Today. Exam 1. The Electric Force Work, Energy and Power. Comments on exam extra credit. What do these pictures have in common?

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on exam extra credit What do these pictures have in common? Each question you get up to 4 tries before CAPA marks it incorrect.

1 Today Exam 1 Announcements: The average on the first exam was 31/40 Exam extra credit is due by :00 pm Thursday February 18th. (It opens on LONCAPA today) The Electric Force Work, Energy and Power Number Score ISP09s10 Lecture ISP09s10 Lecture Comments on exam extra credit What do these pictures have in common? Each question you get up to 4 tries before CAPA marks it incorrect. The online test is not exactly the same as the in-class one, but it is rather close. This is why I didn t collect your copies of the exam. There are only 1 questions, but some of these are weighted more so that the total # of points is 40. Say you got a 30/40 on the in-class exam, and a 40/40 on the extra Credit exam. Then you get (40-30)*.30 = 3 extra points added to Your original score. All are instances of Static Electricity. What is electricity? ISP09s10 Lecture ISP09s10 Lecture 10-4-

2 A new Force! Electrical charge is a property of matter. It is measured in Coulombs C. It comes in types ( positive and negative ) Like charges repel, unlike charges attract. Coulomb s Law of Electric Force kq Q F = k = 8.99E 9 N! m r 1 ISP09s10 Lecture C - + r 1 Questions to ponder Coulomb s law looks like Newton s Law of gravity. Why? Why does charge come in two types and mass only came in one type? Why do we always get r? I hate squares. Why is k = 8.99E9 Nm /C so much bigger than G = Nm /kg? We ll come back to these mundane? s later and see that they are actually pretty profound and cutting edge ISP09s10 Lecture Example 1 Two 1 kg balls with a charge of +1 C are a distance 1 m from each other. What is the electrical force? kq Q F = k = 8.99E 9 N! m r 1 C Example Two 1 kg balls with a charge of +1 C are a distance 1 m from each other. What is the gravitational force? Gm m F =! r ; G = 6.673E 11 Nm kg The math is identical to Gravitational problems! ISP09s10 Lecture The Gravitational Force is 0 orders of magnitude weaker Than the electrical force! Gravity is a VERY weak force. ISP09s10 Lecture 10-8-

3 If the distance between charges suddenly doubles, how does the electric force change? A) It is 1/4 the initial force B) It is 4 times the initial force C) It is 1/ the initial force D) It is times the initial force E) Nothing changes If the distance between charges suddenly doubles, how does the electric force change? A) It is 1/4 the initial force B) It is 4 times the initial force C) It is 1/ the initial force D) It is times the initial force E) Nothing changes ISP09s10 Lecture ISP09s10 Lecture Energy Energy and Power Energy is the ability to do work Energy comes in two forms Kinetic (KE) energy of motion Potential (PE) energy of position There are many variants on these type main types, e.g. chemical, nuclear, thermal, ISP09s10 Lecture Energy is the ability to do work: Work = force x distance = F d Energy comes in two forms Kinetic (KE) energy of motion Potential (PE) energy of position KE = mv m - mass v - velocity Gravitational GPE = m (gh); g = 9.81 m/s on Earth, h height Power (measured in W = J/s) is the rate of change (or use) of energy ISP09s10 Lecture 10-1-

4 Work: Using a Force to Move Something In common English, work refers to any kind of effort you put into performing a task, whether physical or mental. In physics, work is done whenever an object is pushed or pulled through a distance; there must be both force and motion. work = force! distance = Fd ISP09s10 Lecture Under what condition can a force act on an object and yet do no work on that object? (A) If the object moves with no acceleration. (B) If the object does not move. (C) If the net force on the object is zero. (D) If the force operates at constant (unchanging) power. (E) Nonsense--any force on an object must do work on that object. ISP09s10 Lecture Under what condition can a force act on an object and yet do no work on that object? (A) If the object moves with no acceleration. (B) If the object does not move. (C) If the net force on the object is zero. (D) If the force operates at constant (unchanging) power. (E) Nonsense--any force on an object must do work on that object. ISP09s10 Lecture Work and Energy: A simple Example Slowly lift your book some height h and then lower it. You did work on the book while lifting it; the book did work on you while you lowered it. The raised book has an increased ability to do work, and does this work as you lower it. What if you lift your book some height h and then drop it? Can it still do work? ISP09s10 Lecture

Therefore, the gravitational potential energy (GPE) of an object is its weight multiplied by its height, GPE = mgh The kinetic energy (KE) of an object in motion can be derived from Newton s laws KE

5 Work and Energy: A simple Example Work requires motion It accelerates downward, acquiring kinetic energy as it loses height. You could have the dropped book do work by driving a thumbtack into the floor. (remember, W = F x d) ISP09s10 Lecture An object s energy is defined as the amount of work it can do. Therefore, the gravitational potential energy (GPE) of an object is its weight multiplied by its height, GPE = mgh The kinetic energy (KE) of an object in motion can be derived from Newton s laws KE = 1/ * MV Quantitative Look at Energy ISP09s10 Lecture Quantitative Look at Energy GPE when you let it drop from height h is the same as its KE just before it hits the floor. Quantitative Look at Energy If you add the gravitational energy and the kinetic energy at any point in the book s fall, you will find that the sum stays the same. Energy is conserved. ISP09s10 Lecture ISP09s10 Lecture 10-0-

