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Unit 1-B Forces and Energy

Unit 1-B Forces and Energy. What is a Force?. A Force is an Influence or Power that may cause an object to Change (or maintain) its Position. What are some examples of Forces?. Examples of Forces include: Gravity , Magnetism , and Electrical Charge.

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Unit 1-B Forces and Energy

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  1. Unit 1-B Forces and Energy What is a Force? A Force is an Influence or Power that may cause an object to Change (or maintain) its Position. What are some examples of Forces? Examples of Forces include: Gravity, Magnetism, and Electrical Charge. What combination of Forces result in Attraction? “Opposite Forces Attract” What combination of Forces results in Repulsion? “Like (Similar) Forces Repel” What has happened here? Extra Electrons (neg. charge) cause each hair to repel all others hairs. Why does a charged comb attract water molecules (demo)? Water Molecules have Pos. (+) and Neg. (-) ends = Polar. Neg. charge of comb attracts Pos. part of Water Molecules.

  2. Kinetic Energy What is Energy? Energy is the Ability to do Work or to Transfer Heat. What is Kinetic Energy? Kinetic Energy is the energy of Movement. How is Kinetic Energy Determined? Kinetic Energy = ½ Mass x Velocity2 Units: (Joules) = (Kilograms) X (meters/seconds)2 How is the Kinetic Energy of Particles of Matter Determined? Temperature describes the Average Kinetic Energy of a group of particles. How does a Thermometer work? A Thermometer shows the expansion of liquid particles inside the thermometer when they have the Same Average Kinetic Energy as the Surrounding particles. Temperature = Average Kinetic Energy

  3. Potential Energy What is Potential Energy? Potential Energy is Stored Energy: The Energy an object has due to its Position relative to an Attractive (or repulsive) Force. Describe the Force and the Position with Greater Potential Energy  Gravity is the Force of Attraction toward the center of the earth. Water Above the Waterfall has Greater Potential Energy than water below. Uphill Position has Greater Separation from earth and gravity.  Describe the Force and the Position with Greater Potential Energy. There are Attractive Forces between particles that form the bow as a solid. Bending the bow results in a Position of Partial Separation between Particles as increased Potential Energy.

  4. Energy is Conserved How is Energy Conserved in Physical and Chemical Changes? Law of Conservation of Energy: Energy can be neither created nor destroyed, it can simply change from one form to another. Describe the Energy Changes below. Animation courtesy of: http://www.sharp.org/physics/stuff/swings.html As the pendulum falls Potential Energy is transformed into Kinetic Energy. As the pendulum rises Kinetic Energy is transformed into Potential Energy.

  5. Energy Transformations How do we differentiate the System from the Surroundings in physical and chemical changes? The System refers to the Particles of Matter and the Surroundings are everything else; the container, the air in the room, and you. What is an Exothermic Process? Exothermic = The System (particles) Releases heat (thermal energy) to the Surroundings. “Heat Exits the System.” An Exothermic process “Feels Warm.” Describe the characteristics of an Endothermic Process. The Surroundings are Cooled as thermal energy is transferred to the system. Particles of the system increase their Potential Energy (not Kinetic Energy). Describe the energy transformations expected in terms of the concepts above. Chemical potential energy of rocket fuel will be released to surroundings in an exothermic chemical change. The Rocket and other surroundings will have increased kinetic and potential energy.

  6. Phase Changes Explain, in terms of attractive forces between particles, why particles in the Gas Phase have the Greatest Potential Energy. Particles in gas phase are in the Greatest Position of SeparationRelative to an Attractive Force. These Phase Changes are Endothermic   These Phase Changes are Exothermic Potential Energy Increasing

  7. Energy in Phase Changes How can the system absorb heat and not get hotter? Gas Liquid Solid When particles go through an “upward” Phase Change they increase their Potential Energy (stored) not their Kinetic Energy (temp). Link to Phase Changes of Water Indicate increases in KE vs. PE for the system in the graph below. Potential Energy How can melting pt. and boiling pt. be identified? Note the two plateaus. Increased Temp. = Increased KE Liquid to Gas = Increased PE

  8. Thermal Energy Compare the average Kinetic Energy of the molecules in the two samples below. Same Temp. = Same Average Kinetic Energy Compare the Thermal Energy or Heat Energy of the two samples. The Pool of Water contains much, much more Thermal Energy than the Coffee. Distinguish between Heat and Temperature. Heat is a Form of Energy that can be transferred by conduction, convection or radiation. Difference in temperature determines direction of heat flow. Temperature indicates the Kinetic Energy of an average particle in a sample. What does Specific Heat describe? Water has a higher specific heat than most materials. Specific Heat describes the amount of heat energy required to change 1 gram of a substance by 1oC. Unit: J/goC Which has the higher specific heat? 

