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Electric Potential and Electric Circuits

Electric Potential and Electric Circuits . Electric Potential . Total electrical potential energy divided by the charge Electric potential = Electric potential energy Charge. VOLT. SI unit for electric potential – VOLT Named after Allesandro Volta (1745-1827)

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Electric Potential and Electric Circuits

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  1. Electric Potential and Electric Circuits

  2. Electric Potential • Total electrical potential energy divided by the charge • Electric potential = Electric potential energy Charge

  3. VOLT • SI unit for electric potential – VOLT • Named after Allesandro Volta (1745-1827) • 1 volt = 1 joule/coulomb

  4. Voltage • Same as volts • One volt takes 1 joule of energy per coulomb to add it to the conductor • Rub a balloon in your hair • It may take several thousand volts • HOWEVER – it is very few coulombs – less than one millionth • SO – the energy it very low

  5. Capacitors • Electrical storage • Uses • In computers to store “1” and “0” • In keyboards • Photoflash bulb • Giant lasers • Electric fences

  6. Simple Capacitors • Conducting plates separated by a small distance • Connected to battery which moves electrons from battery to plate • Complete when voltage on plates = battery voltage • Advantage – Discharged in large quantity • Can be dangerous – even when appliances are not powered.

  7. Formula • C= Q/V • C – capacitance (Farads or Coulombs/Volt) • Q – charge (coulombs) • V – voltage difference (volts)

  8. Electric Terms • Voltage – electric “pressure” • Current – flow of charge • Resistance – Restrains the electron flow • DC (Direct current) – flows in one direction • AC (Alternating current) – flows back and forth • Power – Rate at which energy is transferred

  9. Flow of Charge • When ends of electric conductor (example-wire) have different potential energy the charge flows • Flow continues until it reaches the same potential • Examples • Shock • Flashlight

  10. Electric Current • Flow of electric charge • Electrons carry the charge • Protons are bound to atoms • In solutions, ions can carry the charge • Measured in amperes (amps) • SI unit (A) • Amps = 1 coulomb/second • Example - 5 amps wire carries 5 coulombs of charge pass a point in 1 second • When there is no current, the wire has no charge

  11. Formula for Current • I = ΔQ/Δt • I – current (amperes or amps) • Q – charge (coulombs) • T – time (seconds) • 1 amp = 1 coulomb/second

  12. Voltage Sources • Electron pump • Needs to slowly release the electrons • Not all at once – like in a shock • Steady flow • Dry cells – chemical energy • Wet cells – chemical energy • Generator – converts mechanical energy • Voltage provides “pressure” to move electrons between terminals

  13. Generators • Average home outlets are AC • Pressure of 120 volts • This means there is 120 joules of energy forcing the coulombs through the wire • Voltage does not FLOW through the wire • The electrons are pushed by the voltage

  14. Electric Resistance • Current depends on • Voltage • Resistance • Resistance – slows the flow • Depends on • Conductivity of material – silver best • Thickness – more thick, less resistance • Length of wire - longer, more resistance • Temperature – hotter, more resistance

  15. Drawing Currents Water line Electric circuit hose resistance valve switch pump battery

  16. Ohm’s Law • Current in a circuit is directly proportional to the voltage and inversely proportional to the resistance • Current = voltage/resistance • I = V/R • Units  1 ampere = 1 volt/ohm • Double the voltage – double the current • Increase the resistance – decrease the current

  17. Typical resistance • Cord – less than 1 ohm • 100 W light bulb – 100 ohms • Iron – 15 ohms • Electric toaster – 15-20 ohms • Low resistance = larger current which produces lots of heat

  18. Ohm’s Law Formula • V=IR • V – voltage (volts) • I – Current (amps) • R – resistance (ohms – Ω)

  19. Resistors • Radios and TV electronics • Current is regulated by resistors • Ranges from few ohms to millions

  20. Question • What is the resistance of an electric frying pan that draws a 12 ampere current when connected to a 120-volt circuit? • R = V/I = 120 V/12 A = 10 Ω • How much current is drawn by a lamp that has a resistance of 100 Ω when a voltage of 50 V is impressed across it? • I = V/R = 50 V/ 100 Ω = 0.5 amps

  21. Electric Shock • What effects us – current or voltage? • Damage comes from current through the body • Current depends on voltage and resistance in body • Resistance varies from 100 Ω (covered in salt water) to 500,000 Ω (very dry skin) • Usually can’t feel 12 volts and 24 volts would tingle • If wet – 24 volts could be very uncomfortable

