Electricity and Energy Resources Unit Notes Chapter 7 Electricity
I. Electric Charge
a. Positive and Negative Charge—When an atom is electrically neutral, the number of protons equal the number of electrons. Positive charge equals negative charge.
i. Transferring Charge—electrons are bound more tightly to some objects than to others. For example, electrons in carpet are bound less tightly to their protons than the electrons on the soles of your shoes. If conditions are right, when you rub your soles across the carpet, the electrons from the carpet will rub off and accumulate on the soles of your shoes. Static electricity is when excess charge accumulates on an object.
ii. Conservation of Charge—Charge cannot be created nor destroyed (electrons are matter). The Law of Conservation of Charge says that charge cannot be created nor destroyed and can be transferred.
iii. Charges Exert Forces—Clinging clothes are proof that charges can exert force on each other. Opposite charges attract each other. Like charges repel each other. The force between two objects that are electrically charged depends on the amount of charge on each object. The force between electric charges depends on the distance between the charges (like gravity).
iv. Electric Fields—An electric field surrounds every electric charge and exerts the force that cause other electric charge to be attracted or repelled. Any charge placed in an electric field will experience forces of push or pull. Positive fields are usually represented by outward pointing arrows from the positive object. Negatively charged sources are represented by inwardly pointing arrows. v. Comparing Electric and Gravitational Forces—the electric force between the
proton and electron in a hydrogen atom is 1039 times stronger than the
gravitational forces between the two particles. This is the same for chemical bonds.
1. Most objects around you are however, electrically neutral. Therefore the electric forces between two large neutral objects are nonexistent. If a small amount of charge is transferred between them, then the electric forces will be more noticeable. Transferring one-trillionth of the
electrons in a hair to a comb is enough to overcome gravity and pull the hair toward the comb.
i. Conductors—a material in which electrons are able to move easily. ii. Insulators—a material in which electrons are not able to move easily. c. Charging Objects—transferring charge by rubbing or touching is called charging by
contact.
i. Charging at a Distance—The rearrangement of electrons on a neutral object caused by a nearby charge object is called charging by induction. Ex. Balloons sticking to a sweater.
ii. Lightning—When enough charge builds up in a storm cloud a static discharge will occur between oppositely charged objects (cloud to ground or cloud to cloud). As the electric charges move through air, they collide with atoms and molecules. These collisions cause the atoms and molecules in air to emit light. iii. Grounding—by providing a path to the Earth for charges to follow, you can
avoid damaging electric spark. The Earth is a large, neutral object that is also a conductor of charge. A wire that connects an object to the Earth is called a ground because it provides a path for electrons to follow to the “ground”. d. Detecting Electric Charge—Electroscopes can detect electric charge. When uncharged,
the leaves hang down. When charged, they repel each other.
Self Check p. 199 Answers: 1) Static electricity is the accumulation of excess electric charge on an object. 2) Lightning occurs when excess negative charge in clouds discharges to regions of positive charge on the ground or in other clouds. 3) Electrically neutral objects can become charged when they gain or lose electric charges. 4) The leaves of the electroscope would remain apart. 5) Excess electric charge remains on an object in dry air, which is an insulator. Humid air is a better conductor and allows any excess charge to drain away into the air. 7) Use F=ma, 0.20 Newtons.
II. Electric Current
a. Current and Voltage Difference—The net movement of electrical charge from one place to another is called an electric current. In a metal wire, electrical charge is in constant motion in all directions (no net motion). When an electrical current moves through a wire, the electrons drift in the direction of the current flow (in addition to any random movement that they undertake).
ii. Electric Circuits—flowing water can perform work…that is that the potential energy (GPE) of the water can be converted into another form of energy (KE) by pumping water up to the top of a large container. On its way down out of the container, the GPE can be converted to KE. The pump that pumped it up is similar to a battery. A battery provides the voltage difference that keeps electric current flowing just like the pump provides the energy to put the water with enough energy (KE) to get to a high height (GPE). Just like water won’t flow through a pipe that is closed. Electrons will not flow through a wire that is not connected to another wire. When a closed path is available for electrons to flow the closed path is called a circuit.
b. Batteries—Batteries provide the voltage difference required for electrons to flow through a circuit. Current flows as long as there is a closed path that connect one battery terminal to the other battery terminal.
