THERMAL ENERGY

THERMAL ENERGY Thermal energy results from the random

movement of particles in a substance

FACTORS AFFECTING THE THERMAL ENERGY OF A SUBSTANCE

Factor Factor Variation Result

Number of Particles

More particles Increased thermal Energy

Less particles Less thermal energy

Temperature Higher Temperature

Increased thermal energy

Lower Temperature Reduced thermal energy

THERMAL ENERGYTHERMAL ENERGY is the energy contained

in a substance, determined by the number of particles in the substance and their temperature

Thermal energy can be transferred from an environment where the temperature is high to an environment where the temperature is lower until the two environments have the same temperature

VIDEOS http://www.youtube.com/watch?

v=YWJHNG5y5F0&feature=related

http://www.youtube.com/watch?v=rU-sPzshVnM&feature=related

TEMPERATURE & HEAT People often confuse temperature and

heat When the temperature is high people

say “ it’s hot!” Temperature takes into account only the

speed of particles in a substance Heat depends not only on the speed of

particles ( degree of agitation) but also on their mass (number of particles

QUESTION If a cup of coffee and a red popsicle

were left on the table in this room what would happen to them? Why?

HEAT Heat is the transfer of thermal energy

between 2 environments with different temperatures.

Heat always passes from the warmer to the cooler environment

HEAT ENERGYHow much heat energy an object has depends on its temperature AND how much of it there is…

The sparkler is at a higher temperature than the bath, but the bath has a large volume of water, so it stores more heat energy.

Q. Which has the highest temperature – spark/kettle of water?

Q. Which has the most heat energy – spark/kettle of water?

Look at the picture of the kettle and the fire.

Think about:1)one of the sparks from

the fire and 2)the kettle full of

boiling water.

HEAT TRANSFER Heat can transfer 3 ways

CONDUCTION

Conduction involves the transfer of heat through direct contact

Heat conductors conduct heat well, insulators do not

CONVECTION Takes place in liquids and gases as

molecules move in currents Heat rises and cold settles to the bottom As a gas or liquid is heated it warms,

expands and rises because it is less dense

When the gas or liquid cools, it becomes denser and falls.

As the gas or liquid warms and rises, or cools and falls, it creates a convection current.

RADIATION Electromagnetic waves travel through empty space

PRACTICE Wrap a fur coat around a thermometer.

Will its temperature rise? Explain why or why not.

If you hold one end of a metal nail against a piece of ice, the end in your hand soon becomes cold. Does cold flow from the ice to your hand? Explain.

ANSWER No, the thermometer will not heat up

because it is the same temperature as the coat. There has to be a difference in temperature in order for heat transfer to take place

No, cold does not flow. Heat flows from your hand to the nail to the ice. Since your hand is losing heat, it feels cold. HEAT is the only thing that flows.

PRACTICE QUESTION

Explain why “firewalkers” can walk safely across a bed of red-hot coals in bare feet.

ANSWER Because the rate of heat transfer is

slow, because coals are not very good conductors. If they walk fast enough, there is only a small amount of heat transferred to their feet, and they do not burn.

PRACTICE QUESTION If heat transfers from hot to cold, and

stops when the two objects reach the same temperature, explain why it is possible for the inside of your car to be hotter than the air outside on a hot summer day.

ANSWER The greenhouse effect. Electromagnetic

waves from the sun enter your car through the windows and are absorbed by the seats and other objects in the car. They then re-emit electromagnetic radiation of a different wavelength that cannot pass through glass. All of that energy is therefore trapped inside the car, causing the temperature to rise.

PRACTICE QUESTION Since a hot cup of tea cools more

rapidly than a lukewarm cup of tea, would it be correct to say that a hot cup of tea will cool to room temperature before a lukewarm cup of tea will?

ANSWER No, because as it cools, the rate of heat

transfer slows down until it is just as slow as the lukewarm cup of tea. The only way to make it cool faster is by introducing convection.

PRACTICE QUESTION The Concorde supersonic airplane is 20

cm longer when in flight. Explain why.

ANSWER As the jet flies, it experiences a huge

amount of friction, which heats it up significantly. This causes thermal expansion, and the plane lengthens.

PRACTICE PROBLEM If you drop a hot rock into a pail of

water, the temperature of the rock and the water will change until both are equal. The rock will cool and the water will warm. Is this true if the rock is dropped into the Atlantic Ocean?

ANSWER Yes, yes, yes!!! Even though the

Atlantic ocean is huge, the heat that leaves the rock enters the water (law of conservation of energy). This means that the temperature will increase. Since the ocean is a huge body of water, it will be an incredibly small change, but a change nonetheless.

PRACTICE PROBLEM When you step out of a swimming pool

on a hot, dry day, you feel quite chilly. Why?

PRACTICE PROBLEM The water will start evaporating in the

air. Evaporation requires energy (heat) and that heat will come from your body. Since you are losing heat, you will feel cold.

PRACTICE PROBLEM Does a gas give off or absorb energy

when it turns into a solid?

ANSWER: It gives off energy, because it is going

from high energy to low energy.