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Thermal Energy. Thermal Energy. Thermal energy results from the random movement of particles in a substance. Factors Affecting the thermal Energy of A SUbstance. Thermal Energy.
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Thermal Energy • Thermal energy results from the random movement of particles in a substance
Thermal Energy THERMAL 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
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. Heat Energy How much heat energy an object has depends on its temperature AND how much of it there is…
Look at the picture of the kettle and the fire. • Think about: • one of the sparks from the fire and • the kettle full of boiling water. Q. Which has the highest temperature – spark/kettle of water? Q. Which has the most heat energy – spark/kettle of 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.