Science 9.1 PDF - Electric Force, Fields, and Energy

# Electric Force, Fields, and Energy - Did you know that there are electric charges, forces, and fields inside your body? You might not see them or feel them, but they are in every atom, everywhere! - Atoms are made up of protons, neutrons, and electrons. - Protons are _positively_ charged pa...

# Electric Force, Fields, and Energy - Did you know that there are electric charges, forces, and fields inside your body? You might not see them or feel them, but they are in every atom, everywhere! - Atoms are made up of protons, neutrons, and electrons. - Protons are _positively_ charged particles. - Electrons are _negatively_ charged particles. - Neutrons are _neutral_, meaning they do not have a charge. - Most objects are made of atoms in which the number of protons is equal to the number of electrons. As a result, the positive and negative charges cancel out and the atoms are _neutral_. - Electrons can move from one atom or object to another. - If an object loses electrons, it is left with more protons than electrons. It has an overall _positive_ charge. - If an object gains electrons, it will have an overall _negative_ charge. - If you have seen a dramatic display of electric charges, like a lightning storm, you have seen _moving electrons_. ## Academic Vocabulary Charge is a basic property of matter that creates a force and accounts for electric interactions. Some particles and atoms have no charge, so they are neutral. Is the atom in Figure 1 neutral or charged? ## Model of an Atom - Figure 1: Charged particles make up atoms. | Particle | |---| | Electron | | Neutron | | Proton | ## Lightning Storm Figure 2: Lightning bolts can travel from clouds to the ground. They can also travel within a cloud and between clouds. These streaks of light are the result of the movement of electric charges. # Electric Field Lines - Figure 3: Images A and B Show the field lines around single charges. Image C shows the field lines around a positive charge and a negative charge next to each other. - Where field lines are closer together, the electric field is stronger. ## Electric Force - The force between charged particles or objects is called electric force. - If a proton and an electron come close together, the opposition of their positive and negative forces creates an attraction that draws them together. - If two electrons come close together, they repel each other because they both are negativity charged. - The electric force causes them to move apart. - In general, opposite charges attract, and like charges repel. - The strength of the electric force depends on the distance between the charges. - For example, when a positively charged particle or object is close to another positively charged particle, a strong force between them pushes them away from each other. - As they move apart, the force between them becomes weaker. - The strength of the electric force also depends on the amount of charge present. - When more charge is involved, the electric force is stronger. For instance, three protons attract an electron more strongly than one proton alone. ## Electric Fields - Two charged particles will experience electric forces between them without even touching. - An electric charge has an invisible electric field around it - a region around the charged particle or object where the electric force is exerted on other charged particles or objects. - Electric fields can be represented by _field lines_. - Figure 3: They point in the direction that the force would push a positive charge. - Field lines point away from the object around a positively charged object, indicating that the object would repel a positive charge. - Field lines point toward the object around a negatively charged object, indicating that the object would attract a positive charge. - When multiple charges are in the same area, the field lines show a slightly more complicated combination of the two fields. # Charges and Potential Energy - When forces are in action, you have a system that consists of two opposite charges and their interaction. - Suppose you pull the opposite charges away from each other, the potential energy of the system increases. - You can understand this by comparing it to gravitational potential energy. - Gravity is an attractive force. - When you lift an object higher above the ground, you apply a force and transfer energy to it. - The object's gravitational potential energy increases. - When you drop the object, the force of gravity pulls the object to the ground and its gravitational potential energy decreases. - The force between opposite charges is also an attractive force. - As you apply a force to move opposite charges away from each other, the electric potential energy of the system increases. - When the electric force between opposite charges pulls them together naturally, the potential energy of the system decreases, as shown in Figure 4. - Potential energy changes in a different way between two like charges. - Two like charges naturally repel each other. - An outside force is not needed to move them apart. - Therefore, as the electric force between two like charges pushes them away from each other, the potential energy of the system decreases. ## Gravitational PE - Increasing PE - Decreasing PE ## Electric PE - Increasing PE - Decreasing PE ## Potential Energy - Figure 4: Electric potential energy behaves a lot like gravitational energy. ## Question It! - Students are conducting an experiment to provide evidence that electric fields exert forces on objects even when the objects are not in contact. They use pith balls hanging from strings. Pith balls are small balls that pick up charge easily. These pith balls have been charged by touching another charged object. The students drew this diagram to show the result of their experiment. - **1. CCC Cause and Effect**: When the two pith balls have opposite charges, they are naturally pulled together due to the attractive electric force between them. If you pull the two pith balls away from each other, what happens to the potential energy of the system? Explain. - **2. Cite Textual Evidence**: How do the results of this experiment provide evidence that electric fields exert forces on the pith balls, even when they are not in contact? # Electric Current and Circuits - Electric charges play a major role in daily life. - Any time you use electricity, you are using energy from electric charges that are in motion. - Electric charges flow through materials like water flows down a stream. - The continuous flow of charge is known as electric current. - Current is measured as a rate in units called amperes. - The abbreviation for this unit is A. - The number of amperes describes the amount of charge that passes by a given point each second. - Current flows through paths known as circuits. - A circuit is a path that runs in a loop. - A basic electric circuit contains a source of energy connected with wires to a device that runs on electricity. - Current flows from the source of energy, through the wires, through the