AP Physics-B ElectroStatics Electric Charges: Subatomic Particles and Electricity: atoms subatomic particles protons neutrons electrons nucleus

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1 AP Physics-B ElectroStatics Electric Charges: It made my hair stand on end! Perhaps you are familiar with this expression, which is often used to describe a frightening or startling experience. According to biologists, it is possible for human hair to stand on end in moments of extreme fear. But there is another force that can make hair stand on end. You have probably experienced it on a cold, dry day when your hair seemed to fly all around as you tried to comb or brush it. What you experienced was electricity. Electricity may also give you a shock if you walk along a carpet and then touch a metal doorknob. Electricity enables you to rub a balloon on your sleeve and make it stick to the wall. Electricity produces the awesome flashes of lightning in the sky. What is electricity? Where does it come from? How does it move? To answer these questions, you must first understand atoms and charges, which are both related to electricity. Subatomic Particles and Electricity: All matter is made up of atoms. An atom is the smallest particle of an element that has all the properties of that element. An element contains only one kind of atom. For example, carbon is made of only carbon atoms. Gold is made of only gold atoms. Atoms are made of even smaller particles called subatomic particles. These subatomic particles include protons, neutrons, and electrons. Protons and neutrons are found in the nucleus, or center, of an atom. Protons and neutrons account for most of the mass of an atom. Whirling around the nucleus is a cloud of electrons. Electrons occupy different energy levels, depending upon their distance from the nucleus. Both protons and electrons have a basic property called electric charge. The kind of charge, however, is not the same for both particles. Protons have a positive charge, which is indicated by a plus symbol (+). Electrons have a negative charge, which is indicated by a minus symbol (-). Neutrons are neutral. Neutrons have no electric charge. Charge and Force: When charged particles come near one another, they give rise to two different forces. A force is a pull or push on an object. A force can pull objects together or it can push objects apart. A force that pulls objects together is a force of attraction. A force of attraction exists between oppositely charged particles. So negatively charged electrons are attracted to positively charged protons. This force of attraction holds the electrons in the electron cloud surrounding the nucleus. A force that pushes objects apart is a force of repulsion. A force of repulsion exists between particles of the same charge. So negatively charged electrons repel one another, just as positively charged protons do. Electric charges behave according to the simple rule: Like charges repel each other, unlike charges attract each other. From your experience, you know that when you sit on a chair, pick up a pen, or put on your jacket, you are not attracted or repelled by these objects. Although the protons and electrons in the atoms of these objects have electric charges, the objects themselves are neutral. Why? The number of electrons in an atom is equal to the number of protons in an atom. So the total negative charge is equal to the total positive charge. The atom is neutral. It has no overall charge. How, then, do objects such as a balloon and strands of hair develop an electric charge if these objects are made of neutral atoms? The answer lies in the fact that electrons, unlike protons, are free to move. In certain

2 materials, the negative electrons are only loosely held by the positive protons. These electrons can easily be separated from their atoms. When two objects are rubbed together, one object loses electrons while the other object gains electrons. The object that gains electrons has an overall negative charge. The object that loses electrons has an overall positive charge. Remember that only the electrons move (not the protons). A neutral object develops an electric charge when it either gains or loses electrons. If you rub a balloon against a piece of cloth, the cloth loses some electrons and the balloon gains these electrons. The balloon is no longer a neutral object. It is a negatively charged object because it has more electrons than protons. As the negatively charged balloon approaches the wall, it repels the electrons in the wall. The electrons in the area of the wall nearest to the balloon move away, leaving that area of the wall positively charged. Using the rule of the charges, can you explain why the balloon now sticks to the wall? Electric Fields: If two charged particles come close to each other, they will experience a force. If the two particles are alike in charge, the force will be one of repulsion. If the two particles are opposite in charge, the force will be one of attraction. The repulsion and attraction of particles occurs because charged particles have electric fields around them. An electric field is the region surrounding a charged particle in which an electric force affecting other charged particles is noticeable. The electric field is strongest near the charged particle. It is weakest far away from the charged particle. The strength of an electric field depends upon the distance from the charged particle. As the distance from a charged particle increases, the strength of the electric field decreases. Electrostatics: When you read that the loss or gain of electrons produces an electric charge and electricity, you may have noticed that the words electron and electricity are similar. This similarity is no accident. Electricity depends upon electrons. In fact, electricity can now be declined as the energy associated with electrons that have moved from one place to another. You are probably most familiar with electricity that flows through electric wires. But the movement of electrons is not always a continuous flow through a wire. Sometimes electrons can move from one object to another and then remain at rest. The type of electricity is called static electricity, or more properly electrostatic charge. The word static means not moving, or, stationary. An electrostatic charge is the buildup of electric charges on an object. The electric charges build up because electrons have moved from one object to another. Once built up, however, the charges do not flow. They remain at rest.

