Gravitational Potential Energy

Transcription

1 Name: Directions: Read and answer the following questions. You can then go on to my web page and check your answers. At the conclusion, go to schoology.com and complete the PE assignment. Gravitational Potential Energy Work Waiting to Happen In energy terms, the work waiting to happen is called potential energy. We define potential energy (PE) as the energy of position or a stored energy of position. In most of our examples the position refers to how high in the air something is or how far something gets pulled back. Gravitational Potential Energy Energy of position has a special name, gravitational potential energy. This type of potential energy depends on an object s mass, height, and gravity. For example, when you lift a book into the air, this book will have potential energy because it has height from being lifted off the ground. This is similar to elastic potential energy but instead of pulling back on a rubber band, here we are lifting something up in the air. Just like elastic PE has an equilibrium or zero point, gravitational PE also has a zero point. In this case, it s the ground or the floor. Calculating Gravitational Potential Energy The formula for calculating gravitational potential energy is as follows: Potential Energy, PE = gravity x mass x height Gravity refers to the force of gravity pushing down on all of the objects on earth. Depending where you are in the solar system, gravity can change, but here on good old earth, gravity is approximately the same everywhere, 9.8. What about the height? Height above what? For example, if I am standing on a desk at school (breaking science classroom rules J), is my height in relation to the floor of the classroom or outside on the ground? Basically, it s the closest point where you will stop falling. If I am standing on a desk, this would be the classroom floor. If I was jumping off the desk and out the window, my height would be measured from the top of the desk down to the ground. So understanding my ground location is extremely important for potential energy. An Example Let s do an example. If I am standing on a desk 1 m off the floor and weigh 100 kg, what is my PE? Just as we did with kinetic energy, let s set up the problem. PE = gravity x mass x height = 9.8 x 100 kg x 1 m = 980 J Just as kinetic energy is measured in joules (J) so is potential energy. What if I was standing on the desk and jumping out the window (my height is now 21 m)? PE = gravity x mass x height = 9.8 x 100 kg x 21m = 20,580 J So just by increasing my height above ground and what I call ground, the PE increases up to 20,580 J. Questions 1. Name two ways that you can change your gravitational potential energy while still on earth.

2 2. Calculate the gravitational potential energy if your weight is 50 kg and you jump from a tree swing reaching a height of 5 meters off the ground. 3. Once you land on the ground, what is your new potential energy? Chemical Potential Energy Just like magnetic PE, chemical PE is due to the attraction or repulsion that objects or their parts have for each other. Atoms are made of charged particles (negatively charged electrons and positively charged nuclei) that attract and repel each other. Since opposite charges attract, negatively charged electrons (-) and the positively charged (+) nucleus within atoms attract each other. However, since like charges repel each other, electrons repel other electrons and the nucleus of an atom (+) will repel another atom nucleus (+). Chemical bonds are the attractive forces between atoms that enable the group to act as a unit. In the figure, straight lines represent the bonds between atoms in the molecule for water. There is no actual physical attachment between the atoms, only an attractive force due to differences in the charged particles in the atoms. The potential energy that substances have due to their bonds is called chemical potential energy or simply chemical energy. During a chemical reaction, one or more chemicals called reactants are changed into one or more new substances called products. The changes during a chemical reaction are due to the breaking and making of chemical bonds, which causes changes in chemical potential energy. There are two easy ways to measure this energy using a thermometer. The first type is called an exothermic reaction. When materials react, heat is given off. If the reaction occurs in a container, the temperature of the mixture would rise due to this released heat. In this case, the potential energy of the two reacting materials is greater than the material it creates. The other type of is called an endothermic reaction. In this case, the potential energy of the product is greater than that of the materials you are mixing together. Energy is not created; instead, energy, such as heat energy, is taken in and transformed into chemical energy. The products of an endothermic reaction are cooler than the reactants. If the reaction occurs in a container, the temperature of the mixture would decrease due to the heat being removed and used to form bonds. Question 1. How do you determine the amount of chemical potential energy in a material?

3 Elastic Potential Energy As you know, there are two different kinds of energy: potential energy, which is stored energy, and kinetic energy, which is energy in motion. We ve come to realize that the two work in opposite directions. As one is increasing, the other is usually decreasing. A great example of the difference between kinetic and potential energy is from the classic "snake-in-a-can" prank. This is an old joke where you give someone a can of peanuts and tell them to open it, but inside is actually a long spring that pops out when the lid is twisted off. Because the spring is usually decorated to look like a snake, this prank usually causes the victim to jump back and shout in surprise! When the snaky spring is compressed and secured inside the unopened can, it has lots of potential energy and no speed the kinetic energy is low (or zero). But when the can is opened, the potential energy quickly converts to kinetic energy as the fake snake jumps out. The snake is in the can, not moving and holding on to all of the EPE. The snake is on the move increasing in speed as the spring is released. 1. Create a rule that helps to determine when something elastic has the highest potential energy. 2. A spring is stretched out to location A and released. Using your knowledge from potential energy, draw a bar chart for the ball when it is at positions A and B.

