Chapter 20 Electricity

20.1 Electric Charge and Static ElectricityElectricity 20.1 Electric Charge and Static Electricity

Electricity Electric Charge  Electric energy is the energy associated with electric charges.  Electric charge is a property that causes subatomic articles to attract or repel each other.  The atom is neutral because it has an equal number of protons and electrons.

20.1 Electric Charge and Static ElectricityElectricity  An excess or shortage of electrons produces a net electric charge.  The SI unit of electric charge is the coulomb (C) which equivalent to 6.24×10 18 electrons.

20.1 Electric Charge and Static ElectricityElectricity Electric Forces  Like charges repel and opposite charges attract.  The force of attraction or repulsion between electrically charged objects is electric force.

20.1 Electric Charge and Static ElectricityElectricity  Coulomb discovered that electric forces obey a law similar to the law of universal gravitation.  The electric force between two objects is directly proportional to the net charge on each object and inversely proportional to the square of the distance between them.

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Electric Fields  The effect an electric charge has on other charges in the space around it is the charge’s electric field.  The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.

20.1 Electric Charge and Static ElectricityElectricity  In figure 4, the lines representing the field are closer together near the charge, where the field is stronger.  An electric field exerts forces on any charged object placed in the field.  The force depends on the net charge in the object and on the strength and direction of the field at the object’s position.

20.1 Electric Charge and Static ElectricityElectricity Fields of Positive and Negative Charges

20.1 Electric Charge and Static ElectricityElectricity Static Electricity and Charging  Static electricity is the study of the behavior of the electric charges, including how charge is transferred between objects.  Charge can be transferred by friction, by contact, and by induction.

20.1 Electric Charge and Static ElectricityElectricity  Whenever there is a charge transfer, the total charge is the same before and after the transfer occurs.  The law of conservation of charge says the total charge in an isolated system is constant.

20.1 Electric Charge and Static ElectricityElectricity Charging by Friction  Rubbing a balloon through hair is an example of charging by friction.  Electrons move from the hair to the balloon because atoms in rubber have a greater attraction for electrons than atoms in hair.

20.1 Electric Charge and Static ElectricityElectricityInduction  Induction occurs when charge is transferred without contact between materials.

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20.1 Electric Charge and Static ElectricityElectricity Static Discharge  Static discharge occurs when a pathway through which charges can move forms suddenly.  Lightning is an example of static discharge.

20.2 Electric Current and Ohm’s LawElectricity 20.2 Electric Current and Ohm’s Law

Electricity Electric Current  The continuous flow of electric charge is an electric current.  The SI unit of electric current is the ampere (A), or amp, which is equal to 1 coulomb per second.  In direct current charge flows only in one direction.

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20.2 Electric Current and Ohm’s LawElectricity  In alternating current the flow of electric charge regularly reverses its direction.

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20.2 Electric Current and Ohm’s LawElectricity  In direct current, electrons flow from the negative terminal of a battery to the positive terminal.

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20.2 Electric Current and Ohm’s LawElectricity  However, scientists define current as the direction in which positive charges would flow.

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20.2 Electric Current and Ohm’s LawElectricity Conductor and Insulators  An electrical conductor is a material through which charge can flow easily.  Material through which charge cannot flow easily is called an electrical insulator.

20.2 Electric Current and Ohm’s LawElectricityResistance  Resistance is opposition to the flow of charges in a material.  The SI unit of resistance is the ohm (Ω).  A material’s thickness, length, and temperature affect its resistance.

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20.2 Electric Current and Ohm’s LawElectricity  Resistance is lowered in a thicker wire because more electrons can flow.  Resistance is more in a longer wire because charges travel farther.  As temperature increases, resistance increases because electrons collide more often.

20.2 Electric Current and Ohm’s LawElectricity  A superconductor is a material that has almost zero resistance when it is cooled to low temperatures.  The best superconductor yet found must be cooled to about 138 K.

20.2 Electric Current and Ohm’s LawElectricityVoltage  In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy.

20.2 Electric Current and Ohm’s LawElectricity Potential Difference  Charges flow spontaneously from a higher to a lower potential energy.  The potential energy of a charge depends on its position in an electric field.

20.2 Electric Current and Ohm’s LawElectricity  Potential difference is the difference in electrical potential energy between two places in an electric field.  Potential difference is measured in joules per coulomb, or volts.  Potential difference is also called voltage.

20.2 Electric Current and Ohm’s LawElectricity Voltage Sources  A source of voltage such as a battery does work to increase the potential energy of electric charges.  Three common voltage sources are batteries, solar cells, and generators.  A battery is a device that converts chemical energy to electrical energy.

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20.2 Electric Current and Ohm’s LawElectricity Ohm’s Law  The unit of resistance is the ohm.  Ohm discovered that voltage is not the same everywhere in a circuit and hypothesized that resistance reduces the voltage.  He found a mathematical relationship between voltage, current, and resistance.

