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Physical Science: Chapter 20. Electricity GA Performance Standards SPS10. Students will investigate the properties of electricity and magnetism. a. Investigate static electricity in terms of Friction Induction conduction b. Explain the flow of electrons in terms of Alternating and direct current The relationship among voltage, resistance, and current Simple series and parallel circuits 20.1 Electric Charge and Static Electricity A. Electric Charge Electrical energy is the energy associated with electric charges Electric charge is a property that causes subatomic particles such as protons and electrons to attract or repel each other Protons have a positive charge and electrons have a negative charge and electrons have a negative charge Electric charges attract one another in clothes taken from the dryer An atom has a cloud of negatively charged electrons surrounding the positively charged nucleus. The atom is neutral sense it has equal numbers of protons and neutrons If an atom loose an electron it becomes positively charged An excess or shortage of electrons produces a net electric charge SI unit of electric charge is the coulomb [C] and it takes about 6.24 x 1018 electrons to produce a single coulomb B. Electric Forces Like charges repel, and opposite charges attract. Electric force is the force of attraction or repulsion between electrically charged objects Charles-Augustin de Coulomb discovered that electric force between two objects is proportional to the net charge on each object, and inversely proportional to the square of the distance between them Example: doubling the charge on one object doubles the electric force Example: doubling the distance between the objects the electric force is one fourth as strong Inside an atom electric forces are stronger then gravitational force, but on large scale matter is mostly neutral and electric forces are close to zero. C. Electric Fields Electric field is a field in a region of space that exerts electric forces on charged particles the electric field is produce by electric charges or by changes magnetic fields the strength of an electric field depends on the amount of charge that produces the field and on the distance from the field. The more net charge an object has the greater is the force on it The electric field around a positive charge points outward The electric field around a negative charge points inward D. Static Electricity and Charging Two types of electricity: Static Electricity and Electric Circuit Static electricity = sock sticking on your pants. Electric circuit= wall outlet. Static electricity is the study of the behavior of electric charges, including how charg is transferred between objects. Charge can be transferred by friction, by contact, and by induction The law of conservation of charge states that the total charge in an isolated system is constant When a charge is transferred the total charge is the same before and after the transfer occurs 1. Charging by friction o Rubbing a balloon on your hair is an example of charging by friction o Electrons move from your hair to the balloon because atoms in rubber have a greater attraction for elections o The balloon picks up a net negative charge and your hair becomes positively charged 2. Charging by Contact o A Van de Graff generator has charged a metal sphere, when you touch the sphere you can acquire enough charge to make your hair stand on end o The sphere is still charged but its net charge is reduced. 3. Charging by induction o Induction is the transfer of charge without contact between materials o Example: when a positive charged object is placed near a neutral object the electrons move closer to the charged object. The net charge is still neutral, but charge has moved within it E. Static Discharge Static discharge occurs when a pathway through which charges can move forms suddenly Air can become charged suddenly when the gap between your fingers and a doorknob becomes very small Lighting is an example of discharge Charge can build up in storm clouds from friction between moving air masses Negative charge in lower part of the cloud induces a positive charge in the ground. Eventually the air becomes charged forming a pathway for electrons to travel from the cloud to the ground 20.2 Electric Current and Ohm’s Law A. Electric Current Electric current is the continuous flow of electric charge The SI unit of electric current is the ampere (A). 1 A = 1 C/s two types of current are direct current and alternating current direct current (DC) is where the charge flows only in one direction Alternating current (AC) is where the flow of electric charge regularly reverses its direction. Scientists define current as the direction in which positive charges would flow B. Conductors and Insulators Electrical conductor is a material though which charge can flow easily Electrical insulator is a mater through which charge cannot flow easily Metals such as copper and silver are good electrical conductors. Wood, plastic, rubbers are good electrical insulators. C. Resistance Resistance is opposition to the flow of charges in a material The SI unit of resistance is the ohm A material’s thickness, length, and temperature affect its resistance Resistance is greater in a longer wire because the charges have to travel farther As temperature increases the resistance increases because electrons collide more often As the thickness increases the resistance decrease because the electrons can flow more easily Superconductor is a material that has almost zero resistance when it is cooled to low temperatures D. 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 1. Potential difference Charges flow spontaneously from a higher to lower potential energy Potential energy of a charge depends on its position in an electric field Potential difference is the difference in electrical potential energy between two places in an electric field Potential difference is measured in joules/coulomb and equals 1 volt Potential difference is commonly called voltage 2. Voltage sources Three common voltage sources are batteries, solar cells and generators Battery is a device that converts chemical energy to electrical energy Voltage sources have terminals that connect to wires in a circuit One thermal is positive and the other is negative The voltage drop (potential difference) is maintained across the terminals Example 9-volt battery has potential difference of 9 volts E. Ohm’s Law The unit of resistance it the ohm The voltage is not the same everywhere in a circuit. George Ohm hypothesized that resistance reduces the voltage Ohm’s Law is that the voltage (V) in a circuit equals the product of the current(I) and the resistance (R) V=I x R Current is measured in amperes, resistance in ohms, and voltage in volts Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current 20.3 Electric Circuits A. Circuit Diagrams Electric circuit is a complete path through which charge can flow. Electrician uses circuit diagrams to keep track of how elements in a circuit are connected Circuit diagrams use symbols to represent part of a circuit, including a source of electrical energy and devices that are run by the electrical energy A circuit diagram shows on or more complete paths in which charge can flow Switches show places where the circuit can be opened If a switch is open the circuit is not complete loop and the current stops If the switch is closed the circuit is complete and charge can flow (called a closed circuit) The + and – on the battery symbol indicate the positive and negative terminals Arrows show the direction of current, from positive to negative Direction of current is defined as the direction in which positive charges would flow B. Series Circuits Series circuit is where charge has only one path through which it can flow If one elements stops functioning in a series circuit, non of the elements can operate Adding bulbs to a series circuit increases the resistance and decreases current, resulting in each bulb shining less brightly C. Parallel Circuits 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 can operate. D. Power and Energy Calculations Power is the rate of doing work Electric power is the rate at which electrical energy is converted to another from of energy Uinit of electric power is Joule/second or watt (W) Electric power can be calculated by multiplying voltage by current Power (in watts) = I (in amps) x V (in volts) To find electrical energy used by an appliance multiply power by time Energy = Power x Time E. Electrical Safety Correct wiring, fuses, circuit breakers, insulation, and grounded plugs help make electrical energy safe to use.’ 1. Home Safety o Fuse prevents current overload in a circuit o A wire in the center of the fuse melts if too much current passes through it o The fuse must be replaced before the circuit can carry a current again o Circuit breaker is a switch that opens when current in a circuit is too high o The circuit breaker must be reset before the circuit can be used again 2. Personal Safety o Insulation protect people and if it is damaged you may accidentally touch the bare wire and get a shock o Insulation also prevent short circuits o Grounding is the transfer of excess charge through a conductor to Earth o
"Physical Science Chapter 20"