2 Electricity Objectives Describe how electric charge exert forces on each other.Compare the strengths of electric and gravitational forces.Distinguish between conductors and insulators.Explain how objects become electrically charged.Describe how voltage difference causes current to flow.Explain how batteries produce a voltage difference in a circuit.List the factors that affect an objects electrical resistance.Define Ohm’s Law.Describe the difference between series and parallel circuits.Recognize the function of circuit breakers and fuses.
3 Electric ChargeProtons have positive charge; electrons have negative charge.In most atoms, the charges of protons and electrons cancel each other out and the atom has no net charge.Atoms become charged by gaining or losing electrons3. Static electricity-the accumulation of excess electric charges on an object.
4 B. Electrically charged object obey the following rules. 1. Law of conservation of charge -charge may be transferred from object to object, but it cannot be created or destroyed.2. Opposite charges attract and like charges repel.Charges can act on each other even at a distance, because any charge that is placed in an electric field will be pushed or pulled by the field.The force decreases the farther apart the chargesElectrons move more easily through conductors like metals.
5 5. Electrons do not move easily through insulators such as plastic, wood, rubber and glass. The force of gravity between you and Earth seems to be strong.The force of gravity is much weaker than electric forces.Attractive electric force between two hydrogen atoms is 1036 times larger than the gravitational force between them.D. Transferring electric charge.Charging by contact is the process of transferring charge by touching or rubbing.a. example-feet rubbing the carpet
6 2. Charging by inductionThe rearrangement of electrons on a neutral object caused by a nearby charged object.Example: a negatively charged balloon near your sleeve causes an area of your sleeve to become positively charged.3. Static dischargeA transfer of charge through air between two objects because of a buildup of static electricity.Example: lightning4. Grounding -using a conductor to direct an electric charge into the ground.The presence of electric charges can be detected by an electroscope.
7 DiscussionWhat do you think happens when you get static electricity in your hair?AnswerUsually it happens after your hair rubs against something, like a sweater or brush, causing the object to gain a negative charge and your hair a positive charge, attracting your hair to the object. Some hair may also repel each other since the hair strands are the same charge.
8 Electricity Review (Write Q and A) Define static electricity.Describe how lightning is produced.Explain why if charge cannot be created or destroyed, electrically neutral objects can become electrically charged.Predict what would happen if you touched the knob of a positively charged electroscope with another positively charged object.Humid air is a better electrical conductor than dry air. Explain why you’re more likely to receive a shock after walking across a carpet when the air is dry than when the air is humid.
9 Electric CurrentThe flow of charges through a wire or conductor is called electric current.1. Current is usually the flow of electrons2. Electric current is measured in amperes (A)3. Charges flow from high voltage to low voltagea. A voltage difference is the push that causescharges to move.b. A voltage difference is measured in volts (V)
10 4. For charges toflow, the wiremust always beconnected in aclosed path orcircuitSources of electricity1. A dry cell battery produces a voltage differencebetween its zinc container and its carbonsuspension rod, causing current to flowbetween thema. Examples- AAA, AA, A, C, D, 9-V
11 A wet cell battery contains two connected plates made of different metal in a conducting solution. Wall sockets have a voltage difference across the two holes of an electrical outlet and a generator at a power plant provides this voltage difference.
12 Resistance-the tendency for a material to oppose the flow of electrons, changing electrical energy into thermal energy and light.1. All materials have some electrical resistance,with the exception of superconductors at lowtemperatures.a. Cu is an excellent conductor and is used inhousehold wiring.2. Resistance is measured in Ohms (W)3. Making wires thinner, longer, or hotterincreases the resistance.
13 Ohm’s Law-the current in a circuit equals the voltage difference divided by the resistance. 1. shows the relationship between voltagedifference, current, and resistance in a simpleelectric circuit.When the wires areconnected, current flowsin the closed pathb. When the wires are not connected, current willnot flow in the open path
14 Ohm’s Law Ohm’s Law can be written as a formula: current (in amperes) = voltage difference (in volts)resistance (in ohms)orI = VR-make sure you understand the relationship between current, voltage and resistance, such as what happens if you decrease resistance (will increase current) etc.
15 Ohm’s Law Practice Show all work and units: Calculate the voltage difference in a circuit with a resistance of 52 W if the current in the circuit is 0.5 A.A current of 0.5 A flows in a 60 W lightbulb when the voltage difference between the ends of the filament is 120 V. What is the resistance of the filament?A toy car with a resistance of 20W is connected to a 3 V source. How much current flows in the car?
16 Electric Current Discussion What property of electric current causes light bulbs to give light?AnswerResistance. As electrons flow through the filament, the filament resists their flow and changes electrical energy into thermal energy.
17 Electric Current (Write Q and A) What is current?How do charges flow?What is the difference between a dry cell and a wet cell battery.How do you increase the resistance of a material?Compare and contrast the flow of water in a pipe to the flow of electrons in a wire.Identify two ways to increase the current in a simple current.Explain how a dry cell battery produces a voltage difference between the positive and negative terminals.
18 Electrical CircuitsA. Circuits rely on generators at power plants to produce a voltage across the outlet, causing the charge to move when the circuit is complete.1. Series circuit-the current has only one loop toflow througha. The parts of a series circuit are wired oneafter another so the amount of current is thesame through every part.b. Open circuit-if any part of a series circuit isdisconnected, no current flows through thecircuit.c. Example-strings of holiday lights
19 for current to move through 2. Parallel circuit- contains two or more branchesfor current to move througha. Individual parts can be turned off withoutaffecting the entire circuit.b. Example-electrical system in a houseSeriesCircuitParallelCircuit
20 Electrical CircuitsB. Household circuits use parallel circuits connected in a logical network.1. Each branch receives the standard voltagedifference from the electric company, which inthe US is usually 120Va. In some branches that use electric stoves orelectric clothes dryers, the voltagedifference is 240V.2. They guard against overheating electric wires.a. Electric fuse-contains a small piece of metalthat melts if the current becomes too high,opening the circuit and stopping the flow ofcurrent.
21 b. Circuit breaker-contains a small piece of metal that bends when it gets hot,opening the circuit and stopping the flow ofcurrent.C. Electrical energy is easily converted to mechanical,thermal, or light energy1. Electrical power – rate at which electricalenergy is converted to another form of energya. Electrical power is expressed in watts (W)b. Power = current x voltage differenceP = IV
22 2. To calculate the amount of energy an appliance uses:a. The unit of electrical energy is the kilowatt-hour, which equals 1000watts of powerused for one hour.b. Energy = power x timeE (kWh) = P (kW) x t (h) with unitsE = PtDiscussionDoes your home have a fuse box or circuit breaker? Why is it there? How does it work?
23 Circuits Practice Show all work and units. 1. The current in an electric clothes dryer is 15 A when it is plugged into a 240 V outlet. How much power does it use?2. A VCR that is not playing still uses 10.0 W of power. What is the current if the VCR is plugged into a 120 V outlet?3. A microwave oven with a power rating of 1200W is used for 0.25 hrs. How much electrical energy is used by the microwave?4. A TV with a power rating of 200 W uses 0.8kWh in one day. For how many hours was the tv on during this day?
24 Circuits Review (Write Q and A) 1. How are electric power and electrical energy related?2. Why are fuses and circuit breakers used in household circuits?3. What determines the current in each branch of a parallel circuit?4. Should a fuse or circuit breaker be connected in parallel or in series to the circuit it is protecting? Why?5. A parallel circuit consisting of 4 branches is connected to a battery. Explain how the amount of current that flows out of the battery is related to the amount of current in the branches of the circuit.
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