ENERGY

1. ENERGY • A decrease in the mechanical energy (KE&PE) of an isolated system is equal to an increase in the internal energy of the system • OR/ Internal energy of a system can be transformed into mechanical energy

2. Internal Energy U • Internal energy is the energy associated with microscopic components of a system, (atoms & molecules). Internal energy is made up of translational, rotational and vibrational motion of particles and the potential energy in the bonding between particles.

3. Heat Q • Heat is the transfer of energy between a system and its environment due to a temperature difference. • Q represents the energy transfer to a system • Calorie is the energy necessary to raise 1 g of water 1 degree celcius. C = 1000c • BTU is the energy to raise 1lb of water 1 degree F 1c = 4.186J

4. Specific Heat • Specific heat is the amount of energy needed to be transferred to a given mass of substance to change its temperature by  T c = Q/mT or Q = mc T • Q is positive if energy flows into the system • Q is negative if energy flows out of the system

5. Thermals • A thermal is an air current caused by heat differences in the air. As hot air rises cooler air moves in to replace it. A circulation pattern develops called thermals. • Ex. Birds can glide higher if they use thermal currents

6. Calorimetry • Raise the temperature of a known mass of a substance and place into a known mass and temperature of water. When the system (substance & water) reach equilibrium the new temperature is taken and the specific heat of the substance can be calculated. • Qcold = -Qhot ie. mcT = -mc T • Qhot is -ve because energy flows out.

7. Systems involving more than 2 objects • Consider a piece of iron in water in a beaker • Qw + Qi + Qb = 0 then mwcw(T-Tw)+mici(T-Ti)+mbcb(T-Tb) = 0

8. Latent Heat • When the transfer of energy to a system does not result in a temperature change, usually the characteristics of the substance changes. ie. a phase change solid to liquid • Q = mL Where L = Latent Heat of substance • +ve when energy absorbed, -ve removed • Lf = heat of fusion Lv = heat of vaporization

9. Phase Diagram 1= ice Q= mcice T 2= melting Q=mLf 3=water Q= mcwater T 4=boiling Q=mLv 5=steam Q= mcsteam T temp 5 4 3 2 1 energy added

10. Conduction • Conduction is the transfer of energy by the exchange of kinetic energy as particles of less kinetic energy collide with particles of greater kinetic energy. Vibration of particles increase as more energy is transferred and the amplitude of vibration increases from particle to particle. An increase in vibration represents an increase in temperature.

11. Rate of Conduction • The rate of conduction depends on the properties of the material. Metals are good conductors due to the large number of free moving electrons. Gases are poor conductors due to the large space between particles. Conduction only occurs if there is a difference in the temperature of materials.

12. Rate of Transfer • P = Q/t proportional to A t/ x • that is proportional to the cross-sectional area A and inversely proportional to the thickness x. P is in watts Q in joules and t in seconds • Consider a rod length L with temperatures at either end Th and Tc then P = kA(Th-Tc)/L k is thermal conductivity constant

13. R-Value • Poor conductors have small values of k good conductors have large k values • Let R = L/k • then Q/t = A(Th-Tc)/(Li/ki) • where Li/ki is the summation of thickness divided by k for a number of materials • therefore Q/t = A(Th-Tc)/(Ri)

14. Convection • Convection is the transfer of energy through the movement of a substance. When movement results from density differences it is called natural convection. When the substance is forced to move (fan), it is called forced convection. Convection currents help in the boiling of water and the heating by a radiator.

15. Radiation • All objects radiate energy continuously through electromagnetic waves due to thermal vibrations of their molecules. The radiation rate is proportional to the fourth power of its absolute temperature.

16. Stefan’s law • P = AeT4 • where P = power in watt  = Stefan- Boltzmann constant = 5.6996*10-8 A = surface area of the object e = constant of emissivity (0<e<1) T = the temperature in Kelvin

17. The Sun • 1340J/m2 of the sun’s radiation passes through the top of the atmosphere every second. Although the energy is primarily visible light, there is a large amount of infrared and ultraviolet light also. • An object in equilibrium with its surroundings radiates and absorbs energy at the same rate so its temperature is unchanged.

18. Net Radiation • Let the object temperature be T and the surrounding temperature be To, then the net rate of energy gained or lost each second is • Pnet = Ae(T4-To4) • An ideal absorber reflects no light and is called a black body. A perfect black body has an e = 1 The sun is a black body because the light is emitted not reflected.

19. Thermography/ Thermogram • Thermography is a method of measuring an objects radiated energy • A thermogram is the image of the varying radiation levels of an object. The brightest image being the warmest. • An ideal reflector has an e = 0. It absorbs no energy reflecting it all.