Thermal Energy

Atomic Motion  The macroscopic properties of a system are based on the microscopic behavior of atoms. Macroscopic equation of state: pressure, volume, temperature Microscopic laws of motion: position, velocity, energy

Ideal Gas Energy  Kinetic theory supplies the energy for an ideal gas. K av = (3/2)kTK av = (3/2)kT  Convert this to the total system energy. U = (3/2)NkTU = (3/2)NkT U = (3/2)nRTU = (3/2)nRT  This energy is internal to the gas. Energy based on temperature and number of molecules

Internal Energy  The kinetic energy of an ideal gas is its internal energy. Also called thermal energy  Internal energy does not include energy from external measures. Center of mass translation Rigid body rotation Potential from external force F ext v CM

Vibrations  Mean particle speed is not the only source of internal energy.  Real molecules can have internal motion. Rotation Vibration  These effects add to internal energy. Rotation around center of mass Vibration at center of mass

Energy Transfer  Thermal energy changes in a system. It flows in and out Due to changes in macroscopic properties  When it is due to temperature changes it is called heat. Heat flow

Heat and Work  Work is a process that represents the change in the mechanical energy. Force acting through a distanceForce acting through a distance Measured in joules like energyMeasured in joules like energy  Heat is a process that represents the change in the thermal energy. Due to a difference in temperature Measured in joules like energy

Mechanical Equivalent  In the 1800’s James Joule demonstrated that mechanical work could cause a change in temperature.  Heat, like work, represented a change in energy.

Calorie  Heat was measured by a change in temperature. 1 calorie (cal) is the amount of heat needed to raise 1 g of water by 1  C.1 calorie (cal) is the amount of heat needed to raise 1 g of water by 1  C. 1 kcal = 1000 cal, and is what we call a “Calorie” for food.1 kcal = 1000 cal, and is what we call a “Calorie” for food.  Calories (or kcal) are often used to measure heat or internal energy.  Since SI has a unit of energy (J) it remains the unit for all forms of energy. Work Mechanical energy Potential energy Dissipative energy Heat Thermal energy Internal energy  1 cal = J

Burn It  You gobble too much ice cream and decide to walk up stairs to burn the additional 500 Calories.  If your mass is 60 kg, what vertical height must you climb?  Convert to joules from kcal. W = (500 kcal)(1000 cal/kcal)(4.186 J/cal) W = 2.1 x 10 6 J.  Work climbing is W = mgh. h = W / mg = (2.1 x 10 6 J) / (60 kg)(9.8 m/s 2 ) h = 3600 m.  Real humans are inefficient and h is closer to 700 m. next