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Chapter 20: Electricity. Jennie Borders. Warm-Up Jan. 8. What particle is responsible for electricity? What is the difference in alternating and direct current? How is a resistor drawn in a circuit diagram?. Section 20.1 – Electric Charge and Static Electricity.
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Chapter 20: Electricity Jennie Borders
Warm-Up Jan. 8 What particle is responsible for electricity? What is the difference in alternating and direct current? How is a resistor drawn in a circuit diagram?
Section 20.1 – Electric Charge and Static Electricity • Electric charge is a property that causes subatomic particles such as protons and electrons to attract or repel each other. • The SI unit of electric charge is the coulomb (C). • Protons have a positive charge and electrons have a negative charge.
The Atom • A cloud of negatively charged electrons surrounds the positively charged nucleus. • The atom is neutral because it has an equal number of positive and negative charges. • If an atom gains electrons, it becomes a negatively charged ion. • If an atom loses electrons, it becomes a positively charged ion.
Electric Forces • Like charges repel, and opposite charges attract. • The force of attraction or repulsion between electrically charged objects is electric force. • 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.
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 charges.
Electric Field • The more net charge an object has, the greater is the force on it. • The direction of each field line shows the direction of the force on a positive charge.
Static Electricity • Static electricity is the study of the behavior of electric charges, including how charge is transferred between objects. • The law of conservation of charge states that the total charge in an isolated system is constant. • Charge can be transferred by friction, contact, and induction.
Charging by Friction • Rubbing a balloon on your hair is an example of charging by friction. • Electrons move from your hair to the balloon because atoms in rubber have a greater attraction for electrons than atoms in your hair.
Charging by Contact • When a girl touches the Van de Graaff generator sphere, she acquires a charge large enough to make her hairs stand on end.
Charging by Induction • You pick up extra electrons when you walk across a carpet, so your hand is negatively charged. • The net negative charge in your hand repels the electrons in the metal doorknob. • Overall, the doorknob is still neutral, but charge has moved within it.
Charging by Induction • A transfer of charge without contact between materials is induction.
Static Discharge • The spark you feel when touching a doorknob is static discharge. • Static discharge occurs when a pathway through which charges can move forms suddenly. • Lightning is static discharge that occurs because charge can build up in a storm cloud from friction between moving air masses.
Section 20.1 Assessment • How is a net electric charge produced? • What determines whether charges attract of repel? • Name two factors that affect the strength of an electric field. • List three methods of charge transfer. • Explain how static discharge occurs.
Section 20.1 Assessment • How does electric force depend on the amount of charge and the distance between charges? • What is the law of conservation of charge? • When a glass rod is rubbed with neutral silk, the glass becomes positively charges. What charge does the silk now have?
Warm-Up Jan. 9 What is the law of conservation of charge? What charge does a proton have? Draw the electric field lines for a positive ion.
Section 20.2 – Electric Current and Ohm’s Law • A continuous flow of electric charge is an electric current. • The SI unit of electric current is the ampere (A), or amp, which equals 1 coulomb per second. • The two types of current are direct current and alternating current.
Direct Current • Charge flows only flows in one direction in direct current (DC). • A flashlight and most battery-operated devices use direct current.
Alternating Current • Alternating current (AC) is a flow of electric charge that regularly reverses its direction.
Current Flow • Electrons flow from the negative terminal of a battery to the positive terminal of a battery. • The current is in the opposite direction because current is the direction in which positive charges would flow.
Conductors and Insulators • An electrical conductor is a material through which charge can flow easily. Examples include copper, silver, and most metals. • A material through which charge cannot flow easily is called an electrical insulator. Examples include wood, plastic, rubber, and air.
Metal Conductors • Metals tend to be electrical conductors because they are made up of an ion lattice. • The positive metal ions cannot move, but the electrons can move. • This mobile electron lattice is known as the electron sea model.
Resistance • As electrons move through a wire, they collide with other particles which converts some kinetic energy into thermal energy. • Resistance is opposition to the flow of charges in a material. • The SI unit for resistance is the ohm (W).
Resistance • A material’s thickness, length, and temperature affect its resistance. • Resistance is greater in a longer wire because the charges move farther. • As temperature increases, a metal’s resistance increases because electrons collide more often.
Superconductor • A superconductor is a material that has almost zero resistance when it is cooled to low temperatures.
Voltage • 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.
Potential Difference • 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 (V). • Potential difference is also called voltage. • Charges flow from higher to lower potential energy.
Voltage Sources • Three common voltage sources are batteries, solar cells, and generators. • A battery is a device that converts chemical energy to electrical energy.
Ohm’s Law • According to Ohm’s law, the voltage (V) in a circuit equals the product of the current (I) and the resistance (R). V = I x R V = voltage (volts) I = current (amps) R = resistance (ohms)
Section 20.2 Assessment • List the two types of current. • Name two good electrical conductors and two good electrical insulators. • What variables affect the resistance of a material? • What causes charge to flow? • According to Ohm’s law, how is voltage related to resistance and current?
Section 20.2 Assessment • Suppose you have two wires of equal length made from the same material. How is it possible for the wires to have different resistances? • Use Ohm’s law to explain how two circuits could have the same current but different resistances.
Section 20.3 – Electric Circuits • An electric circuit is a complete path through which 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 the electrical energy.
Circuit Diagrams • Switches show places where the circuit can be opened. • If the switch is open, the circuit is not a complete loop, and the current stops. This is called an open circuit. • When the switch is closed, the circuit is complete and charge can flow. This is called a closed circuit.
Circuit Diagrams • The + and – on the battery symbol indicate the positive and negative terminals.
Series Circuit • In a series circuit, 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. • The more bulbs you have, the less brightly they shine.
Parallel Circuit • 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 can still operate.
Electric Power • 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 (W). • Electric power can be calculated by multiplying voltage by current.
Electric Power P = I x V P = power (W) I = current (A) V = voltage (V)
Sample Problem • An electric oven is connected to a 240 volt line, and it uses 34 amps of current. What is the power used by the oven? P = I x V P = I x V P = ? P = 34A x 240V I = 34A P = 8200W V = 240V
Practice Problems • A clothes dryer uses about 27 amps of current from a 240 volt line. How much power does it use? • A camcorder has a power rating of 2.3 watts. If the output voltage from its battery is 7.2 volts, what current does it use? P = I x V P = 27A x 240V P = 6500W P = I x V I = P/V I = 2.3W/7.2V I = 0.32A
Practice Problems • A power tool uses about 12 amps of current and has a power rating of 1440 watts. What voltage does the tool require? P = I x V V = P/I V = 1440W/12A V = 120V
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.
Electrical Safety • The transfer of excess charge through a conductor to Earth is called grounding.
Section 20.3 Assessment • Name two elements included in a circuit diagram. • What is the difference between a series circuit and a parallel circuit? • Write the equation for calculating electric power. • Name five safety devices used with electric current. • You plug in a string of holiday lights and notice that the entire string turns off when you remove one bulb. Explain why this happens.
