Heat, Work and Energy

Tutorial to heat, work and energy - essentials as specific heat and specific heat capacity

Heat

Heat energy is transferred as a result of a temperature difference. 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 temperature difference 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 1oF) and the Calorie (the amount of heat to raise 1 gram of water by 1oC (or 1 K)).

  • check temperature for more info about degrees Celcius and degrees Kelvin

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

1 cal = 4.184 J

Units of energy used may be calorie (cal), Joule (J, SI unit) or Btu. For comparing units, check the unit converter for more information!

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 common to use kiloJoules (kJ) (1 000 Joules).

Specific enthalpy is a measure of the total energy of a unit mass. The unit commonly used is kJ/kg.

Heat Capacity

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

Specific Heat

Specific heat 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/lboF. For comparing units, check the unit converter for more information!

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

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

Unless the pressure is extremely high the work done by applied pressure on solids and liquids can be neglected, and enthalpy can be represented by the internal energy component alone. Constant-volume and constant-pressure heats can be said to be equal.

For solids and liquids

cp = cv

The specific heat  represents the amount of energy required to raise 1 kg by 1oC (or 1 K), 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 = cp m dT         (2)

where

Q = amount of heat (kJ)

cp = specific heat (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 (kJ/kg.oC):

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

    = 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 

or 

W = F · l       (3)

where 

W = work (Nm, J)

F = force (N)

l = length or distance (m)

Example - Work done by Force

The work done by a force 100 N moving a body 50 m can be calculated as 

W = (100 N) (50 m)

  = 5000 (Nm, J)

 

Work 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

Example - Work due to Gravitational Force

The work done when lifting a mass of 100 kg an elevation of 10 m can be calculated as 

W = m g h

  = (100 kg) (9.81 m/s2) (10 m)

  = 9810 (Nm, J)

where

g = acceleration of gravity 9.81 (m/s2)

h = elevation (m)

Example - Work due to Change in Velocity

The work done when a mass of 100 kg is accelerated from a velocity og 10 m/s to a velocity of 20 m/s

W = (v22 - v12) m / 2

  = ((20 m/s)2 - (10 m/s)2) (100 kg) / 2

  = 15000 (Nm, J)

where

v2 = final velocity (m/s)

v1 = initial velocity (m/s)

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

Ek =1/2 m  v2         (4)

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.

Related Topics

  • HVAC Systems - Heating, Ventilation and Air Conditioning systems
  • Heating - Heating systems - capacity and design of boilers, pipelines, heat exchangers, expansion systems and more
  • Thermodynamics - The effects of work, heat and energy on a system

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