3 Methods of Charging: An object can become charged in three ways: friction, conduction, and induction. Rubbing a balloon with a piece of cloth is an example of charging an object by friction. The motion of the cloth against the balloon causes charges on both objects to separate. Since the electrons in the cloth are more loosely held than the electrons in the balloon, electrons move from the cloth to the balloon. What is the resulting charge on the cloth? On the balloon? If a hard rubber rod is rubbed with fur, friction separates charges on both the rod and the fur. Electrons are transferred from the fur to the rod. Because the rubber rod has gained electrons, it is negatively charged. The fur, which has lost electrons, is positively charged. If a glass rod is rubbed with silk, electrons are transferred from the glass rod to the silk. The glass rod, which has lost electrons, is positively charged. What is the charge on the silk? Charging by conduction involves the direct contact of objects. In conduction, electrons flow through one object to another object. Certain materials allow electrons to flow freely. Materials that permit electric charges to move easily are called conductors. Most metals are good conductors of electricity. Silver, copper, aluminum, and mercury are among the best conductors. Materials that do not allow electrons to flow freely are called insulators. Insulators do not conduct electric charges well. Good insulators include rubber, glass, wood, plastic, and air. The rubber tubing around an electric wire and the plastic handle on an electric power tool are examples of insulators. What do these insulators do? An object can acquire a charge by induction. Induction involves a rearrangement of electric charges. For induction to occur, a neutral object need only come close to a charged object. No contact is necessary. For example, a negatively charged rubber rod can pick up tiny pieces of paper by induction. The electric charges in the paper are rearranged by the approach of the charged rubber rod. The electrons in the area of the paper nearest to the rod are repelled, leaving the positive charges near the rod. Because the positive charges are closer to the negative rod, the paper is attracted. In order to induce a permanent charge on an object you need to temporarily ground the object while inducing the charge on it and then remove the source of the charge.

4 The Electroscope: Uncharged electroscope Charged electroscope An instrument called an electroscope can detect an electric charge. A typical electroscope consists of a metal strip with a metal plate at the top and an aluminum foil straw which pivots on the metal strip at the bend. The picture Above shows this electroscope. In an uncharged electroscope, the aluminum straw hangs straight down. When a charged object touches the metal plate, electric charge travels down the strip and onto the straw. The presence of an electric charge is indicated when straw pushes away from the strip and rotates at the pivot point. This happens due to the fact that the charge on both the strip and the straw is the same, like charges repel. An electroscope can be charged by conduction. A charged object is brought in direct contact with the metal plate of the electroscope. For example, if a negatively charged rubber rod touches the metal plate, electrons from the charged rubber rod move to the metal plate of the electroscope and then down the metal strip to the aluminum straw. The straw gains a negative charge and repels from the metal strip. If a positively charged glass rod touches the metal plate of the electroscope, free electrons in the straw and the metal strip are attracted by the glass rod. The metal strip and metal plate conduct the electrons out of the electroscope to the glass rod. The loss of electrons causes the straw to become positively charged and repel from the metal strip. Whether indicating a negative or positive charge, the electroscope behaves the same. Lightning: Electrons that move from one object to another and cause the buildup of charges at rest, or static electricity, eventually leave the object. Usually these extra electrons escape into the air. Sometimes they move onto another object. The charged object loses its electrostatic charge and becomes neutral. The loss of electrostatic charge as electric charges move off an object is called electric discharge. Sometimes the discharge is slow and quiet. Sometimes it is very rapid and accompanied by a shock, spark of light, or a crackle of noise. One of the most dramatic examples of the discharge of electrostatic charge is lightning. During a storm, particles contained in clouds are moved about by the wind. Charges become separated, and there are buildups of positive and negative charges. If a negatively charged cloud forms near the surface of the earth, objects on the earth become electrically charged by induction. Soon electrons are jumping from the cloud to the earth. The result of this transfer of electrons is a giant spark called lightning. Lightning can also occur as electrons jump from cloud to cloud. As electrons jump through the air, intense light and heat are produced. The light is the bolt of lightning you see. The heat causes the air to expand suddenly. The rapid expansion of the air is the thunder you hear. Lightning contains dangerously high amounts of electric energy. An average lightning bolt transfers 6 billions electrons between a cloud and the earth.

5 One of the first people to understand lightning as a form of electricity was Benjamin Franklin. In the mid s, Franklin performed experiments that provided evidence that lightning is a form of electricity, that electricity moves quickly through certain materials, and that a pointed surface attracts electricity. Franklin suggested that pointed metal rods be placed above the roofs of buildings as protection from lightning. These rods were the first lightning rods. Luckily for Franklin, he put a lightning rod on his roof. Shortly afterward, lightning struck his home! Lighting rods work according to a principle called grounding. A discharge of electrostatic charge usually takes the shortest path from one object to another. So lightning rods are attached to the tops of buildings and a wire connects the lightning rod to the ground. When lightning strikes the rod, which is taller than the building, it travels through the rod and the wire harmlessly into the earth. Unfortunately, other tall objects such as trees can also act as grounders. That is why it is not a good idea to stand near a tree during a lightning storm. Why do you think it is also not a good idea to stand in an open field during an electric storm? Review Questions: 1.) What are the charged particles in an atom and what are their charges? 2.) What is the rule of electric charges? 3.) What is the underlying cause of an object's electric charge? 4.) What are the three methods by which an object can acquire an electric charge? 5.) How can you tell if an object touching an electroscope is neutral or has a charge? 6.) What is lightning? 7.) What would happen if a lightning rod were made out of an insulator rather than a conductor? 8.) What is an electric field? Draw a diagram of lines of force of an electric field. 9.) Where is an electric field the strongest? weakest? 10.) Upon what does the strength of the electric field depend? 11.) What is electricity? 12.) How are the processes of conduction and induction different? alike? 13.) What is an electroscope? Describe how an electroscope works. 14.) What is electric discharge? How does electric discharge occur? 15.) Describe the grounding of an object? 16.) How does a lightning rod work?