6 An Aside on Semantics A system is a distinct collection of objects and/or fields that are interacting in some way. Any system having the ability to do work is said to have energy. A system with energy need not do work, but a system that does work must have had energy. Some Forms of Energy Kinetic energy: energy of motion Gravitational energy: energy associated with a raised Object (aka potential energy ) Elastic energy: energy of a stretched or deformed object Thermal energy: energy in the form of heat due to the random microscopic motion of atoms and molecules ISP09s10 Lecture ISP09s10 Lecture Forms of Energy Electromagnetic energy energy associated with electric and magnetic fields Radiant energy energy of electromagnetic waves such as light, infrared, and X-rays Chemical energy energy involved in chemical reactions Nuclear energy energy involved in nuclear reactions Physically, the type of energy represented by the water behind a dam is (A) kinetic. (B) chemical. (C) thermal. (D) electromagnetic. (E) Gravitational potential energy. ISP09s10 Lecture ISP09s10 Lecture 10-4-

7 Physically, the type of energy represented by the water behind a dam is (A) kinetic. (B) chemical. (C) thermal. (D) electromagnetic. (E) Gravitational potential energy. ISP09s10 Lecture The Law of Conservation of Energy Experiments have found that energy is always conserved, although it may change its form. The total energy of all the participants in any process remains unchanged throughout that process. That is, energy cannot be created or destroyed. Energy can be transformed (changed from one form to another), and it can be transferred (moved from one place to another), but the total amount always stays the same. ISP09s10 Lecture The Work-Energy Principle Another way of stating the conservation of energy is what the book calls The work-energy principle: Work is an energy transfer. Work reduces the energy of the system doing the work and increases the energy of the system on which work is done, both by an amount equal to the work done. ISP09s10 Lecture Transformations of Energy Where is all that energy now? What happens after it hits the floor? It s gone into heat both your book and the floor are now slightly warmer. ISP09s10 Lecture 10-8-

8 Transformations of Energy One way to visualize energy transformations is through the use of an energy flow diagram. The one below is for the dropped book. Chemical energy is transformed into gravitational energy when (A) a car accelerates (on a level surface). (B) a leaf undergoes photosynthesis. (C) a block slides downhill. (D) water evaporates. (E) a person walks uphill. ISP09s10 Lecture ISP09s10 Lecture Some Example Problems Chemical energy is transformed into gravitational energy when (A) a car accelerates (on a level surface). (B) a leaf undergoes photosynthesis. (C) a block slides downhill. (D) water evaporates. (E) a person walks uphill. Examples: A mass of 1.0 kg is raised 1.0 m. How much work was done? W =!GPE = mg!h = 1.0 kg x 9.81m/s^ x 1.0 m = 9.81 J A 90.0 kg ISP09 professor walks up two flights of stairs. How much did his/her potential energy increase? DATA 1 flight of stairs = 3.00 m!gpe = 90.0 kg x 9.81m/s x flights x (3 m/flight) = 5.9 kj ISP09s10 Lecture ISP09s10 Lecture 10-3-

9 Conservation of Energy In nature certain quantities are conserved. Energy is one of these quantities. Charge is another. Example: Ball on a hill A 1.00 kg ball is rolled toward a hill with an initial speed of 5.00 m/s. If the ball roles without friction, how high, h, will the ball go? m KE = mv PE = mgh ; g = 9.80 s v ( 5 m / s) mv = mgh " h = = = 1.8 m g m! 9.80 s ISP09s10 Lecture Power Power is the rate of change of energy Power = (change in energy)/(change in time) Power is a scalar and is measured in watts. Light bulbs are measured in watts Sun (a big light bulb) !10 6 W ISP09s10 Lecture Information Horsepower 746 W = 1 horsepower In fourteen hundred and ninety-two Columbus sailed the ocean blue. And if you divide by two You get watts in a horsepower too Food energy is measured in kcal 1 (food) Calorie = 1000 (physics) calorie 1 Calorie = 4184 Joule 1 Calorie = amount of energy required to raise the temp. of 1 kg of water by 1 degree Celsius ISP09s10 Lecture Example Problem How many kcal are burned by doing 1500 J of work? DATA: The human body is 10% efficient in converting food energy to work. cal = energy' 1500 J ' 1cal J 1cal J & ' $ % & 1 # ' $! % efficiency " 1 #! = 3590.cal = 3.59 kcal 0.1" ISP09s10 Lecture

F=ma. Exam 1. Today. Announcements: The average on the first exam was 31/40 Exam extra credit is due by 8:00 am Friday February 20th.

F=ma. Exam 1. Today. Announcements: The average on the first exam was 31/40 Exam extra credit is due by 8:00 am Friday February 20th. Today Exam 1 Announcements: The average on the first exam was 31/40 Exam extra credit is due by 8:00 am Friday February 0th. F=ma Electric Force Work, Energy and Power Number 60 50 40 30 0 10 0 17 18 0