Chapter 3 Section 4

1. Chapter 3 Section 4 Rate in Thermal Systems

2. Objectives • Define heat flow rate & its SI & English units. • Describe the heat transfer processes of conduction, convection & radiation. • Solve heat transfer problems using the heat conduction equation.

3. Heat Flow Rate • Heat flow rate = heat transferred/ time • Q =Q/t

4. Units • Heat flow rate can be measured in a number of different units. • Basically, any unit of energy divided by any unit of time can be a heat flow rate.

5. Energy or heat British thermal units (BTU) Calories (cal) Joules (J) Foot – pounds (ft-lbs) Time Hours (hr) Minutes (min) Seconds (s) Units – cont.

7. Heat conduction • Heat conduction is the transfer of energy (heat) between different parts of a body due to a temperature difference. • It is caused by molecules of a higher kinetic energy colliding with slower molecules and speeding them up. • Thermal conductivity is a measure of a substance’s ability to conduct heat.

8. Heat conduction rate • The rate of heat conduction through a substance depends upon: • Its thermal conductivity • Its cross sectional area • Its thickness and; • The temperature difference.

9. Heat conduction rate • Q = kADT / Dxwhere: • Q = heat flow rate • K= thermal conductivity • A = cross sectional area • Dx = thickness • DT = temperature difference

10. Other forms of heat transfer • Convection – heat transfer by moving large quantities of fluid (liquid or gas) • Radiation – caused by electromagnetic radiation (light). No medium is required.

11. Summary • Heat flow rate is the amount of thermal energy transferred per unit of time. • Heat is transferred from high temperature areas to low temperature areas. • Heat conduction is the transfer of heat within an object due to a temperature difference within the object. • Thermal conductivity is a measure of an object’s ability to conduct heat. • Convection is the transfer of heat by moving fluids (liquid or gas). • Radiation is the transfer of heat by electromagnetic waves.