  22. Effect of Various Electric Current

  23. Questions • If resistance of your body were 100,000 ohms, what would be the current in your body when you touched the terminals of a 12-volt battery? • Current = V/R = 12 v/100,000 Ω = 0.00012 A • If your skin were wet, your resistance would be 1000 Ω, what would you feel on a 24 volt battery? • Current = 24 V/ 1000 Ω = 0.024 A

  24. Shocked • 120 V in house • normally our shoes provide resistance • Standing barefoot in bathtub – very little resistance to ground – don’t use appliances in bathtub!! • Birds sitting on wire • Same potential on both sides of body • Only get it if they touch wires with different potential

  25. Grounded • Most appliances are grounded • Helps prevent potential differences • All ground wires are connected together • Provides a direct route to the ground • What causes shock – current or voltage? • The initial cause is the voltage, but the current does the damage

  26. Types of current • DC – Direct current • Flow of charge is always in one direction • Source – batteries (wet or dry) • Move from negative terminal to positive • AC – Alternating current • Charge moves back and forth in the line • In North America – frequency is 60 cycles per second – 60 Hz

  27. Voltage • North America • Small appliances – 110-120 volts • Large appliances – 220 volts • Obtain by “combining voltage” • Europe – 220 volts (more efficient) • Electrons do NOT travel from power plants, they oscillate in place • They sell energy, you provide the electrons in your wiring

  28. Diode • One way valve in a circuit • Only allows electrons to move one way • Can be used to convert AC to DC current

  29. Power • Rate at which electric energy is converted to another energy form • Electric power = current x voltage • Units – 1 watt = 1 amp x 1 volt • 1 kilowatt = 1000 watts • 1 kilowatt hour = amount of energy consumed in 1 hour at rate of 1 kilowatt

  30. Calculations • How much power is used by a calculator that operates on 8 volts and 0.1 amps? If it is used for one hour, who much energy does it use? • Power = A x V = 0.1 A x 8 V = 0.8 W • Energy = Power x time = 0.8 W x 1 hour = 0.8 watt-hours or 0.0008 kilowatt-hours • FYI – Speed of e- in wire = 0.01 cm/s

  31. Lighting a bulb – which one works? Must have a closed loop!!

  32. Electric Circuits • Need • Pathway • Voltage source (battery or AC) • Resistance (light, fan, bell, etc) • Optional • switch – stops and starts current • Capacitor – hold a large voltage for quick release • Diode – one-way valve • Transformer and capacitors– controls current flow

  33. Types of Circuits • Series – single pathway for all electrons to flow • Parallel – branched pathway for electrons to flow

  34. Series circuit • When switch is closed, current runs through all lamps in same path • Open – when one break occurs, all flow stops • Break from switch, broken wire, burned out light-bulb

  35. Series Circuit Features • Single pathway – same current • Total resistance = sum of individual resistance • Current = voltage/ total resistance (Ohms law) • Voltage drop across each device depends on resistance of the device

  36. Disadvantage of Series • If one device fails, the whole circuit ceases

  37. Parallel Circuits • Devices are connected to the same points in the circuit • Each resistor has its own path to the voltage source • If one device fails, it does not interupt the other devices

  38. Features of Parallel Circuits • Each device connects directly to the voltage source. Therefore the voltage is the same across each device. • Total current divides between devices. The one with the lowest resistance gets the most current (Ohm’s law) • Total current = sum of current in the devices • As number of devices increases, resistance decreases

  39. Schematic Diagrams Resistance Battery (2) http://www.rkm.com.au/ANIMATIONS/animation-electrical-circuit.html

  40. Combined Compound Circuits

  41. Calculating Current in Compound Circuits • When resistors are in • Series • The resistance equal sum of devices • -----^^^^^-----^^^^^^------- = -----^^^^^------ 8Ω 8Ω 16 Ω • Parallel (2 devices with same resistance) • The resistance equal half of one device • ----^^^^^------ = ----^^^^----- 8Ω 4 Ω ----^^^^^------- 8Ω

  42. Another Example ----^^^^^----- 8Ω ----^^^^^----  ----^^^^^-----^^^^^-- 8Ω 8Ω 4Ω ----^^^^^----- 8Ω  ---^^^^^----- 12Ω

  43. Overloading • The more devices on a line, the more current it draws as resistance lowers • Overloaded – line carrying more than a safe amount of current • Fuses – put in a current in SERIES • Cuts the line if it overheats • Overheating caused by • Short circuit (cuts the resistance) • Too many devices on a line

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