2. • During this odd state of
water there are small
domains within the liquid
that still have the very
open, ordered solid
structure.
• The decrease in density
is caused by these
“melting”.
4. Phase Changes:
• We know the distribution of particle energies
changes with temperature.
• We also know from evaporation that particles
with more energy than can be held will escape.
• So temperature of the liquid reaches an upper
limit and thereafter does not change as a liquid
boils away.
• What about melting?
5. Boiling
• Molecules attain enough
energy to escape liquid.
• Temperature of liquid
does not change while
boiling.
• Energy is still absorbed!
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. Energy and Phase Change
• Although temperature does
not change (c*m*ΔT = 0)
heat energy is still
absorbed/released.
• Energy to melt/freeze is
called heat of fusion.
• Energy required to
boil/condense is called heat
of vaporization.
8. Conclusion:
Around Freezing water has unusual thermal expansion
characteristics.
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 is
underway.
All heat energy transferred into (out of) a system during a
phase change is used to change the phase. Process similar to
H = m*c*ΔT, instead H = m*L: heat = mass time latent heat
(fusion/vaporization).
10. Work and the Electric Force
• Work: force taking place
over distance.
– W = F * d
• Imagine two particles of
opposite charge.
– If we separate them have
we done work?
q1 q2
d1
q1 q2
d2
11. Electric Potential Energy
• Unlike gravitational
potential energy, we
have different charges.
• By pushing two of the
same charge (both
positive or both
negative) closer together
we store energy.
12. What is a Volt?
(Measuring Electric Potential Energy)
• A volt is a way of measuring how much
potential energy is present per charge.
– Potential (voltage) = Electric Potential Energy
amount of charge
– A Volt is a Joule per Coulomb
When we think about a 1.5 Volt battery, what is it the
number is telling us?
• Each Coulomb of charge passing through the battery
carries 1.5 Joules of energy.
13. High Voltage
• Voltages between earth and
various objects may be quite
high.
– 5,000 V on a balloon
– 100,000 V on a van de graaf
generator
• Why aren’t these high
voltages dangerous?
– Very few charges involved,
much less than one coulomb
(~10^18 electrons)
– Even if you get shocked the
current is very small.
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5,000 V
14. The Flow of Electric Energy
• Just as heat flows from warm to cold and as
material flows from high to low pressure
electric charges will flow from high potential to
low potential.
• Charges move from one pole of a battery to
another, trying to make the potential difference
(voltage difference) smaller.
15. • A plumbing analogy can be very useful when thinking
about circuits.
– Pressure = voltage
– Flow = current
– Size of pipe = resistance.
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 leave
at the other end..
– Net charge remains zero.
18. Electric Current
• The electrons which flow through a wire are already
present.
• Electric Potential simply forces them to flow.
• Current is measured in Amperes
– One Amp means One Coulomb of charge passes through a
point on the wire each second. (Coulomb/second)
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 the
potential, however, is near the speed of light! (This is why
electric appliances will react almost instantaneously to being
switched on/off.)
20. DC and AC
• Direct Current: the current
remains constant over time.
• Alternating Current: the
direction the current flows
reverses.
– Alternating current has the
advantage of easy conversion
of voltages: low voltage to high
and high to low.
– We’ll talk more about this later.
21. Electrical Resistance
• Current in a circuit depends both on Voltage
and on Resistance
– Resistance is sort of like a constriction in a pipe.
– Less water flows through a clogged drain despite
having the same pressure.
• Resistance is measured in Ohms.
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 low
temperature.
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 the
current.
24. Example Problem
• The bulbs are identical.
• Which bulb will be
brighter.
• How much more current
will flow in the 2nd
circuit?
6 V
12 V
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, return
trip.
Experiment time: each group will receive one bulb,
one battery and ONE wire. Find ways to connect
them which light the bulb (there are 4 ways, all very
similar).
26. Circuits
Circuits can have two major forms of
organization:
Series: everything comes one after the other
Parallel: different paths through each device
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 the
size of its resistance.
28. Experiment: Simple Series
• Two bulbs in series
• What happens if we unscrew one bulb?
12 V
There is only one path for electrons to take.
Unscrew one bulb and you break the road
they were following: both go out.
29. Example Problem
• All 3 bulbs are identical.
• Which will be brighter,
the bulb in the top circuit
or the bulbs in the
bottom circuit?
• Why?
12 V
12 V
30. Experiment: Different Resistance
Bulbs 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. Different Resistances in Series
• One bulb will be brighter than the other.
• It isn’t the one you might expect. Why not?
12 V
100 Ohm 200 Ohm
Total Resistance = 300 Ohm
Current = V/R = 40 mA
Find the voltage drop across each.
We know the resistance of each bulb and
we know the current through both bulbs
must be the same (only one path for the
flow to take).
First bulb: v = I*R = 0.040Amp*100Ohm
4 Volts
Second bulb: V = 0.040Amp*200Ohm
8 Volts
Note: the sum is 12. The total voltage around
a loop must equal the voltage of the supply
(battery in this case).
Result: 200Ohm bulb is brighter.
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 the
resistance in the branch.
• As number of branches increases overall resistance
decreases.
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 paths
the electron flow may take, so stopping one up does not affect the other.
34. Example Problem
• What happens when a
car headlight burns out?
• Does this imply the lights
are in a series or parallel
circuit?
• Why?
12 V
12 V
Homework problem, so if you’re not sure
ask in class.
35. What fuses do
• Fuses prevent the total
current through a circuit from
exceeding the rating of the
wire.
• If total current through circuit
too high, the fuse will burn
out first.
• Circuit breakers (as you
likely have at home) work
like fuses, but are resettable.
36. Demo
Thin 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, protecting
the circuit from having too much current flow for the size of wire used (too
small a wire will heat up and may catch fire).
37. Power
• P = I * V
– P = Power
– I = Current
– V = Voltage
• Power is the amount of energy being used by
that component per second. Higher power
tends to mean hotter or brighter.
12 V
I = 1 Amp
38. Example Problem
If the voltage drop across the bulb is 12 Volts
and the current is 1 Amp, what is the power
drawn?
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. 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 the
other. If one component burns out no current can flow.
• Parallel Circuits: each component has its own loop
between the same two points. If one component blows,
others continue to function.
