1. PHY 231 1 PHYSICS 231 Lecture 23: Temperature Remco Zegers Walk-in hour: Tue 4-5 pm Helproom

2. PHY 231 2 Chapter 10 Temperature R Potential Energy 0 R 2 atom/molecules -E min The curve depends on the material, e.g. E min is different for water and iron Kinetic energy ~ T

3. PHY 231 3 Solid (low T) R Potential Energy 0 Kinetic energy ~ T -E min The temperature (and thus kinetic energy) is so small that the atoms/molecules can only oscillate around a fixed position R min R min

4. PHY 231 4 Liquid (medium T) R Potential Energy 0 Kinetic energy ~ T -E min R min On average, the atoms/molecules like to stick together but sometimes escape and can travel far.

5. PHY 231 5 Gas (high T) R Potential Energy 0 Kinetic energy ~ T -E min R min The kinetic energy is much larger than E min and the atoms/molecules move around randomly.

6. PHY 231 6 What happens if the temperature of a substance is increased? R 0 Kinetic energy ~ T -E min R min =R ave (T=0) T=0: Average distance between atoms/molecules: R min R ave (T>0) > R min T>T o : The average distance between atoms/molecules is larger than R min : the substance expands

7. PHY 231 7 Temperature scales Conversions T celsius =T kelvin -273.5 T fahrenheit =9/5*T celcius +32 We will use T kelvin. If T kelvin =0, the atoms/molecules have no kinetic energy and every substance is a solid; it is called the Absolute zero-point. Kelvin Celsius Fahrenheit

8. PHY 231 8 Thermal expansion  L=  L o  T L0L0 LL T=T 0 T=T 0 +  T  A=  A o  T  =2   V=  V o  T  =3  length surface volume Some examples:  =24E-06 1/K Aluminum  =1.2E-04 1/K Alcohol  : coefficient of linear expansion different for each material

9. PHY 231 9 Water: a special case Coef. of expansion is negative: If T drops the volume becomes larger Coef. Of expansion is positive: if T drops the volume becomes smaller Ice is formed (it floats on water)

10. PHY 231 10 Ice liquid ice  (g/cm 3 ) 1 0.917 Phase transformation Ice takes a larger volume than water!

11. PHY 231 11 expanding ice Someone puts a bottle of water of 1 liter (1E-03 m 3 ) in the freezer. If there is 25cm 3 space left in the bottle without water, what will happen when it freezes?

12. PHY 231 12 Thermal equilibrium Low temperature Low kinetic energy Particles move slowly High temperature High kinetic energy Particles move fast Thermal contact Transfer of kinetic energy Thermal equilibrium: temperature is the same everywhere

13. PHY 231 13 Zeroth law of thermodynamics If objects A and B are both in thermal equilibrium with an object C, than A and B are also in thermal equilibrium. There is no transfer of energy between A, B and C

14. PHY 231 14 Thermal expansion: an example In the early morning (T=30 o F=272.4K) a person is asked to measure the length of a football field with an aluminum measure and finds 109.600 m. Another person does the same in the afternoon (T=60 o F=289.1K) using the same ruler and finds 109.556 m. What is the coefficient of linear expansion of the ruler?

15. PHY 231 15 A heated ring A metal ring is heated. What is true: a)The inside and outside radii become larger b)The inside radius becomes larger, the outside radius becomes smaller c)The inside radius becomes smaller, the outside radius becomes larger d)The inside and outside radii become smaller

16. PHY 231 16 Demo: bimetallic strips Application: contact in a refrigerator  top  bottom  top <  bottom if the temperature increases, The strip curls upward, makes contact and switches on the cooling.

17. PHY 231 17 Demo’s Bimetallic coil Expanding and contracting rod