20.2 Electric Current and Ohm’s LawElectricity  Ohm’s law states the voltage in a circuit equals the product of the current and the resistance, or V=IR.  When the current is in amps and resistance is in ohms, the voltage is in volts.  Increasing the voltage increases the current.  Keeping the same voltage and increasing the resistance decreases the current.

20.3 Electric CircuitsElectricity 20.3 Electric Circuits

Electricity Circuit Diagrams  An electric circuit is a complete path through which a charge can flow.  Circuit diagrams use symbols to represent parts of a circuit, including a source of electrical energy and devices that are run by electrical energy.  See figure 12.

20.3 Electric CircuitsElectricity Series Circuits  In a series circuit, the charge has only one path through which it can flow.  If one element stops functioning in a series circuit, none of the elements can operate.  Adding bulbs to a series circuit increases the resistance, decreases the current, and each bulb shines less brightly.

20.3 Electric CircuitsElectricity Series Circuit

20.3 Electric CircuitsElectricity Parallel Circuits  A parallel circuit is an electric circuit with two or more paths through which charges can flow.  If one element stops functioning in a parallel circuit, the rest of the elements still operate.

20.3 Electric CircuitsElectricity Parallel Circuit

20.3 Electric CircuitsElectricity Power and Energy Calculation  Power is the rate of doing work.  The rate at which electrical energy is converted to another form of energy is electric power.  The unit of electric power is the joule per second, or watt.

20.3 Electric CircuitsElectricity  Power is often measured in kilowatts (kW).  Electric power can be calculated by multiplying voltage by current (P=IV).  To find the electrical energy used by an appliance, multiply power by time (E=Pt).  Unit is kilowatt hours (kWh).

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20.3 Electric CircuitsElectricity Electrical Safety  Correct wiring, fuses, circuit breakers, insulation, and grounded plugs help make electrical energy safe to use.  A fuse prevents current overload in a circuit.  A circuit breaker is a switch that opens when current in a circuit is too high.  The circuit breaker must be reset before the circuit can be used again.

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20.4 Electronic DevicesElectricity 20.4 Electronic Devices

Electricity Electronic Signals  The science of using electric current to process or transmit information is electronics.  An electronic signal is information sent as patterns in the controlled flow of electrons through a circuit.  Electronics conveys information with electrical patterns called analog and digital signals.

20.4 Electronic DevicesElectricity Analog Signals  An analog signal is a smoothly varying signal produced by continuously changing the voltage or current in a circuit.

20.4 Electronic DevicesElectricity  Analog signal  changes continuously,  has many voltage values,  resembles the pattern of the original signal,  and is easily distorted.

20.4 Electronic DevicesElectricity Digital Signals  A digital signal encodes information as s string of 1’s and 0’s.  Digital signals are more reliable than analog signals.

20.4 Electronic DevicesElectricity  Digital signal  changes abruptly,  has only two voltage values,  does not resemble the original signal,  and is not easily distorted.  A DVD encodes digital signals as a series of pits on the DVD surface.

20.4 Electronic DevicesElectricity

20.4 Electronic DevicesElectricity Vacuum Tubes  To create an electronic signal, the flow of electrons must be controlled.  A vacuum tube was used to control electron flow in early electronic devices.

20.4 Electronic DevicesElectricity  Vacuum tubes can  change alternating current into direct current,  increase the strength of a signal, or  turn a current on or off.

20.4 Electronic DevicesElectricity

20.4 Electronic DevicesElectricitySemiconductors  A semiconductor is a crystalline solid that conducts current only under certain conditions.  Most semiconductors are made with silicon or germanium.

20.4 Electronic DevicesElectricity  There are two types of semiconductors  In n-type semiconductors, the current is a flow of electrons.  In p-type semiconductors, it appears as though positive charge flows.

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20.4 Electronic DevicesElectricity Solid-State Components  Most modern electronic devices are controlled by solid-state components.

20.4 Electronic DevicesElectricityDiodes  A diode is a solid-state component that combines an n-type and p-type semiconductors.  Because the current can be in only one direction, a diode can change alternating current to direct current.

20.4 Electronic DevicesElectricityTransistors  A transistor is a solid-state component with three layers of semiconductors.  A transistor can be used as a switch because the small current can turn current on or off.  Transistors can also be used as amplifiers.  Small voltage on one side produces large voltage on the other side.

20.4 Electronic DevicesElectricity Integrated Circuits  An integrated circuit is a thin slice of silicon that contains many solid-state components.  Integrated circuits are also called chips or microchips.  Integrated circuits are fast compared to vacuum tubes.  The current does not have to travel far to get from point to point in the circuit.

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