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.

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