Thermal Energy, Specific Heat and Heat Transfer Chapter 6

Temperature and Thermal Energy What’s the difference? Temperature IS NOT HOW HOT OR COLD SOMETHING IS! Temperature is defined as the measure of the average kinetic energy of the individual particles in an object/substance. What is kinetic energy? What does average mean?

Temperature The higher the average KE, the higher the temperature Which substance has a lower temperature?

Temperature and Thermal Energy What’s the difference? Thermal energy is defined as the total energy of all the particles in an object/substance. What differences do you see in these definitions?

Temperature and Thermal Energy TEMPERATURE IS a measure of the average kinetic energy of the molecules in a substance. Thermal energy is defined as the total energy of all the particles in a substance Adding thermal energy to a substance increases the average kinetic energy of the molecules and therefore causes a rise in temperature. Higher temperature = faster molecular motion Lower temperature = slower molecular motion

Temperature VS Thermal Energy You have two containers with water at 90ºC One container has a mass of 10 g, and the second has a mass of 5 g. ( there’s more substance in one container than another!) They are at the same temperature, but the 10 gram sample has more thermal energy TE depends upon mass, but temperature does not.

CELSIUS SCALE is used to measure temperature in the metric system. CELSIUS FAHRENHEIT boiling 100 degrees point 212 degrees (water) human 37 degrees body 98.6 degrees temp freezing 0 degrees point 32 degrees (water)

A Question to Consider… The water in both of the beakers below is at a temperature of 30 degrees Celsius. Which has more thermal energy? Explain your answer. B A 100 mL 50 mL

a. The flow of thermal energy from one object to another. Cup gets cooler while hand gets warmer Heat a. The flow of thermal energy from one object to another. b. Heat always flows from warmer to cooler objects. Ice gets warmer while hand gets cooler

3 Types of Heat Transfer Conduction Convection Radiation

Conduction Conduction heat transfer is the flowing of heat energy from a high-temperature object to a lower-temperature object.

Why does metal feel colder than wood, if they are both at the same temperature? Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

Convection

Water movement Cools at the surface Convection current Hot water rises Cooler water sinks

Radiation

The third method of heat transfer How does heat energy get from the Sun to the Earth? There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection. RADIATION ?

Radiation The transfer of energy by electromagnetic waves. Examples: Sun Fire Light bulb

Land heats up and cools down faster than water Specific Heat a. Some things heat up or cool down faster than others. Land heats up and cools down faster than water

b. Specific heat is the amount of heat required to raise the temperature of 1 kg of a material by one degree (C or K). 1) C water = 4184 J / kg C 2) C sand = 664 J / kg C This is why land heats up quickly during the day and cools quickly at night and why water takes longer.

Why does water have such a high specific heat? water metal Water molecules form strong bonds with each other; therefore it takes more heat energy to break them. Metals have weak bonds and do not need as much energy to break them.

How to calculate changes in thermal energy Q = m x T x Cp Q = change in thermal energy m = mass of substance (kg or g) T = change in temperature (Tf – Ti) Cp = specific heat of substance (J/kgC or J/gC)

Calculating Specific Heat It takes 487.5 J to heat 25 grams of copper from 25 °C to 75 °C. What is the specific heat in Joules/g·°C? Q = m x T x Cp

Thermal Expansion The increase in the volume of a substance because of an increase in the temperature. As a substance gains thermal energy the particles move faster and spread out causing the substance to expand.

Other Thermal Expansion Examples Expansion joints on the highway Thermostats Hot Air Balloons Read p. 278 – 279 and describe in a few sentences how each of these examples is related to thermal expansion.

Expansion Joints and Thermal Expansion Engineers leave space in the road to allow for thermal expansion of the pavement during the summer time. During the summer the road expands and closes these gaps. In the winter the road contracts and gaps appear.

Thermostats and Thermal Expansion A bimetallic strip is made up of two types of metal that gain thermal energy at different rates. As the bimetallic strip gains thermal energy it begins to expand and because the two metals do not expand at the same rate the strip will bend. As it bends it works to open and close the circuit to turn the heater on and off.

Hot Air Balloons and Thermal Expansion What types of objects float in water? Objects less dense than water float in water. This is the same reason hot air balloons float! The gas in the balloon is heated and becomes less dense than the air around the balloon so the balloon begins to float!