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Unformatted text preview: Thermal Physics Linear Expansion and Heat Transfer RHJansen Temperature and Expansion
As a substance is heated the molecules move or vibrate faster. They hit each other harder causing the particles to spread farther apart. In gases it results in higher pressure and greater volume, which we will investigate in greater detail in the next presentation. Liquids and solids are held together by strong intermolecular forces, so there is a only a slight expansion. The expansion of liquids and solids is important since it can tear apart the things we build. If a soda bottle is filled all the way to the top and heated it will shatter, so we leave a little bit of easily compressible gas in the container to allow for the liquid to expand safely. If the materials we build a sky scrapper or bridge out of were to expand at different rates, then the differing expansion would tear the structure apart. Walk outside the class room and you will see that the concrete has not been poured in one continuous slab. Instead there are gaps between small sections. RHJansen Linear Expansion l Change in length (m) Linear expansion coefficient (1/oC) = 0 All substances have unique properties and expand at different rates. Common coefficients of expansion can be found in a table in the text. l o Initial Length (m)
If the temperature goes up, then the change in temperature is positive and the object will increase in length. If the temperature goes does, then the change in temperature is negative and the object will decrease in length. T Change in temperature (oC) RHJansen Example 1 A 5.0 m steel rod is heated from 10oC to 45oC. a. What is the change in length? = 0 DT = (12 10 )(5.0)( 45 - 10) = 0.0021 m
-6 b. What is its new length? Simply add the change in length to the new length. = 0.0021 + 5.0 = 5.0021 m RHJansen Bimetallic Strip
A bimetallic strip is two pieces of metal with different coefficients of expansion that are fused together. When heated or cooled they change length at different rates causing the strip to bend. When heated, the metal with the higher expansion coefficient expands faster and is on the outside of the bend. When cooled, the metal with the higher expansion coefficient contracts quicker and in now on the inside of the bend. A common application of bimetallic strips is a thermostat to control heating and air conditioning. As an example: Metal is a good conductor of electricity. A bimetallic strip can be part of a circuit containing an air conditioner. On a hot day the strip will bend and if it bends enough to touch another piece and completing a circuit electricity will flow to the air conditioner turning it on automatically. When it gets cool enough the metal bends away and breaks contact turning off the air conditioning. RHJansen Heat
Heat is a transfer of energy from one substance to another Its variable is Q and its units are Joules (J) Mechanical Equivalent of Heat
Joule demonstrated that the energy of masses moving in mechanics (mechanical) was equivalent to the energy of heat A mass pulled by gravity started a rod with paddles attached to it rotating in a liquid. The rise in temperature of the liquid was recorded by a thermometer. He realized that heat is just another form of energy. m RHJansen Heat Transfer
The three methods of heat transfer can be imagined using a campfire. Convection involves fluids. If you hold your hand above the fire you will feel heat from the rising convection currents of hot air. Conduction involves direct contact (touch). If you grab a burning log you will experience heat due to conduction. RHJansen Radiation involves light. When you stand near the fire your front side is warm, but your back is not. A type of light (radiation) called infrared is given off by objects. It travels in straight lines so it hits the side facing the fire. How Heat Transfers Heat energy has to do with the random motion of molecules and atoms. When two objects touch the object that is hotter has faster moving molecules. The faster moving molecules in the hot object collide with the slower moving molecules in the colder object. The collisions get the first layer of molecules in the cold substance moving faster. The first layer then collides with the second layer causing that layer to move faster. The process continues from one side of the cold object to the other. Over a period of time the entire cold object speeds up and gains energy becoming hotter. The hot object losses energy to the cold object and it slows and cools. When they reach thermal equilibrium (equal temperature) the process of heat transfer stops. The natural direction of heat transfer has to be from hot to cold. RHJansen Rate of Heat Transfer
H k Rate of Heat Transfer (J/s) Thermal conductivity kADT H= L Every substance has unique properties and tranfers heat at different rates. The thermal contivity of common substances is in a table in the text. A Area (m2) T Change in Temperature (oC) L Length (thickness) that the heat moves through (m) Be careful! The variable H is not heat. It is the rate of heat (rate means divide by time). The variable for heat is Q . Q H= t Which changes the above equation to RHJansen Q kADT = t L Example 2
A frying pan made of Aluminum sits on top of a burner on the stove. Assume heat is delivered evenly over the bottom surface of the pan. The pan has a radius of 10 cm and is 0.3 cm thick. When the stove is turned on the top of the pan is at 25oC and the bottom of the pan is at 200oC. How much heat transfers to the food every 10 seconds. kADT H= L
Q kADT = t L (240)(p (0.10) )(200 - 25) Q = (10) (0.003) RHJansen 2 Q = 4.40 10 J 5 ...
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