4 3. A spring is pulled downward (position A) and then let go. Draw a bar chart for the block attached to the spring when it is at positions A and B. 4. A nurf gun spring is compressed and then fired. Draw an energy bar chart for the nurf ball at positions A and B. 5. Describe how this scenario is different than problems 2 and How would your rule change if you are stretching a rubber band now instead of a spring?

5 Electrical PE Voltage is another word for electrical potential energy. When you have voltage coming out of a wall outlet, before you plug something into it, it has electrical potential energy. Voltage refers to how much work is available in the outlet. For example, in the U.S., we have 110V outlets. This means we have 110V of electrical potential energy waiting to come out and do some work. But what is it that is waiting to do the work. The answer is electrons. Remember that electrons are negatively charged particles. When a charged particle sits around all by itself, it doesn't have any motivation to do work. When we gather a group of charged particles together, it's called a charge collection. The most common example of a charge collection is a battery, which is simply a bunch of electrons packed into a tight space. If we follow the same ideas that we did above, you might see that there are similarities between the gravitational potential energy described above and electric potential energy. Let s say you place a positive charge near the positive plate in an electric field between two parallel plates. At the top, we can say that the charge has maximum electric potential energy. The charge doesn't want to be up there, so if you let it start to move it will be repelled away from the positive plate and attracted towards the negative plate. It will speed up all the way down. While it is falling we know that the electric potential energy is being converted to kinetic energy. When it reaches the negative plate (its reference point) it has no electric potential energy remaining. It's all changed to kinetic energy. We can demonstrate this just like we did with the two magnets. 1. A cart on a track has a large, positive charge and is located between two sheets of charge. Initially at rest at point A, the cart moves from A to C. 2. Draw a bar chart for the cart when it is at positions A, B, and C. EPE KE EPE KE EPE KE

6 3. For each situation below, create a bar chart of the electrical PE and KE at the initial and final locations. ElPE KE ElPE KE ElPE KE ElPE KE ElPE KE ElPE KE ElPE KE ElPE KE ElPE KE ElPE KE

7 Magnetic Potential Energy A little review before we introduce a new term in magnetism. We ve been discussing all the wonderful things about magnets. 1. We ve talked about magnets and magnetic materials being made of metal. 2. We ve discussed and accomplished how to make a magnet using both electricity (electromagnet) and by rubbing a piece of metal (a needle) against something that already is magnetized. 3. All magnets, no matter the shape or size, have two regions where their magnetic force is strongest. These regions are called magnetic poles. We drew the fields using a compass and bar magnet and saw how the fields all end near the poles. 4. And finally, the law of magnetic poles states that like poles of a magnet (north and north or south and south) repel each other while unlike poles of a magnet (north and south) attract each other. Magnetism describes all the above effects of a magnetic field, including something new called magnetic potential energy. Recall, potential energy is the ability of an object to do work because of its position. The energy of an object that has the ability to do work because of its position in a magnetic field is called magnetic potential energy. Magnetized objects move in the direction that reduces their magnetic potential energy. This is no different than the skate park. It basically says that as potential energy decreases, kinetic energy increases and vice versa (as one goes up the other goes down). In the picture to the right, as the magnet starts to move, the speed increases. Think about the skate park. As the skater falls down the half pipe, the speed increases. As the speed increases, the KE increases. As the KE increases, the PE decreases. It s a vicious circle going the opposite way as PE increases only to start over when the skater starts to fall. As we already mentioned, the magnetic PE gets larger the further away the magnetic material moves from the magnet. Remember, we are not measuring the strength of the attraction just how far away it can still pull an object. The mass of the magnet can play a role. The bigger the magnetic field is the larger its magnetic potential energy can be. The location on the magnet plays a role on how much magnetic potential energy an object will have. Assignment: take two magnets and observe what happens as you slowly move them closer together. Questions 1. Magnetic potential energy is dependent on... a. the mass of a magnet and how fast the magnet is moving. b. the shape and size of a magnet. c. the position of magnetic objects relative to one another. d. the distance a magnet has moved from its resting position.

8 2. One of the components on the front mentioned location on the magnet playing a role in magnetic PE. Draw what a magnetic field would look like on a typical bar magnet: N S 3. Jane places two bar magnets on her desk next to each other with like poles facing. She then sprinkles a few iron filings on and around the magnets. Which of the following best shows how the iron filings will arrange themselves? 4. Using what you learned from the skate park lab, draw how a plot of KE and PE would look in the following scenario: 5. Compare Magnetic PE to Gravitational PE. How are they similar? 6. For GPE, the ball wants to fall to the lowest energy state possible. Where would the lowest energy state be for two magnets? Why?