Chapter 7: Electricity

Section 1: Electric Charge

Positive and Negative Charge * The center of an atom contains protons and neutrons. Electrons swarm around the atom’s center. * Protons and electrons have electric charge, and neutrons have no electric charge. * Protons have positive electric charge and electrons have negative electric charge. * The amount of positive charge on a proton equals the amount of negative charge on an electron. * An atom contains equal numbers of protons and electrons, so the positive and negative charges cancel out and an atom has no net electric charge. * Objects with no net charge are said to be electrically neutral. * Electrons are bound more tightly to some atoms and molecules. * Static Electricity: The accumulation of excess electric charge on an object * Law of Conservation of Charge: charge can be transferred from object to object, but it cannot be created or destroyed. * Positive and negative charges exert forces on each other. * Just as for two electric charges, the force between any two objects that are electrically charged decreases as the objects get farther apart. * This force also depends on the amount of charge on each object. * Surrounding every electric charge is an electric field that exerts forces on other electric charges. * Any charge that is placed in an electric field will be pushed or pulled by the field. * All atoms are held together by electric forces between protons and electrons that are tremendously larger than the gravitational forces between the same particles.
Conductors and Insulators * Conductor: A material in which electrons are able to move easily * The best electrical conductors are metals. * Insulator: A material in which electrons are not able to move easily * Most plastics are insulators * The plastic coating around wires is an insulator. A damaged electrical cord is hazardous when the conducting wire is exposed. * Rubbing two materials together can result in a transfer of electrons. * Charging By Contact: The process of transferring charge by touching or rubbing * Because electrical forces act at a distance, charged objects brought near a neutral object will cause electrons to rearrange their positions on the neutral object. * Charging By Induction: The rearrangement of electrons on a neutral object caused by a nearby charged object * Lightning is a large static discharge. * A static discharge is a transfer of charge between two objects because of a buildup of static electricity. * The sensitive electronics in a computer can be harmed by large static discharges. * Connecting an object to Earth with a conductor is called grounding.
Detecting Electric Charge * The presence of electric charges can be detected by an electroscope.

Section 2: Electric Current

Current and Voltage Differences * When a spark jumps between your hand and a metal doorknob, electric charges move quickly from one place to another. * Electric Current: The net movement of electric charges in a single direction * In a metal wire, or any material, electrons are in constant motion in all directions. * Electric current is measured in amperes. * The movement of an electron in an electric current is similar to a ball bouncing down a flight of stairs. * Voltage Difference: related to the force that causes electric charges to flow. * Voltage difference is measured in volts. * Water or electric current will flow continually only through a closed loop. If any part of the loop is broken or disconnected, the flow stops. * Circuit: A closed path that electric current follows
Batteries * Chemical reactions in batteries produce a voltage difference between the positive and negative terminals. * A battery can provide the voltage difference that is needed to keep current flowing in a circuit. * A cell consists of two electrodes surrounded by a material called an electrolyte. * The electrolyte enables charges to move from one electrode to the other. * The electrolyte is a moist paste containing several chemicals. * The cell is called a dry cell because the electrolyte is a moist paste, and not a liquid solution. * When the two terminals of a dry-cell battery are connected in a circuit, such as in a flashlight, a reaction involving zinc and several chemicals in the paste occurs. * A wet cell contains two connected plates made of different metals or metallic compounds in a conducting solution. * A wet-cell battery contains several wet cells connected together. * Most car batteries are lead-acid batteries. * A lead-acid battery contains a series of six wet cells made up of lead and lead dioxide plates in a sulfuric acid solution. * A voltage difference is provided at electrical outlets, such as a wall socket.
Resistance: the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light. * Flashlights use dry-cell batteries to provide the electric current that lights a lightbulb. * Electric current loses energy as it moves through the filament because the filament resists the flow of electrons. * Electrical conductors have much less resistance than insulators. * Resistance is measured in ohms (Ω). * Copper is an excellent conductor and has low resistance to the flow of electrons. * The electric resistance of most materials usually increases as the temperature of the material increases.
The Current in a Simple Circuit * A simple electric circuit contains a source of voltage difference, such as a battery, a device, such as lightbulb, that has resistance, and conductors that connect the device to the battery terminals. * The amount of current flowing through a circuit is related to the amount of resistance in the circuit. * The voltage difference, current, and resistance in a circuit are related. * Ohm’s Law: the current in a circuit equals the voltage difference divided by the resistance. * Ohm’s law provides a way to measure the resistance of objects and materials. * An object is connected to a source of voltage difference and the current flowing in the circuit is measured. The object’s resistance then equals the voltage difference divided by the measured current.

Section 3: Electrical Energy

Series and Parallel Circuits * Circuits usually include three components. * One is a source of voltage difference that can be provided by a battery or an electrical outlet. * Another is one or more devices that use electrical energy. * Circuits also include conductors such as wires that connect the devices to the source of voltage difference to form a closed path. * Series Circuit: Current has only one loop to flow through * Series circuits are used in flashlights and some holiday lights. * When any part of a series circuit is disconnected, no current flows through the circuit. This is called an open circuit. * Parallel Circuits: contain two or more branches for current to move through. * Parallel circuits have several advantages. * When one branch of the circuit is opened, such as when you turn a light off, the current continues to flow through the other branches.
Household Circuits * The wiring in a house must allow for the individual use of various appliances and fixtures. * Parallel circuits branch out from the breaker or fuse box to wall sockets, major appliances, and lights. * In a house, many appliances draw current from the same circuit. * When you hear that somebody has “blown a fuse,” it means that the person has lost his or her temper. This expression comes from the function of an electrical fuse which contains a small piece of metal that melts if the current becomes too high. * A circuit breaker is another device that prevents a circuit from overheating and causing a fire.
Electric Power: The rate at which electrical energy is converted to another form of energy * The reason that electricity is so useful is that electrical energy is converted easily to other types of energy. * Appliances that have electric heating elements, such as ovens and hair dryers, usually use more electric power than other appliances. * The electric power used depends on the voltage difference and the current. * The unit for power is the watt (W). * The cost of using the appliance can be computed by multiplying the electrical energy used by the amount the power company charges for each kWh.