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Electricity Basics: Electric Charge, Static Electricity, and Ohm's Law

Learn about electric charge, static electricity, and Ohm's Law, including concepts such as voltage, current, resistance, conductors, and insulators.

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Electricity Basics: Electric Charge, Static Electricity, and Ohm's Law

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  1. Ch 20 Electricity 20.1 Electric Charge and Static Electricity

  2. Let’s Review: • Protons have a positive charge • Electrons have a negative charge • When atoms lose electrons, they have a positive charge • When atoms gain electrons, they have a negative charge.

  3. Electric Charge • The SI unit for electric charge is the coulomb (C) • It takes 6.24 x 1018 electrons to produce 1 C • Lightning = 10-20 coulombs • Flash Camera = 0.025 C • An excess or shortage of electrons produces a net electric charge.

  4. What is happening in A? B? C?

  5. Electric Forces • Electric Forces - the attraction or repulsion between electrically charged objects • Electric forces obey a law similar to the law of universal gravitation • Reminder of Newton’s Law • 2x the mass = 2x the gravitational force • 2x the distance = ½ the gravitational force

  6. Electric Fields • Electric Fields - the effect an electric charge has on other charges in the area around it. Said another way: an area which is effected by a charged particle • Lines closer together equal a stronger field Electric Field

  7. Static Electricity and Charging • Static Electricity- this term is used to explain how charges can be transferredbetween objects • Law of Conservation of Charge- the total charge beforeequals the total charge after. • Let’s discuss some ways charges can be transferred…

  8. 1. CHARGING BY FRICTION • Ex: rubbing a balloon on your hair. • Electrons move from your hair to the balloon because atoms in the rubber have a greater attraction for the electrons.

  9. 2. CHARGING BY CONTACT *Not rubbing…simply touching • A Van De Graaff generator has a charged metal sphere. When the girl touches the sphere, the charge moves to her

  10. 3. CHARGING BY INDUCTION (aka NOT touching…just getting close) • A transfer of charge without contact between the two materials. In review…

  11. Static Discharge • Static Discharge – This is when a pathway suddenly opens that electric charges can move through • Charges don’t travel through air… But . . . • Sometimes air becomes charged suddenly when the gap between the two objects is small. • Like when you “SHOCK” someone!

  12. Lightning

  13. 20.2 Electric Current and Ohm’s Law • Electric Current- the rate (how fast) of continuous flow of electric charge. • There are two types of current. • Direct Current (DC)– electric charges flow in only 1 direction. • Ex: flashlights an other battery operated devices. • Alternating Current (AC)- electric charges that regularly reverse its direction. Ex: electric in your house. • Electrons always flow from negative to positive. However, current is measured in the opposite direction (positive charge to negative charge) because current is defined as the direction in which positive charges would flow.

  14. Electric Current

  15. Conductors and Insulators **These 2 terms mean the same as in our thermal energy unit • Conductor– a material that allows charges to flows easily through • Insulator – material that block the flow of charges • The coating around wires helps to control the current and keep it where it’s needed.

  16. Resistance • Resistance- when a material that gives opposition to the flow of charges. • Resistors – a material that slows down the flow of moving particles. • Resistors are in all of our electronics… ie: turning down the music volume is actually turning UP the electrical resistance in the speaker • Electrons have collisions as they travel through wires. These collisions convert KE to thermal energy (heat.) Since less energy is available, current is reduced.

  17. Resistance • SI unit for resistance is the ohm (Ω) • A material’s thickness, length, and temperature affect resistance. • Thicker wires lower resistance because it’s easier for electrons to move. • Shorter wires have lower resistance because the electrons travel a shorter path so they are less like to get into collisions on their journey. • Lower temperatures mean lower resistance. Electrons travel slower so they are less likely to collide. • Superconductors- a material that has almost no resistance when it is cooled to low temperatures.

  18. Voltage * a slightly complex concept… • In order for charge to flow through a wire, it must be connected in a complete loop that includes a source of electrical energy. • Charges flow spontaneously from a higher to a lower PE. • Potential difference is the difference in electrical PE between two places in an electric field. • Potential difference is measured in joules per coulomb or volts. Potential difference is also called voltage.

  19. Voltage Sources • There are three common voltage sources: batteries, solar cells and generators. • Batteries convert chemical energy to electrical energy. A voltage is maintained across the terminals.

  20. Ohm’s Law • Ohm’s Law is the relationship the mathematical relationship between voltage, current and resistance. • Resistance reduces voltage. V = IR • V = voltage • I = current • R = resistance • Current is measured in amperes. • Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current.

  21. Math Practice • What is the voltage if the current is 4 amps and the resistance is 10 ohms? • You try! • Find the voltage if the current is 0.2 A and the resistance is 2 ohms. V = IR V = 4 x 10 V = 40 Volts V = IR V = V =

  22. Math Practice • The resistance of an object is 4 ohms. If the current in the object is 9 A, what voltage must be used. • An object has the resistance of 20 ohms. Calculate the voltage needed to produce a current of 0.5 A. V = IR V = V = V = IR V = V =

  23. Nuclear Proliferation

  24. How to make a Nuclear Bomb:A fun-filled, family weekend activity

  25. 6/5 Do Now: • Name as many types of Frictional forces as you can • Explain Newton’s 1st law • What is the equation for calculating force? • What force opposes an object’s motion? • What’s the difference between concave lenses and concave mirrors?

