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.