Heat, Work and Energy

A brief tutorial to heat, work and energy. Including essentials as specific heat and specific heat capacity

Heat

Heat is energy transferred as a result of a temperature differences. Energy as heat passes from a warm body (with higher temperature) to a cold body (with lower temperature).

The transfer of energy as a result of the difference in temperature alone is referred to as heat flow. The watt, which is the SI unit of power, can be defined as 1 J/s of heat flow.

Other units used to quantify heat energy are the British Thermal Unit - Btu (the amount of heat to raise 1 lb of water by 1^oF) and the Calorie (the amount of heat to raise 1 gram of water by 1^oC). Units of energy used may be calorie (cal), Joule (J, SI unit) or Btu. For comparing units, check the unit converter for more information!

Calorie is defined as an amount of heat required to change temperature of one gram of liquid water by one degree Celsius.

1 cal = 4.184 J

Specific Enthalpy

This is the term given to the total energy, due to both pressure and temperature, of a fluid (such as water or steam) at any given time and condition. More specifically it is the sum of the internal energy and the work done by an applied pressure.

The basic unit of measurement is the joule (J). Since one joule represents a very small amount of energy, it is usual to use kilojoules (kJ) (1 000 Joules).

The specific enthalpy is a measure of the total energy of a unit mass, and the units are usually kJ/kg.

Heat Capacity

Heat Capacity of a system is the amount of heat required to change temperature of the whole system by
one degree.

Specific heat capacity

Specific heat capacity is the amount of heat required to change temperature of one kilogram of a substance by one degree. Specific heat may be measured in kJ/kg K or Btu/lb^oF. For comparing units, check the unit converter for more information!

Specific heat capacities for different materials can be found in the Material Properties section.

Since the enthalpy of a fluid is a function of its temperature and pressure, the temperature dependence of the enthalpy can be found by measuring the rise in temperature caused by the flow of heat at constant pressure. The constant-pressure heat capacity - c_p - is a measure of the change in enthalpy at a particular temperature.

Similarly, the internal energy is a function of temperature and specific volume. The constant volume heat capacity - c_v - is a measure of the change in internal energy at a particular temperature and constant volume.

Because the specific volumes of solids and liquids are generally smaller, then unless the pressure is extremely high, the work done by an applied pressure can be neglected. Therefore, if the enthalpy can be represented by the internal energy component alone, the constant-volume and constant-pressure heat capacities can be said to be equal.

For solids and liquids

c_p == c_v

The specific heat capacity represents the amount of energy required to raise 1 kg by 1^oC, and can be thought of as the ability of a substance to absorb heat. Therefore the SI units of specific heat capacity are kJ/kg K (kJ/kg ^oC). Water has a very large specific heat capacity (4.19 kJ/kg ^oC) compared with many fluids.

• Water is a good heat carrier!

Amount of Heat Required to Rise Temperature

The amount of heat needed to heat a subject from one temperature level to an other can be expressed as:

Q = c_p m dT (2)

where

Q = amount of heat (kJ)

c_p = specific heat capacity (kJ/kg.K)

m = mass (kg)

dT = temperature difference between hot and cold side (K)

Example Heating Water

Consider the energy needed to heat 1.0 kg of water from 0 ^oC to 100 ^oC when the specific heat of water is 4.19 kJ/kg K:

Q = 4.19 (kJ/kg.K) 1.0 (kg) (100 - 0)(K)

    = 419 (kJ)

Work

The amount of mechanical work done can be determined by an equation derived from Newtonian mechanics

Work = Force x Distance moved in the direction of the force

It can also be described as the product of the applied pressure and the displaced volume:

Work = Applied pressure x Displaced volume

The unit of work is joule, J, which is defined as the amount of work done when a force of 1 newton acts for a distance of 1 m in the direction of the force.

1 J = 1 Nm

Energy

Energy is the capacity to do work (a translation from Greek-"work within"). The SI unit for work and energy is the joule, defined as 1 Nm.

Moving objects can do work because they have kinetic energy. ("kinetic" means "motion" in Greek).

The amount of kinetic energy possessed by an object can be calculated as

E_k =1/2 m v² (1)

where

m = mass of the object (kg)

v = velocity (m/s)

The energy of a level position (stored energy) is called potential energy. This is energy associated with forces of attraction and repulsion between objects (gravity).

The total energy of a system is composed of the internal, potential and kinetic energy. The temperature of a substance is directly related to its internal energy. The internal energy is associated with the motion, interaction and bonding of the molecules within a substance. The external energy of a substance is associated with its velocity and location, and is the sum of its potential and kinetic energy.

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