Section 1: Electric Charge
- Positive and Negative Charge
* Atoms contain particles called protons, neutrons, and electrons.
* 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
* 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.
* The force between any two objects that are electrically charged decreases as the objects get farther apart.
* Surrounding every electric charge is an electric field that exerts forces on other electric charges.
* Many of the forces that act on objects are due to the electric forces between atoms and molecules
* The frictional force between two surfaces in contact is due to the attractive electric forces between the atoms on the two surfaces. - Conductors and Insulators
* As you walk across a carpeted floor, excess electrons can accumulate on your body. When you reach for a metal doorknob, electrons flow from your hand to the doorknob and you see a spark.
* 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
* The plastic coating around wires is an insulator. A damaged electrical cord is hazardous when the conducting wire is exposed - Charging Objects
* 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.
* A thundercloud is a mighty generator of static electricity.
* The electrical energy in a lightning bolt rips electrons off atoms in the air and produces great amounts of thermal energy.
* The sensitive electronics in a computer can be harmed by large static discharges.
* Earth is a large, neutral object that is also a conductor of charge.
* Connecting an object to Earth with a conductor is called grounding.
* A lightning rod directs the charge from a lightning bolt safely to the ground. - Detecting Electric Charge
* The presence of electric charges can be detected by an electroscope.
* If a glass rod is rubbed with silk, electrons move away from the atoms in the glass rod and build up on the silk.
Section 2: Electric Current
- Current and Voltage Difference
* When a spark jumps between your hand and a metal door- knob, electric charges move quickly from one place to another.
* Electric Current: The net movement of electric charges in a single direction
* Electric current is measured in amperes.
* In a metal wire, or any material, electrons are in constant motion in all directions.
* The movement of an electron in an electric current is similar to a ball bouncing down a flight of stairs.
* Electric forces in a material cause electric current to flow.
* 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
* 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.
* Chemical reactions in batteries produce a voltage difference between the positive and negative terminals.
* The cell is called a dry cell because the electrolyte is a moist paste, not a liquid solution.
* 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. - Resistance: the tendency of a material to oppose the flow of electrons, changing electrical energy into thermal energy and light.
* As electrons move through the filament in a lightbulb, they bump into metal atoms. Due to the collisions, the metal heats up and starts to glow.
* 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 that has resistance, such as a lightbulb, 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.
* Ohm’s Law: the current in a circuit equals the voltage difference divided by the resistance.
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.
* To use electrical energy, a complete circuit must be made.
* Series Circuit: the current has only one loop to flow through
* A series circuit provides only one path for the current to follow.
* 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 Circuit: contain two or more branches for current to move through.
* In parallel circuits, the current follows more than one path.
* The current can flow through both or either of the branches.
* Houses, automobiles, and most electrical systems use parallel wiring so individual parts can be turned off without affecting the entire circuit. - Household Circuits
* The wiring in a house must allow for the individual use of various appliances and fixtures.
* In a house, many appliances draw current from the same circuit.
* To protect against overheating of the wires, all household circuits contain either a fuse or a circuit breaker.
* Too many appliances in use at the same time is the most likely cause of the overheating of the circuit.
* A circuit breaker contains a piece of metal that bends when the current in it is so large that it gets hot. The bending causes a switch to flip and open the circuit, stopping the flow of current. - Electrical Power: The rate at which electrical energy is converted to another form of energy
* The reason why electricity is so useful is that electrical energy is converted easily to other types of energy.
* The electrical power used depends on the voltage difference and the current.
* Electrical Power Equation: electrical power (in watts) = current (in amperes) X voltage difference (in volts)
* The cost of using an appliance can be computed by multiplying the electrical energy used by the amount the power company charges for each kWh.