electric device, and back to the source. ## Voltage - Why do charges flow through a circuit? - They move because of differences in potential energy. - Current flows from a point of higher potential energy to a point of lower potential energy in the circuit. - For instance, a battery, like the one shown in Figure 5, has one end where current has a higher potential energy per charge than it has at the other end of the battery. - This difference in electric potential energy per charge is called voltage. - The voltage acts like a force that causes current to flow. - Voltage is measured in units of volts. - The abbreviation for this unit is V. ## Current in Circuits - The following circuit shows a battery connected to a light bulb. - Based on potential energy, which direction should the current flow? Explain your answer. ## Energy in Circuits - You can compare a charge in a circuit to an object in the gravitational field of Earth. - When an object falls, the force of gravity pulls the object from a position of higher potential energy to a position of lower potential energy. - You give that potential energy back when you lift the object up to its initial position. - A battery gives energy back to charges as well. - Inside the battery, the energy from chemical substances is converted to electric energy. - That electric energy becomes the potential energy of the charges. - They return to a position of higher potential energy, from which they flow through the circuit. ## Current and Resistance - What are the charges that flow through a circuit? - They are electrons. - Historically, the current is described as flowing in the direction in which positive charges would move. - However, electrons are negatively charged. - So the direction of current is opposite to the direction of electron flow. - Some materials have electrons that are tightly bound to their atoms. - Their electrons are difficult to move. - Those materials, called insulators, do not allow charge to flow. - Therefore, they have a high resistance to electric current. - Some materials have electrons that are more loosely bound to their atoms. - Those materials are conductors - they allow charge to flow more freely (Figure 6). - Just as there are insulators and conductors of heat, there are insulators and conductors of charge. - Insulators of charge are materials such as rubber, wood, and glass, while conductors include materials such as silver, copper, and gold. ## Conductors and Insulators of Charge - Figure 6: Conductors and insulators of charge are all around you. - Label each of these common items as a conductor or an insulator. - SEP Construct Explanations: Which of the materials used to make these objects would you use in a circuit? Explain why. # Static Electricity - Recall that most objects are made of atoms in which the number of protons is equal to the number of electrons. As a result, these atoms are neutral. By the law of conservation of charge, charge cannot be created or destroyed, but it can be transferred. - The transfer of charge happens by moving electrons from one object to another or from one part of an object to another. - When charges build up on an object, they do not flow like current. - Instead, they remain _static_, meaning they stay in place. - This buildup of charge on an object is called _static electricity_. ## Methods of Charging - Objects can become charged by four methods: conduction, friction, induction, and polarization. - Charging by conduction is simply the movement of charge by direct contact between objects. - The object that is more negatively charged transfers electrons to the other object. - Charging by friction occurs when two objects rub against each other and electrons move from one object to the other. - Objects become charged by induction without even touching. - The electric field of one charged object repels the electrons of the other object. - So the second object ends up with a buildup of charge on its opposite side, as in Figure 7. - Polarization is similar except the electrons only move to the opposite side of their atoms rather than to the opposite side of the entire object. - See if you can identify the methods of charging in Figure 8. ## Interactions with Static Electricity - Figure 8: Label the method of charging in each image as conduction, friction, induction, or polarization. # Potential Energy and Static Electricity - If you rub a balloon, you might be surprised that it can pick up bits of paper. - The balloon attracts the paper because of static electricity. - Rubbing the balloon causes electrons to transfer to it. - The charged balloon polarizes the bits of paper. - Because the surface of the balloon is negatively charged and the surface of the paper is positively charged, they attract each other as in Figure 9. - As the bits of paper move toward the balloon, the potential energy between the balloon and paper decreases. - When you pull the bits of paper off of the balloon, you apply a force to them. - The potential energy between the balloon and the paper increases the further you move them apart. ## Static Discharge - Most objects that become charged eventually lose their charge to the air. - Charge transfers to or from the air until the charged object is neutral. - The process of discharging can sometimes cause a spark or shock when the electrons transfer. - If you have ever reached to pet a cat and experienced a shock, it was the result of static discharge. - Lightning is also the result of static discharge. - Water droplets in the clouds become charged due to all of the motion within the air during a storm. - Electrons then move from areas of negative charge to areas of positive charge. - The movement of charge produces the intense spark that we see as a lightning bolt. ## Balloon and Paper - Figure 9: The balloon attracts the paper because of static electricity. Draw the charges on the balloon and on the bits of paper. Then, describe what happens to potential energy as you pull the bits of paper off of the balloon. # Lesson 1 Check - **1. Describe**: Why are conductors better than insulators for the flow of electric current? - **2. Explain**: A proton is placed next to an negatively charged object. In which direction would the proton move? Explain why. - **3. CCC Cause and Effect**: If you move two objects with opposite charges apart, what happens to the potential energy between them? Explain your response. - **4. SEP Develop Models**: After Sandra combs her hair, she notices that her hair moves toward the comb. Draw a model of the comb and Sandra's hair. Show the charges on both the comb and the hair. Describe the types of charges that you think occurred to charge the comb and then to charge the hair. # Quest Check-In - In this lesson, you learned about the interactions of electric charges through forces and fields. - You also discovered how potential energy plays a role in the flow of current. - Additionally, you explored how charges behave in static electricity. - **SEP Design Solutions**: How might electric fields become involved in your levitation device?