Temperature Physics 202 Professor Lee Carkner Lecture 12

PAL # 11 Doppler b)Detector toward, Source toward: f’ = f ( v+v D / v-v S ) f’ = Hz for v D = 3 m/s, f’ higher d) Detector away, Source toward: f’ = f ( v-v D / v-v S ) f’ = Hz for v D = 3 m/s, f’=f c) Detector away, Source away: f’ = f ( v-v D / v+v S ) f’ = Hz for v D = 3 m/s, f’ lower e) Detector toward, Source away: f’ = f ( v+v D / v+v S ) f’ = Hz for v D = 3 m/s, f’=f

How fast would you have to be moving towards a red light (650 nm) to make it look green (550 nm)? a)0.0015c b)0.01c c)0.15c d)0.18c e)100c

The best spectrographs can resolve wavelength shifts of about 5.5X10 -6 nm for lines at 550 nm. What velocity does this correspond to? a)1X10 -6 m/s b)0.01 m/s c)3 m/s d)5 m/s e)1X10 8 m/s

Thermodynamics   Thermodynamics deals with the internal energy of a system   We will examine how this internal energy changes due to changes in heat and work  One of the important concepts in thermodynamics is temperature   How do we experience temperature?

Thermoscope  Changes in temperature cause changes in other properties   the electrical resistance of a wire   We can measure these changes to construct a thermoscope, a device to record changes in temperature   The oven is at a higher temperature than the refrigerator  Temperature changes are caused by an exchange of heat

Thermal Equilibrium  Consider a thermoscope in a cup of water  When the thermoscope stops changing, it and the water are in thermal equilibrium    If we move the thermoscope to a different cup and it reads the same value we know that it and the second cup are also in thermal equilibrium

Zeroth Law of Thermodynamics  What happens if we pour the two cups together?    Zeroth Law of Thermodynamics If two objects are each in thermal equilibrium with a third object then they are also in thermal equilibrium with each other

How Thermal Equilibrium Works

Thermometers   In order for a thermoscope to be a thermometer if needs to be calibrated  For example, the Celsius scale   Them put the tube in boiling water and mark that mercury level “100”   You now have a Celsius thermometer

The Kelvin Scale   The Kelvin scale is designed so that 0 K is at absolute zero and that K is at the freezing point of water  T C = T K

The Triple Point of Water  The Kelvin scale is actually defined not from the freezing point, but from the triple point of water   One Kelvin is defined as 1/ of the difference between absolute zero and the triple point of water

Converting Temperatures T C = T K T F = 9/5 T C +32 

Temperature Scales  Gabriel Daniel Fahrenheit invented the mercury thermometer in 1714   Anders Celsius introduced his scale is 1742   William Thomson, Lord Kelvin, determined from theory that minus degrees Celsius is the coldest it can get

Three Temperature Scales

What’s Hot, What’s Not

Thermal Expansion   Some objects expand more than others when heated   The degree of expansion depends on the change in temperature and the coefficient of expansion

Thermal Expansion of Ruler

Linear Expansion  The degree to which the length of an object changes is given by:  L = L   T   This applies to all dimensions of a solid length, width and height  Example: bimetal strip   As the strip heats up, one side expands more than the other bending the strip 

Bimetal Strip

Volume Expansion   V = V   T  Where  =3    Density in general decreases with increasing temperature  This is what makes a hot air balloon work

Temperature and Heat   Temperature is a measure of the internal (thermal) energy of a system   Heat (Q) is energy transferred between systems due to a temperature difference   Heat can manifest itself in different ways  Heat is not a thing or a condition  Being “hot” does not mean you have a lot of “heat”

Heat   The rate of heat exchange depends on the temperature difference  Run warm water over your hands after being outside on a cold day and the water feels hot   You feel hot on a hot day because your body generates heat and can’t get rid of it quickly enough 

What is Heat?

Heat Units  Heat is a form of energy and so its SI unit is the joule (kg m 2 /s 2 )    Heat is also given in calories  1 calorie is the heat needed to raise the temperature of 1 gram of water 1 deg C  1 cal = J 

Next Time  Read:  Homework: Ch 18, P: 14, 15, 23, 28