Heat, Work and Energy
Heat vs. work vs. energy.
The SI-unit of heat - or energy - is joule (J).
With temperature difference
- heat will transfer from a warm body with higher temperature to a colder body with lower temperature
Other units used to quantify heat 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)).
A calorie is defined as the amount of heat required to change the temperature of one gram of liquid water by one degree Celsius (or one degree Kelvin).
1 cal = 4.184 J
1 J = 1 Ws
= (1 Ws) (1/3600 h/s)
= 2.78 10-4 Wh
= 2.78 10-7 kWh
Heat Flow (Power)
Heat-transfer as result of temperature difference alone is referred to as heat flow. The SI units for heat flow is J/s or watt (W) - the same as power. One watt is defined as 1 J/s.
Specific Enthalpy is a measure of the total energy in a unit mass. The SI-unit commonly used is J/kg or kJ/kg.
The term relates 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 enthalpy is the sum of internal energy and work done by applied pressure.
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 capacity) is the amount of heat required to change temperature of one mass unit of a substance by one degree.
Specific heat may be measured in J/g K, J/kg K, kJ/kg K, cal/gK or Btu/lboF and more.
Never use tabulated values of heat capacity without checking the unites of the actual values!
Specific heat for common products and materials can be found in the Material Properties section.
Specific Heat - Constant Pressure
The enthalpy - or internal energy - of a substance is a function of its temperature and pressure.
The change in internal energy with respect to change in temperature at fixed pressure is the Specific Heat at constant pressure - cp.
Specific Heat - Constant Volume
The change in internal energy with respect to change in temperature at fixed volume is the Specific Heat at constant volume - cv.
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 (1)
The specific heat represents the amount of energy required to raise 1 kg of substance by 1oC (or 1 K), and can be thought of as the ability to absorb heat. The SI units of specific heats are J/kgK (kJ/kgoC). Water has a large specific heat of 4.19 kJ/kgoC compared to many other fluids and materials.
- 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)
Q = amount of heat (kJ)
cp = specific heat (kJ/kgK)
m = mass (kg)
dT = temperature difference between hot and cold side (K)
Example Heating Water
Consider the energy required to heat 1.0 kg of water from 0 oC to 100 oC when the specific heat of water is 4.19 kJ/kgoC:
Q = (4.19 kJ/kgoC) (1.0 kg) ((100 oC) - (0 oC))
= 419 (kJ)
Work and energy are from a technical viewpoint the same entity - but work is the result when a directional force (vector) moves an object in the same direction.
The amount of mechanical work done can be determined by an equation derived from Newtonian mechanics
Work = Applied force x Distance moved in the direction of the force
W = F l (3)
W = work (Nm, J)
F = applied force (N)
l = length or distance moved (m)
Work can also be described as the product of the applied pressure and the displaced volume:
Work = Applied pressure x Displaced volume
W = p A l (3b)
p = applied pressure (N/m2, Pa)
A = pressurized area (m2)
l = length or distance the pressurized area is moved by the applied force (m)
Example - Work done by a 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)
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 = Fg h
= m g h
= (100 kg) (9.81 m/s2) (10 m)
= 9810 (Nm, J)
Fg = force of gravity - or weight (N)
g = acceleration of gravity 9.81 (m/s2)
h = elevation (m)
In imperial units a unit work is done when a weight of 1 lbf (pound-force) is lifted vertically against gravity through a distance of 1 foot. The unit is called lb ft.
An object with mass 10 slugs is lifted 10 feet. The work done can be calculated as
W = Fg h
= m g h
= (10 slugs) (32.17405 ft/s2) (10 feet)
= 3217 lbf ft
Example - Work due to Change in Velocity
The work done when a mass of 100 kg is accelerated from a velocity of 10 m/s to a velocity of 20 m/s can be calculated as
W = (v22 - v12) m / 2
= ((20 m/s)2 - (10 m/s)2) (100 kg) / 2
= 15000 (Nm, J)
v2 = final velocity (m/s)
v1 = initial velocity (m/s)
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.
- more energy units
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)
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.