  9. Measuring Heat Energy How can one Measure the amount of Thermal Energy transferred by a system? “Give the Heat to Water.” Ex. How much heat is absorbed by 150.0 g of water if its temperature increases from 23.2oC to 31.9oC? Calorimeter q = m C ΔT q = 150.0 g x 4.184 J/goC x 8.7 oC www.saskschools.ca/.../1_graphics/coffee_cup.gif q = 5454.9J 5,500 J or 5.5 KJ heat: q = mCΔT Ex. Predict the final temperature for 83.7 g of water starting at 45.5 oC after the sample releases 2,200J of heat energy. q = heat m = mass C = specific heat ΔT = change in temp. q = m C ΔT 2,200 = 83.7 x 4.184 x ΔT ΔT = 6.28812…..  6.3oC final temp. = 39.2oC

  10. Heat of Fusion and Heat of Vaporization What does the Heat of Fusion describe? Heat of Fusion is the heat that must be added to melt 1 gram of a solid or the heat removed to solidify 1 gram of a liquid. Ex. How much heat energy is required to melt 24.3 g of ice at 0oC? q = 24.3 g x 334 J/g q = mHf q = 8,116.2 J  8,120 J or 8.12 KJ Ex. Determine the heat of vaporization of a substance if 2,370 J of heat is required to boil a 125.0 gram sample at its boiling point. q = mHv q = mHv 2370 J = 12.50 g x Hv Hv = 189.6 J/g  190. J/g

  11. q = mCΔT q = 100.0 g x 4.18 J/goC x (57.0 – 22.0 oC) q = 14,630 J  14,600 J or 14.6 KJ % Error = 14.238… 14%

  12. q = mCΔT q = 5.00 x 4.71 x (240. - 210.) q = 707 Joules P.E. Increase while K.E. Remains Constant. q = mHv q = 5.00 g x 1,370 J/goC q = 6,850 J or 6.85 KJ

  13. Absolute Zero How can one predict the Temperature at which Particles of Matter would have No Kinetic Energy (No Pressure) = Absolute Zero? Ex. Compare Temperature vs. Pressure of a Gas. Extrapolate to determine Temperature with Pressure = Zero. Room Temp. . 100 oC 0 oC . . Pressure lbs/ft2 0 5 10 15 20 Vary the Temp. of the gas inside. Determine Pressure. lecturedemo.ph.unimelb.edu.au Extrapolate: _____________________________ Predict beyond experimental data. -300 -200 -100 0 100 -273 oC Temperature oC Absolute Zero

  14. Kelvin Temperature What is the main difference between the Celsius Temp. and the Kelvin Temp? Zero Kelvin is Absolute Zero. Zero Celsius is m.p. of water. Written: oC vs. K How does one convert from one Temp. system to the other? K = oC + 273 ΔT oC = ΔT K Ex. Potassium is a very soft metal. What is its melting point in oC? K = oC + 273 337 K = X + 273 X = 64 oC Ex. What phase is a sample of bromine in ice water (0oC)? Bromine: m.p. = 266 K and b.p. =332 K resources.edb.gov.hk Ice Water = 273 K is above melt. pt. and below boil. pt.  Liquid Phase.

  15. Fire Syringe Demonstration Explain how the fire syringe works. http://www.arborsci.com/CoolStuff/Fire_Syringe.gif www.trippensee.com Recall that Gases have increased Potential Energy due to the Position of Separation relative to their Intermolecular Forces of Attraction (IFA). Squeezing the gas reduces their Position of Separation and therefore, their Potential Energy is reduced. The Gas molecules (system) release their energy as heat to the cotton (surroundings) which may ignite.

  16. Evaporation Explain how making the canvas wet will affect the temperature of the water inside. As the water in the canvas evaporates (endothermic) some heat energy will be absorbed from the water inside thus, cooling the water inside. How does the Drinking Bird work? 1. Bird bends over. Beak gets wet and liquid in “Straw” flows to base causing bird to stand up. 2. Water evaporates from beak cooling the head and causing air in the head (above the “straw”) to decrease pressure. Higher Relative air Pressure in belly pushes liquid up straw causing bird to bend over………. www.escience.ca

  17. Evaporation as a Cooling Process

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