Physics: Forms of Energy PDF

Chapter 2 Forms of Energy  ❑ Relate thermal energy to changes in temperature and changes in state. ❑ Explain that chemical energy is stored in elements and compounds Objectives and may appear in different forms during and after a chemical...

Chapter 2 Forms of Energy  ❑ Relate thermal energy to changes in temperature and changes in state. ❑ Explain that chemical energy is stored in elements and compounds Objectives and may appear in different forms during and after a chemical reaction. ❑ Relate electrical energy to charge and voltage. ❑ Know that nuclear energy is due to nuclear forces. ❑ Relate mass to energy (E = m c2). ❑ Explain that heat energy accompanies all types of energy conversions. ✓ Thermal Energy (Heat): It is the energy transferred between two bodies as a result of the difference in their temperatures. Temperature is an indication whether an object is hot or cold. The unit of temperature is Kelvin (K) or Celsius (0C). Relation between TK and tC is given by: TK = tC + 273 When an object gains (or losses) heat: The temperature increases (or decreases) or the state changes. The unit of thermal energy is Joule (J). ✓ Chemical energy: It is the energy stored in chemicals due to their chemical bonds which transforms into another form during chemical reactions. Exothermic reaction: in which thermal energy is given off. Endothermic reaction: in which thermal energy is absorbed Examples: Gasoline : its combustion changes the chemical energy into heat and then into mechanical energy using in a car Food: its slow oxidation in our body converts chemical energies into heat and chemical energy. Battery: it stores chemical energy and converts it into electrical energy  ✓ Electrical energy: It is the energy which operates the electric appliances by causing a displacement of electrons through an electric conductor. Electrical energy is given by: W = VAB.Q where VAB is the potential difference between A and B an d Q is the quantity of electricity. But Q = I.t  W = VAB.I.t. but electrical power P = VAB.I  W = P. t In S.I : W in J ; Q in C ; VAB in V ; I in A and P in W (watt ). Examples: Electric motor: converts electric energy into mechanical and heat energy. Resistor: converts electric energy into heat. Loudspeaker: converts electric energy into sound energy. Lamp: converts electric energy into 5% radiant and 95% heat. Electrolytic cell: converts electric energy into chemical and heat energy. ✓ Atomic and Nuclear energy : They are the energies that derived from the atom or the nucleus due to the extremely strong force of attraction between the nucleons (p – n). ✓ Equivalence of Mass and Energy : ❖Statement: « The mass of a body is a measure of the amount of energy it contains, that is mass and energy are different forms of each other». ❖ Einstein’s Relation Energy is given by: E = m.c 2 where c is the speed of light in vacuum and m is the mass in kg.. c = 3  10 m / s 8 ✓ Conservation of Energy: ❖Principle of conservation of energy: Energy can neither be created nor destroyed. It is converted during its transformation from one state to another by an energy converter. ❖ Energy converter: It is a device that converts energy from In put energy Output energy one form into another. This energy Converter conversion is accompanied by energy lost as heat Energy loss  ❖ Examples: Electric Fan Mech. Chem. Mech. Car Energy Energy Energy Energy Heat Heat Then : Wlost =Win put −Wout put Or Wlost =Wreceived −Wuseful  ❖ Efficiency of an energy converter: It is the ratio of useful energy (Wu) supplied by the converter to the total energy received by it (Wr). Usefull energy Out put energy Wu Efficiency : k = = =. Re ceived energy In put energy Wr Efficiency k has no unit (ratio → unitless ).