  26. 20.3 Electric Circuits • Electric Circuits- a complete path through which a charge can flow. • Circuit diagrams are used to represent parts of a circuit.

  27. “Series” Circuit • Circuit with only 1 path • If one light bulb burns out, it becomes an open circuit and nothing works. • Adding more bulbs increases the resistance. Current decreases so bulbs are less bright.

  28. Parallel Circuit • A circuit with two or more paths through which it can flow. • If one bulb burns out, charges still flow along the other paths. The other bulbs stay lit.

  29. Power and Energy Calculations • Electric power- The amount of electrical energy depends on the power of the electrical devices and the length of time that those devices are on • (Power unit is the watt) • P = IV • To find the total electrical energy used by an appliance, you can multiply power by time. • E =P x t

  30. Math Practice • A computer monitor draws 1.2A at a voltage of 120V. What is the power rating of the monitor? • You try! • What voltage would a light bulb have if the current is 0.5A and the power is 60W? P = IV P = 1.2 x 120 P = 144 w P=IV P = P=

  31. Math Practice • How much energy is used if a 50W appliance is run for 5 seconds? • You try! • How much energy will be needed for a 200W appliance to run for 10 seconds? E = PT E = 50 x 5 E = 250 Joules E = PT E= 2,000J

  32. Math Practice • What is the power of an appliance that uses 200,000 J of energy to run for 1000 seconds? • You try! • An appliance uses 5000J of energy to run for 4 seconds. What is the power? E = PT P = 1250 Watts E = PT P=200 Watts

  33. Math Practice • How long will a 75W appliance run if it needs 150,000J? • You try! • A 5000J appliance uses 250 W. How long will it run for? T = 2000 seconds T = 20 seconds

  34. 6/11 Warm Up 1. How much energy is used by a 1500w electrical device if it is used for 75 mins? UseE=P x t 2. List the three factors that affect resistance. 3. What is the unit for energy? 4. Is work done when you hold a book over your head? Explain.

  35. 6/11 Agenda: • Do Now and go over Do Now • Chapter 20 Notes - Calculating Cost of Electricity - Electrical Safety Chapt 20 Quiz Wednesday!!

  36. 20.3—Electric Circuits • Electrical cost • Power companies usually charge by the kilowatt hour • Steps to find cost • Find energy: multiply appliance wattage by time (in hours) • Convert to kilowatt hours (either ÷ by 1000 or move decimal 3 times to the left) • Multiply by the cost from the electrical company • Round to two decimal places

  37. 20.3—Electric Circuits

  38. 20.3—Electric Circuits

  39. 20.3—Electric Circuits

  40. Electrical Safety Correct wiring, fuses, circuit breakers, insulation, and ground plugs help make electrical energy safe to use. • In the US, most household circuits are 120V. • Current in a circuit varies depending on the number of devices that are in the circuit. • Each device that is turned on increases the current.

  41. Home Safety • Fuse- Prevents overload in a circuit’s current. A wire in the center of the fuse melts if too much current passes through it. “Blowing a fuse” • Circuit Breaker- A switch opens when current in a circuit is too high. Can be reused. Fuse Circuit Breaker A “switch” Circuit “Breaker Box” Electric Source Outlet/battery Device Bulb, etc Fuses

  42. Personal Safety • Electrical wiring is insulated to protect people. • Don’t touch electrical devices with wet hands because your hands conduct current more readily when they are wet. • Insulation also prevents short circuits.

  43. Short Circuits • Short Circuit- current finds a short path through the circuit with less resistance than the full path through the circuit. • 3-prong plugs- prevents shocks caused by short circuits. • Usually found on devices with metal exteriors • If a short circuit develops, you could get shocked. Instead of entering your body, the current takes an easier path to the ground. • Grounding- transfer of excess charge through a conductor to Earth. • GFI (GFCI) • Ground fault (circuit) interrupter • Monitors current flowing to and from an outlet. If the two currents are not equal, it means current is escaping. The GFI circuit opens.

  44. Electricity and Humans

  45. Final Exam Review Practice #1 • What is the SI unit for electric charge? • If an atom loses electrons, it becomes _________. • Walking across a carpet is an example of _________. • If a neutral metal comb is held near an object with a negative charge, the comb will become negative because of ____________. • An electrical device is attached to a 9 volt battery with a current of 2A, find the resistance. • How much energy does a light bulb with 120 V with a current of 10 A use in 35 minutes? • Draw a complete free body diagram for a falling object. • Sound is a longitudinal or transverse wave?

  46. Final Exam Review Practice #2 • Draw two lamps in a parallel with a battery • A ground fault circuit shuts down if__________? • What is the difference between parallel and series circuit? • Why is it important for electrical wiring to be insulated? • Explain the purpose of a fuse and a circuit breaker? • Explain the difference between gravitational energy and potential energy. • A wheelbarrow lever has a mechanical efficiency of 18%. What is meant by mechanical efficiency?

  47. Final Exam Review Practice #3

  48. Final Exam Review Practice #4

  49. Final Exam Review Practice #5 Answers: B, C, A, D, A, B

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