i. Dry-Cell Batteries—A cell consists of two electrodes surrounded by a material called an electrolyte (!). Electrolytes allow electrons to flow from one electrode to another. (Carbon rod and Zinc container)
ii. Wet-Cell Batteries—two connect plates made of different metals are surrounded by a conducting solution (electrolyte). Several wet cells in a car battery are connected in series.
iii. Lead-Acid Batteries—A series of Lead and Lead dioxide plates in a sulfuric acid solution. Each 2V cell provides a total of 12V to the car. As the car is driven, the alternator recharges the battery by sending current through the battery in the opposite direction to reverse the chemical reaction. The voltage different in a household plug is 120V.
c. Resistance—as electrons flow through a thin filament of metal, they collide with each other and their KE is converted to thermal energy. The thermal energy is high enough to cause the filament to glow.
i. Resisting the Flow of Current—the electrons lose energy as they move through the filament. Resistance is the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light. Almost all materials resist the flow of energy (except some materials that become superconductors at low temperature). Electrical conductors have much less resistance than insulators. Resistance is measure in the unit called an ohm. ii. Temperature, Length, and Thickness—resistance usually increases as the
d. The Current in a Simple Circuit—a simple circuit contains a source of voltage difference, such as a battery, a device (lightbulb), and wires to connect the battery and device.
i. Voltage difference, current, and resistance are related to each other in a circuit. If the voltage is constant (battery), decreasing the resistance will increase the current that can go through the circuit.
ii. Ohm’s law—the relationship between voltage, resistance and current is called Ohm’s Law V = I * R
Current (in amperes) = voltage (in volts)/ resistance (in ohms)
Self Check p. 205 Answers: 1) Both are a movement of electrons from the negative to the positive charge. A circuit has a continuous current provided by a voltage source. A static discharge is a very rapid, noncontinuous transfer of charge. 2) A chemical reaction causes a negative charge on the zinc container and a positive charge on the carbon rod. 3) Increasing the voltage difference or decreasing the
resistance will increase the current. 4) Water flow is the movement of molecules, not just electrons. The larger the diameter of pipe or wire, the greater is the flow of water or electrons. The higher the
pressure, the more water current can flow if the pipe is open. The higher the voltage, the more electrical current can flow if the circuit is complete (closed circuit). 5) A lightbulb filament heats up as an electric current flows through it, causing an increase in the wire’s resistance. According to Ohm’s Law, the higher resistance results in a lower current. 6) 12.5 volts. 7) 240 ohms.
III. Electrical Energy
a. Series and Parallel Circuits—circuits have three parts (source, consumer, conductors). Switch will open the circuit and turn off the current.
i. Series circuit—the current has only one loop to flow through. Series circuits are used in flashlights and some holiday lights.
ii. Open circuit—Because a series circuit is connected together in line, all of the current flows through one wire. If the wire is disconnected at any place (like at an unplugged light in a string of lights) the current cannot flow through the circuit. This is called an open circuit.
b. Household Circuits—Most household circuits have a 120V power source with several parallel branches. For big appliances, the circuits may be 240V. If too many appliances are attached to the same circuit, too much current will make the wires get too hot. If they get hot enough to melt the insulation, the bare wires could cause a fire.
c. Fuses—a small piece of metal will melt inside if the current gets too high. The fuse will break the circuit to avoid damage to your house or other electrical equipment.
d. Circuit Breaker—This works like a fuse except that instead of a temperature sensitive piece of metal melting, the temperature sensitive metal bends and flips a switch that can be easily reset.
IV. Electric Power—The rate at which electrical energy is converted to another form of energy is the electric power. The amount of power that a piece of electrical equipment uses, is labeled on the appliance.
a. Calculating electric power—P = I * V
Power (watts) = Current (in amperes) x Voltage (in volts)
Watts are a small unit of power, so the power is usually measured in kW
Electric Power Equation Practice problems p. 211 Answers: 1) 1,200 W or 1.2 kW. 2) 0.083A. 4) 3V 5) 10A
b. Electrical Energy—using electric power costs money. The electric companies charge on the amount of energy used, not power, so they use a measure called a kWh (kilowatt hour). E = P x t
Electrical energy (kWh) = Electric power (kW) * time (in hours)
c. The Cost of Using Electrical Energy—The electric company will have a price per kWh ($0.10/kWh).
