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Chapter 21 Electricity

Chapter 21 Electricity. Objectives. 21.1 Describe the effects of static electricity. 21.1 Distinguish between conductors and insulators 21.1 Recognize the presence of charge in an electroscope. . Objectives.

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Chapter 21 Electricity

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  1. Chapter 21 Electricity

  2. Objectives • 21.1 Describe the effects of static electricity. • 21.1 Distinguish between conductors and insulators • 21.1 Recognize the presence of charge in an electroscope.

  3. Objectives • 21.2 Explain the occurrence of lightning in terms of induction and static discharge • 21.2 Evaluate the positive and negative aspects of lightning induced forest fires

  4. Objectives • 21.3 Describe how static electricity is different from current electricity • 21.3 Explain how a dry cell is a source of electricity • 21.3 Conceptually and mathematically relate potential difference, resistance, and current.

  5. Objectives • 21.4 Sketch a series and a parallel circuit, and list applications of each type of circuit • 21.4 Recognize the function of circuit breakers and fuses • 21.5 Explain and calculate electrical power • 21.5 Calculate the amount of electrical energy in kW/hrs

  6. Terms • Static Electricity: Net build up of charge • Charge is not created or destroyed, so the loss of electrons for one is the gain for another • Electric Field: A force that attracts or repulses charged objects • Objects not charge feel no force • Insulator: Doesn’t allow electrons to move • Conductor: Allows passage of electrons

  7. Atom (Review) • Composed of protons, neutrons, and electrons. The Protons and Neutrons are stuck but the electrons are capable of moving

  8. Rearranging electrons

  9. Conductors/Insulators • Conductors: Allow their electrons to move freely • Most metals • Insulators: Electrons stuck in place • Plastic, Rubbers

  10. Lightning

  11. Positive charge exerts field away • Thanks to Benjamin Franklin

  12. Electric Fields

  13. More Examples • Doubled both charges

  14. Potential Difference • Also called the Volt • In the gravitational world: How high have you been lifted from the earth? • In the electric world: How far are you away from your opposite charge?

  15. Voltage

  16. Voltage • Same q, differing v Same v, diff q

  17. Voltage Jupiter (lots of PE) Mercury (little PE)

  18. AED • Build up a large static charge, then release

  19. Electric Current • The flow of electrons • Only occurs when there is a potential difference • One location must be at a lower potential than the others • This is produced by batteries, photovoltaic cells (solar panels), generators (spinning coils) • Measured in Amperes

  20. The Ampere • Abbreviated as Amp usually • 1 Amp = 1 C/second through a cross-sectional area

  21. Electric Circuit as related to Amps

  22. Energy Changes • Energy and Charge are conserved

  23. Random Information • Effect of Various Electric Currents on the Body • Current in Amps Effect • 0.001 Can be felt • 0.005 Painful • 0.010 Involuntary Muscle Contractions • 0.015 Loss of Muscle Control • 0.070 If through heart, serious disruption, probably fatal if > 1s

  24. Current • While electrons have random motion, current is the net movement. • The net movement of the electrons is the drift velocity. The drift velocity is actually very small. • As the wire becomes thinner, the drift velocity becomes faster. • Electrons in a current carrying wire move slower than a snail

  25. Resistance • Resistance of a circuit determines how much current will flow • Different materials have different resistances • Metals are low, Non-Metals are high • Insulators have lots of resistance, conductors next to none • Resistance determines the amount of current • High resistance = low current because very few electrons are able to be moved

  26. Ohm’s • Resistance is measured in Ohm’s • Resistors also cause a drop in voltage. They resist the push that is being applied by the voltage. • Is resistance a bad thing? Not all the time. When transporting energy across the country in big power lines = yes. When heating up your curling iron or light bulb = no.

  27. Resistance • Temperature as well influences the resistance. As temperature goes up, atom “jiggling” increases. Temp goes Up, Resistance goes up • The more the atom’s jiggle, the more they interfere with the movement of electrons (the electrons crash into them and don’t move as fast). • At very low temperatures, the atom jiggling is so little that some materials act as superconductors.

  28. Resistance • Ohm’s Law = • Resistance is calculated by how much drop in voltage it causes • Question • 2.0 Amps of current are flowing in a current. A resistor causes a drop of 10.0 V. How many Ohm’s of resistance does the resistor have?

  29. Water Pump Analogy • If we have a water pump that exerts pressure (voltage) to push water around a "circuit" (current) through a restriction (resistance), we can model how the three variables interrelate.

  30. Water Pump Analogy • If the pressure stays the same and the resistance increases (making it more difficult for the water to flow), then the flow rate must decrease:

  31. Water Pump Analogy • If the flow rate were to stay the same while the resistance to flow decreased, the required pressure from the pump would necessarily decrease:

  32. Capacitors • Store up energy. As the electrons flow past a place, the capacitor acts as a holding dock of sorts for the electrons. • You can think of it as a dam, where water flows to the dam, the dam stores up the water and releases it later. • These are used for your windshield wipers. When enough energy is stored up, it causes the wiper to go .

  33. Circuit Breakers/Fuses • Designed to stop fires • Lots of current = lots of electrons bouncing around = lots of friction = lots of heat • Typical setting is 30 amps • Two hairdryers on the same line usually enough to trip the circuit

  34. Power = Current times Voltage • P = IV • Power measured in Watts (J/s) • Electric power is measured by the amount of current flowing and the voltage difference • More current equals more power • More voltage (bigger potential difference) equals more power

  35. Kilo-Watt Hour • The most important thing to take away from this chapter (as far as your every day life is concerned) • It is what electricity is sold in • 1 kW/Hr = A device that runs on 1 kW and runs for one hour • Equivalent to 3.6 million Joules of energy

  36. Typical Ranges and Costs • 60 W light bulb = 0.060 kW run for 1 hour = 0.060 kW/Hr of energy • 3000 W stove = 3 kW run for 1 hour = 3 kW/hrs of energy • Cost is approximately $0.05 to $0.20 per kW/hr (10 cents here in SEMO)

  37. Cost • A computer runs on 500 W. If the computer is run for 5 hours… • A) How many kW/hr’s will be used? • B) If it costs 10 cents per kW/hr, how much does it cost the consumer?

  38. Why turn off the lights? • A certain light fixture has five 60 W light bulbs. If the lights are left on in an unattended room for 8 hours a day for a month, how much does it cost (at 10 Cents per kW/hr) to light the room? • A computer runs at 400 W. If it is left on at night every day for a year, how much will it cost you to leave it on at night (same cost rate)

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