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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.