Chapter 8: Magnetism and Its Uses I. Magnetism
a. Magnets--Minerals found in ancient Magnesia could attract iron. Chinese naval navigators used them in compasses.
i. Magnetic force—magnets exert force on each other. The magnetic force depends on distance. The closer they are the stronger the force.
ii. Magnetic field—a magnetic field exerts a force on other magnets and objects made of magnetic materials. Lines of force are use to designate a magnetic field. Magnetic fields have direction.
iii. Magnetic poles—are where the magnetic force exerted by the magnet is strongest (magnetic field lines are closest together at the poles).
iv. How Magnets Interact—You know that N attracts S (N & N repel, S & S repel). When two magnets are brought close to each other, their magnetic fields combine to produce a new magnetic field.
v. Magnetic Field direction—the compass needle rotates until it lines up with the magnetic field lines. The north pole of a compass points in the direction of the magnetic field. The direction of the field is always pointed away from the N toward the S.
vi. Earth’s Magnetic Field—The Earth’s magnetic core causes the entire planet to have a magnetic field. (Funny, the north pole is the S part of the magnet and the south pole is the N side of the magnet).
vii. Earth’s Magnetic Poles—I just said the main point in parenthesis above. Molten iron and nickel in the Earth’s out core produces Earth’s magnetic field…we think. b. Magnetic Materials—iron, cobalt, nickel can be made into permanent magnets. You can make a nail into a magnet. Iron has an unusual property that allows it to be magnetized.
i. Magnetic Domains—A Model for Magnetism In a magnetic domain, a large group of atoms have their magnetic fields aligned. This groups are called magnetic domains and they act like a small magnet within the piece of metal. ii. Lining Up Domains One iron nail has many domains. If the domains are
other temporarily and the nail become magnetized (until you take the magnet and nail away from each other).
iii. Permanent magnets If a piece of iron or other magnetic material is placed for a long time in a magnetic field, the random motion of atoms in the material will help the domains eventually line up permanently. When they add together, the magnetic properties of the material can be made very strong. Heating a magnet will make it lose its magnetic properties.
iv. Can a Pole be Isolated? No. The broken piece will have a N and S pole.
Self Check p. 230 Answers: 1) the poles attract each other 2) it aligns itself along the field lines 3) Only certain materials have magnetic properties that do not cancel out. The atoms that have the properties have clusters that have similar magnetic orientation. These domains can be aligned to form a temporary magnet. 4) The magnetic poles would still be aligned, but since each piece has fewer domains, the magnetic field of each would be weaker. The new pieces would have a N and S pole. 5) When the magnet is heated, the atoms in the magnet move faster. If the atoms are moving fast enough, collisions with other atoms may cause atoms in domains to lose their alignment. This would reduce the magnetic field. 6) If the metal in the door contains magnetic material, the magnetic domains align when the magnet is brought close so the opposite poles of the domains attract the closest pole of the magnet.
II. Electricity and Magnetism
a. Electric Current and Magnetism—electric current running in a wire cause a magnetic field around the wire that changes based on the direction of the current.
i. Moving charges and magnetic fields I said that above.
b. Electromagnets—My son used to call magnets…”dagmets”. I had a friend in Seattle named Dagmar. Any connection? An electromagnet is a temporary magnet made by wrapping a wire around a metal core (like a nail) and running an electric current through the wire. When the current flows, the magnetic fields in the wire add up and cause the metal core to become magnetized. (a coil of wire that create a magnetic field is called a solenoid).
i. Properties of Electromagnets—because the current effects the magnetic field of an electromagnet, we can make use of electromagnets by controlling the current. We use them in speakers, switches, and electric motors.
voltage in the speaker cone, this vibration vibrates the air molecules that finally reach your ear and vibrate your ear drums.
iii. Making an Electromagnet rotate—If you place an electromagnet in a magnetic field and turn on the current, the magnet will rotate to align its poles up with the magnetic field. If you change the direction of the current in the magnet, it will rotate again to realign with the poles…a motor does this.
iv. Galvanometers—gauges in your car are usually galvanometers. The magnet in a coil receives current from a source. The coil is in a magnetic field. As the current increases in the field, the electromagnet rotates in proportion to the current. c. Electric Motors—changes electrical energy into mechanical energy.
i. Simple electric motor—wire coil, a permanent magnet, electric current source. Conducting pads rotate around so that the current changes in the coil. As the current changes, the electromagnet aligns and realigns in the permanent magnet’s field.
ii. Making the Motor Spin—There are four steps that I briefly outlined in my notes above. A nice diagram can be seen on page 236.
