Ch Energy I. Energy and Work (p )  Energy and Work  Energy  Conservation of Energy

Energy and Work What is energy?  Whenever work is done, energy is transformed or transferred from one system to another  When is work done? Energy is the ability to do work.

REMEMBER: Work is done only when an object moves. BUT energy can be present in an object or a system when nothing is happening. HOWEVER it can only be observed when it is transferred from one object or system to another. How are energy and work related? Energy and Work

Because the amount of energy transferred is measured by how much work is done – energy and work are expressed in the same unit. Joules (J) is the unit of Energy SI Unit of Energy

Energy and Work Work  transfer of energy through motion  force exerted through a distance W = Fd Distance must be in same direction of force! W:work (J) F:force (N) d:distance (m) 1 J = 1 N·m

Brett’s backpack weighs 30 N. How much work is done on the backpack when he lifts it 1.5 m from the floor to his back? GIVEN: F = 30 N d = 1.5 m W = ? WORK: W = F·d W = (30 N)(1.5 m) W = 45 J F W d Energy and Work

A dancer lifts a 40 kg ballerina 1.4 m in the air and walks forward 2.2 m. How much work is done on the ballerina during and after the lift? GIVEN: m = 40 kg d = 1.4 m - during d = 2.2 m - after W = ? WORK: W = F·dF = m·a F =(40kg)(9.8m/s 2 )=392 N W = (392 N)(1.4 m) W = 549 J during lift No work after lift. “d” is not in the direction of the force. F W d Energy and Work

Potential Energy Potential Energy is energy that is stored. You can’t see it but you know it’s there. AKA – Energy of Position

Types of Potential Energy Type of EnergyExample Gravitational Potential Energy (GPE) Energy stored due to position (objects that are above Earth’s surface such as apples in an apple tree). Chemical Energy Energy stored in chemical bonds such as food or fuel. Elastic Energy energy stored by something that can stretch or compress such as a rubber band or spring.

Gravitational Potential Energy Depends on mass and height. GPE = m g h Or GPE = mass x free-fall acceleration x height (mg = weight in Newtons) Mass (m) = kg; Gravity (g) = 9.8 m/s 2 ; Height (h) = m AKA - GPE

Potential Energy (PE)  stored energy  cannot be seen  depends on position or configuration of an object Which boulder has greater gravitational PE? What other ways can an object store energy? Gravitational Potential Energy

Example A 65 kg rock climber ascends a cliff. What is the climber’s gravitational potential energy at a point 35 m above the base of the cliff? Given: m = 65kg h = 35 m g = 9.8m/s GPE = ? 2 GPE = mgh GPE = 65 x 9.8 x 35 GPE = 22,295 J

Kinetic Energy is the energy of motion.

Kinetic Energy Kinetic Energy (KE)  energy in the form of motion  depends on mass and velocity 80 km/h 50 km/h 80 km/h 80 km/h truck 50 km/h motorcycle Which has the most KE? Which has the least KE?

Kinetic Energy KE = ½ mass x velocity OR KE = ½ m v mass (m) = kg velocity (v) = m/s 2 2 AKA = KE

Note: Kinetic energy depends more on speed than on mass.

Example What is the kinetic energy of a 44kg cheetah running at 31 m/s? Given: m = 44 kg v = 31 m/s KE = ? KE = ½ (44) x (31) 2 KE = 22 x 961 KE = J KE = ½ m v 2

Forms of Energy Forms of Energy: Each of these forms of energy can be converted into other forms of energy. KineticPotentialEnergy in Fields MechanicalThermalChemical ElectricalElectromagneticNuclear

ENERGY motion of electric charges bonding of atoms motion of objects internal motion of particles changes in the nucleus The ability to cause change. MECHANICAL ELECTRICAL CHEMICAL NUCLEARTHERMAL joules (J) Energy

Energy Electrical energy: results from the flow of charged particles or electrons. Electric charges can exert forces that do work. Chemical Energy is the energy stored in chemical bonds – when the bonds are broken, the released energy can do work.

Energy Mechanical Energy the energy associated with the motion or position of an object. The sum of potential and kinetic energy in a system (Usually involves movement of an object) Nuclear Energy: energy stored in atomic nuclei – nuclear fission releases energy by splitting nuclei apart, nuclear fusion releases energy by combining 2 nuclei into a larger nuclei.

Thermal Energy: energy given off as heat (friction). The total potential and kinetic energy of all the microscopic particles in an object. Energy Electromagnetic Energy: a form of energy that travels through space in the form of waves (visible light and X-rays are examples).

Energy Conversions The process of changing energy from one form to another.

Law of Conservation of Energy Energy can not be created nor destroyed, it can only be changed. Energy can be transferred to another object/system or to another form.

Conservation of Energy Law of Conservation of Energy  Energy may change forms, but it cannot be created or destroyed under ordinary conditions. Example:  PE  KE  mechanical  thermal  chemical  thermal

E. Conservation of Energy PE  KE View pendulum animation.pendulum animationView roller coaster animation.roller coaster animation

E. Conservation of Energy Mechanical  Thermal View rolling ball animations.rolling ball animationsView skier animation.skier animation

Ch. 5 - Energy II. Thermal Energy  Temperature  Thermal Energy  Heat Transfer

A. Temperature Temperature  measure of the average KE of the particles in a sample of matter.

B. Thermal Energy Thermal Energy  the total energy of the particles in a material.  KE - movement of particles  PE - forces within or between particles due to position.  depends on temperature, mass, and type of substance.

B. Thermal Energy Which beaker of water has more thermal energy?  B - same temperature, more mass 200 mL 80ºC A 400 mL 80ºC B

C. Heat Transfer Heat  thermal energy that flows from a warmer material to a cooler material. Like work, heat is...  measured in joules (J)  a transfer of energy

C. Heat Transfer What are 3 types of heat transfer? Conduction Convection Radiation

D. Conduction Transfer of heat as a result of direct contact

E. Convection

F. Radiation

Summary

Quiz

C. Heat Transfer Why does A feel hot and B feel cold? 80ºC A 10ºC B  Heat flows from A to your hand = hot.  Heat flows from your hand to B = cold.

C. Heat Transfer Specific Heat (C p )  amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin  units: J/(kg·K) or J/(kg·°C)