Lecture 11

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Lecture 11

  1. 1. Water’s Density
  2. 2. • During this odd state ofwater there are smalldomains within the liquidthat still have the veryopen, ordered solidstructure.• The decrease in densityis caused by these“melting”.
  3. 3. Phase Changes and Heat
  4. 4. Phase Changes:• We know the distribution of particle energieschanges with temperature.• We also know from evaporation that particleswith more energy than can be held will escape.• So temperature of the liquid reaches an upperlimit and thereafter does not change as a liquidboils away.• What about melting?
  5. 5. Boiling• Molecules attain enoughenergy to escape liquid.• Temperature of liquiddoes not change whileboiling.• Energy is still absorbed!
  6. 6. • Melting: ordered solid state breaks apart.• Energy absorbed to melt even though, like boiling,temperature does not change.• Energy released when freezing.Melting and Freezing
  7. 7. Energy and Phase Change• Although temperature doesnot change (c*m*ΔT = 0)heat energy is stillabsorbed/released.• Energy to melt/freeze iscalled heat of fusion.• Energy required toboil/condense is called heatof vaporization.
  8. 8. Conclusion:Around Freezing water has unusual thermal expansioncharacteristics.Actually becomes less dense as it gets colder, which is why ice floats.Phase changes are similar to Evaporation/Condensation.No temperature change may occur while a phase change isunderway.All heat energy transferred into (out of) a system during aphase change is used to change the phase. Process similar toH = m*c*ΔT, instead H = m*L: heat = mass time latent heat(fusion/vaporization).
  9. 9. Circuits
  10. 10. Work and the Electric Force• Work: force taking placeover distance.– W = F * d• Imagine two particles ofopposite charge.– If we separate them havewe done work?q1 q2d1q1 q2d2
  11. 11. Electric Potential Energy• Unlike gravitationalpotential energy, wehave different charges.• By pushing two of thesame charge (bothpositive or bothnegative) closer togetherwe store energy.
  12. 12. What is a Volt?(Measuring Electric Potential Energy)• A volt is a way of measuring how muchpotential energy is present per charge.– Potential (voltage) = Electric Potential Energyamount of charge– A Volt is a Joule per CoulombWhen we think about a 1.5 Volt battery, what is it thenumber is telling us?• Each Coulomb of charge passing through the batterycarries 1.5 Joules of energy.
  13. 13. High Voltage• Voltages between earth andvarious objects may be quitehigh.– 5,000 V on a balloon– 100,000 V on a van de graafgenerator• Why aren’t these highvoltages dangerous?– Very few charges involved,much less than one coulomb(~10^18 electrons)– Even if you get shocked thecurrent is very small.-------------------5,000 V
  14. 14. The Flow of Electric Energy• Just as heat flows from warm to cold and asmaterial flows from high to low pressureelectric charges will flow from high potential tolow potential.• Charges move from one pole of a battery toanother, trying to make the potential difference(voltage difference) smaller.
  15. 15. • A plumbing analogy can be very useful when thinkingabout circuits.– Pressure = voltage– Flow = current– Size of pipe = resistance.
  16. 16. Electron Flow in a Circuit• A current carrying wire is not electrically charged– It’s just a conduit.– Whenever a charge is pushed on at one end one will leaveat the other end..– Net charge remains zero.
  17. 17. Exercise: The meaning of CircuitANDWhere do the electrons that flow comefrom?
  18. 18. Electric Current• The electrons which flow through a wire are alreadypresent.• Electric Potential simply forces them to flow.• Current is measured in Amperes– One Amp means One Coulomb of charge passes through apoint on the wire each second. (Coulomb/second)
  19. 19. Electron SpeedThe speed of an electron through a circuit is quite low;measured in cm/sThe speed at which the circuit reacts to changes in thepotential, however, is near the speed of light! (This is whyelectric appliances will react almost instantaneously to beingswitched on/off.)
  20. 20. DC and AC• Direct Current: the currentremains constant over time.• Alternating Current: thedirection the current flowsreverses.– Alternating current has theadvantage of easy conversionof voltages: low voltage to highand high to low.– We’ll talk more about this later.
  21. 21. Electrical Resistance• Current in a circuit depends both on Voltageand on Resistance– Resistance is sort of like a constriction in a pipe.– Less water flows through a clogged drain despitehaving the same pressure.• Resistance is measured in Ohms.
