more internal energy than an object at a higher temperature?
Yes, total internal energy depends on both mass and temperature. For example, a large mass at lower temperature (like an iceberg at 0°C) can have more internal energy than a small mass at higher temperature (like a cup of water at 100°C).
9. How do you convert calories to Joules?
To convert calories to Joules, use:
1 calorie (cal) = 4.186 Joules (J).
Multiply the amount in calories by 4.186 to get the value in Joules.
10. What are the units of specific heat capacity in SI and CGS systems?
Specific heat capacity units:
- SI unit: Joule per kilogram per Kelvin (J/kg·K)
- CGS unit: Calorie per gram per degree Celsius (cal/g·°C)
11. What formula do you use to calculate the heat required to change the temperature of a substance?
The heat required is calculated using: Q = mcΔT,
where Q is the heat supplied (J), m is mass (kg), c is specific heat capacity (J/kg·K), and ΔT is the change in temperature (K or °C).
12. Which method is used to transfer heat in solids, liquids, and gases?
The three methods of heat transfer are:
- Conduction: Heat transfer through direct contact, mainly in solids.
- Convection: Heat transfer by movement of fluids (liquids or gases).
- Radiation: Transfer of heat through electromagnetic waves, without requiring a medium.