Self Check p. 237 Answers: 1) The magnetic field around each bit of wire has the same strength, and when the wire is looped, the fields combine, making the field stronger. 2) by varying the electric current passing through it or by changing the number of loops. 3) The magnetic field would be weaker because aluminum is not a magnetic material. The only magnetic field would be the one created by the wires. 4) When an end of the coil moves past a pole of the permanent magnet, reversing the current cause the end of the coil to be attracted to the other pole of the permanent magnet. Repeating this process keeps the coil continually rotating in the same direction. 5) The bar magnet would have been attracted to the electromagnet because the pole would have been opposite to what it was.
III. Producing Electric Current
a. From Mechanical to Electrical Energy—When a magnet moves near a wire, a current results. If a magnet is moved through a loop of wire, the magnetic field of the magnet causes the electrons in the wire to flow. The mechanical energy associated with the motion of the wire loop or the magnet is converted into electrical energy. …this is called electromagnetic induction.
ii. Switching direction—After the wire coil makes a ½ rotation, the poles reverse. This causes the current to change direction. In a generator, as the coil keeps rotating, the induced current periodically changes direction. The frequency of this current change is regulated by the rotation rate of the generator. In the US it is 60 cycles per second.
iii. Using Electric Generators—an alternator in your car is a generator that uses induced current to recharge the battery in your car and to supply energy to the electrical devices in your car.
iv. Generating Electricity for Your Home—The rotating magnets in the power plant that move inside the wire coils are connected to turbines. Turbines are powered by any source of mechanical energy. In a dam, the mechanical energy is supplied by falling water. In coal fired/petroleum fired power plants, the burning coal, heats water, which boils, the resulting steam is pressurized and forced to turn the turbine.
b. Direct and Alternating Current—Current produced by a battery is different than current produced by a power station. Batteries produce direct current, the kind of current that only moves in one direction through a wire. Alternating current is the kind made by power plants in the USA. The current of this type changes direction at a 60 Hz frequency.
c. Transmitting Electrical Energy—Transmitting energy in power lines is wasteful. It causes heat that ends up lost to the environment. High voltages help this problem…and they are not safe for your home. So we have to use transformers.
d. Transformers—Transformers safely use coils and induction to safely transform electricity at very high voltage to electricity at very low voltage. The voltage in the primary coil is the input voltage…the voltage in the second coil is the output voltage. The output voltage divided by the input voltage = # turns in the secondary coil/# turn in the primary coil.
i. Step-Up transformer—The transformer increase the voltage so that the output is greater than the input voltage. The number of wire turns on the secondary coil is greater than the number of turns on the primary coil.
ii. Step-Down Transformer—The transformer decreases the voltage so that the output voltage is less than the input voltage. The number of wire turns on the secondary coil is less than the number of turns in the primary coil.
because they can be a cool toy and a robot, so if you want to play with the toy you can transform it into the toy. If you want to play with a robot, you can transform it into a robot.
Self Check p. 244 Answers: 1) Water at the top of the falls has GPE, it falls and starts having more KE, the KE powers a wheel that powers the shaft of the generator. The mechanical energy is converted to electrical energy when the magnet on the shaft spins. The coils generate electrical current that is carried away to wherever it needs to be used. 2) They are opposites. A generator converts mechanical energy into electrical energy. A motor turns electrical energy into mechanical energy. 3) only alternating current can induce voltage in a transformer. 4) Higher voltage means more efficient transfer of power. Power companies prefer to transmit power more efficiently at very high voltage. By using alternating current, power companies are able to use transformers to convert the voltage down to a safe level before coming into a house. Direct current would not allow such a step down. 5) Zero. In order to have current, the magnet needs to be moving in the coil.
(As mentioned in the unit helper, in the interest of time and other important factors, we will not discuss chapter 9 in detail. During various times this year, students have been presented with concepts from this chapter. Students may wish to read the material in this section for the benefit of the end of course exam in physical science.)
Chapter 9: Energy Sources I. Fossil Fuels Self Check p. Answers: 1)
II. Nuclear Energy Self Check p. Answers: 1)