  22. 22. Resistance• Electrical resistance depends upon:– Length– Type of material• Copper conducts well, rubber does not– Temperature• In general resistance rises with temperature.• Some materials exhibit zero resistance at very lowtemperature.
  23. 23. Ohm’s Law• There is a simple relationship between Voltage,Current and Resistance in a circuit:– V = I*R or I = V/R• The greater the resistance, the smaller thecurrent.
  24. 24. Example Problem• The bulbs are identical.• Which bulb will bebrighter.• How much more currentwill flow in the 2ndcircuit?6 V12 V
  25. 25. Electric Circuits• The flow of electrons requires a pathway.• Pathway goes from and returns to pump.– This is why it is a circuit: it makes a full circle, returntrip.Experiment time: each group will receive one bulb,one battery and ONE wire. Find ways to connectthem which light the bulb (there are 4 ways, all verysimilar).
  26. 26. CircuitsCircuits can have two major forms oforganization:Series: everything comes one after the otherParallel: different paths through each device
  27. 27. Series Circuits• Current has a single pathway.• The current is resisted by all resistances– In V = I*R, R = R1 + R2 + ...• Each device experiences a ‘voltage drop’ depending on thesize of its resistance.
  28. 28. Experiment: Simple Series• Two bulbs in series• What happens if we unscrew one bulb?12 VThere is only one path for electrons to take.Unscrew one bulb and you break the roadthey were following: both go out.
  29. 29. Example Problem• All 3 bulbs are identical.• Which will be brighter,the bulb in the top circuitor the bulbs in thebottom circuit?• Why?12 V12 V
  30. 30. Experiment: Different ResistanceBulbs In Series• Tall Bulb has higher resistance than short.• Place one tall and one short in a series circuit.• Now try putting them the other way around.Does it make a difference?
  31. 31. Different Resistances in Series• One bulb will be brighter than the other.• It isn’t the one you might expect. Why not?12 V100 Ohm 200 OhmTotal Resistance = 300 OhmCurrent = V/R = 40 mAFind the voltage drop across each.We know the resistance of each bulb andwe know the current through both bulbsmust be the same (only one path for theflow to take).First bulb: v = I*R = 0.040Amp*100Ohm4 VoltsSecond bulb: V = 0.040Amp*200Ohm8 VoltsNote: the sum is 12. The total voltage arounda loop must equal the voltage of the supply(battery in this case).Result: 200Ohm bulb is brighter.
  32. 32. Parallel Circuits• Each device connects across the same two points.• Each device experiences the same voltage drop.• The current in each branch is inversely proportional to theresistance in the branch.• As number of branches increases overall resistancedecreases.
  33. 33. Exercise: Bulbs in Parallel• 2 bulbs in parallel• What happens if we unscrew one bulb?Only one bulb will go out. In a parallel circuit you have two possible pathsthe electron flow may take, so stopping one up does not affect the other.
  34. 34. Example Problem• What happens when acar headlight burns out?• Does this imply the lightsare in a series or parallelcircuit?• Why?12 V12 VHomework problem, so if you’re not sureask in class.
  35. 35. What fuses do• Fuses prevent the totalcurrent through a circuit fromexceeding the rating of thewire.• If total current through circuittoo high, the fuse will burnout first.• Circuit breakers (as youlikely have at home) worklike fuses, but are resettable.
  36. 36. DemoThin piece of metal used as fuse.What happens?If you can get a thin enough piece of steel wool (without cutting yourself on it)and put enough current through it the ‘fuse’ (steel wool) will melt, protectingthe circuit from having too much current flow for the size of wire used (toosmall a wire will heat up and may catch fire).
  37. 37. Power• P = I * V– P = Power– I = Current– V = Voltage• Power is the amount of energy being used bythat component per second. Higher powertends to mean hotter or brighter.12 VI = 1 Amp
  38. 38. Example ProblemIf the voltage drop across the bulb is 12 Voltsand the current is 1 Amp, what is the powerdrawn?This is simpler than a question I might ask on a test.Answer here is 12 Watts.Test problem more likely to be like 12 Volts and 0.75 Amp or such.
  39. 39. Conclude• Electric Potential is the energy moved per coulomb:volts• Ohm’s Law: V(voltage) = I(current)*R(resistance)• Series Circuits: each component follows one after theother. If one component burns out no current can flow.• Parallel Circuits: each component has its own loopbetween the same two points. If one component blows,others continue to function.

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