Physics 1010:
The Physics of Everyday Life
TODAY
• Circuits
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CIRCUITS
• Given a bunch of batteries, light bulbs, and wire, how to design
light bulb: that will burn brightest, or that will last longer, or that will
be dim, or that will turn on and off.
• How can you control and predict current and power in light bulbs?
• All this basic circuit stuff applies to home wiring, hair dryers,
heaters, all home electronics, etc.
Builds on electrostatics (like charges repel, opposite charges
attract)… electrons can move … circuits are all about electron flow
… need to start thinking like an electron!
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Electrons carry charge around
• Coulomb is unit of charge
• Electron has 1.602x10-19 Coulombs of charge
• There are 1/1.602x10-19 = 6.25x1018
electrons in one Coulomb of charge
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In different materials, electrons have
more or less freedom to move
• Conductors: electrons
move freely
• Insulators: electrons
stuck to one place,
but can polarize
• Semiconductors:
insulators that
become conductors
with the addition of
some impurities
+
+
-
+
Conductor
+
-+
-+
-+
Insulator
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Voltage: the work per unit charge
• Gravitational potential energy is Mgh
 Get more for more mass
 Relative to surface: how much kinetic
energy if it fell to that surface?
Mass M
Earth’s surface
• Electrostatic potential energy is QV
(charge x voltage)
 More for more charge
 Relative to surface
Charge Q
- - - - - - - - Charged conductor
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Batteries
• Chemicals inside battery like to
react to move electrons to one side
• Electrons pile up by release of
chemical energy
• Electrons pile up until too much
work to do more: get a voltage
(reaction releases a certain amount
of energy)
Chemical reactions
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-
+
-
+
+
0
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Batteries can add or subtract
• 1.5 V battery
• Each battery adds
1.5 V to electrons,
so 3V
• Increase of
energy from one
battery is
decreased by the
other
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+
+
+
0
0
+
+
+
-
+1.5
-
0
+1.5
+
+
+
+
+ +
+1.5
+
+
+
+3
0
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Resistors: energy is extracted from the
moving charges (current)
• Electron collides with atoms in material
• Energy (from voltage) makes atoms move:
HEAT!
• Enough heat ⇒ light
• Resistance: how hard to go through material
- how much heat is produced
• Current = Charge passing through per
second
• 1 Ampere = 1 Coulomb per second
• Must have current to extract energy!
-
-
+
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Circuit elements
• Voltage source: batteries, wall socket,
clouds (for lightning)
• Resistors (extract energy from moving
charges): light bulbs, electrical ovens,
space heaters, …
• Current: movement of charge (electrons)
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What are basic conditions for electric current to flow and
lightbulb to light up?
1. What will happen when hook up battery to flashlight
bulb with one wire?
a. fully light up
b. barely light up
c. not light up
-
+
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What are basic conditions for electric current to flow and
lightbulb to light up?
1. What will happen when hook up battery to flashlight
bulb with one wire?
b. barely light up
c. not light up
++
++
Now excess positive
charges here and all
along wire because
some electrons left
++
a. fully light up
++
-
+
++
++
Answer is c. Does NOT light up. Nowhere for electrons to
flow!!! Electrons in wire attracted to positive end of battery
but just stop when all parts of wire are equally positive.
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What are basic conditions for electric current to flow and
lightbulb to light up?
1. What about hooking up battery to outside of
flashlight bulb?
a. fully light up
b. barely light up
c. not light up
-
+
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What are basic conditions for electric current to flow and
lightbulb to light up?
1. What about hooking up battery to outside of
flashlight bulb?
a. fully light up
b. barely light up
Now excess negative
charges here and all
along wire because
some electrons left
c. not light up
-
+
Answer is c. Does NOT light up. Still no where for
electrons to flow!!! Negative end of battery has whole pile of
electrons crowded together some push into wire but again
stop when all parts of wire equally negative.
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What are basic conditions for electric current to flow and
lightbulb to light up?
1. What will happen when hook up battery to flashlight
bulb with two wires?
a. fully light up
b. barely light up
c. not light up
-
+
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What are basic conditions for electric current to flow and
lightbulb to light up?
1. What will happen when hook up battery to flashlight
bulb with two wires?
a. fully light up b. barely light up
c. not light up
-
+
a) Now electrons can flow, so flow
through wire and filament, heating
filament to cause light.
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Electrons flowing then there is Current (I)
Not flowing, then no current (I=0 amps)
Battery provides energy to push electrons around circuit.
Electrons have to be able to flow all the way around and
back into battery. Otherwise will pile up and push back
(Coulomb's law) preventing any more from flowing.
Show battery-resistor applet
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Voltage Difference is measure of how much energy you need to add
(work) or how much energy is released when a charge moves across
voltage difference.
Change in Electrostatic PE (EPE) = charge x voltage difference = qV
Voltage difference between ends of battery…
1.5 Volts, 9 Volts, 12 Volts
e
e
e
------
Takes energy
Stage 1
++
e ++
++
++
++
Stage 2
Releases energy
What happens to electron’s
EPE as they flow?
a) EPE is always the same.
b) EPE increases during 1 and
decreases during 2.
c) EPE decreases during 1 and
increases during 2.
Answer is b.
Takes energy to move
electron (negative charge)
Releases energy as it
moves back to positive. 17
If I connect the light bulb as shown:
a.
b.
c.
d.
e.
Light will not light up, No current will flow
Light will light up, Current will flow
Light will barely light up, Current will flow
Light will not light up, Current will flow
Light will light up, No current will flow.
d. Light will not light up, Current will flow …
THINK LIKE AN ELECTRON!
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If I connect the light bulb as shown:
a.
b.
c.
d.
e.
Light will not light up, No current will flow
Light will light up, Current will flow
Light will barely light up, Current will flow
Light will not light up, Current will flow
Light will light up, No current will flow.
a. Light will not light up, No current will flow
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What makes light bulb hot and bright?
a. Electrons pass through really fast, so they
are moving fast, produce light.
b. Electrons run into stuff in filament, transfer
energy.
c. Electrons in filament repelled by electrons
flowing through so escape and produce light.
Answer is b. Electrons run into stuff in filament,
collide … EPE energy to kinetic energy .. then turned
into thermal energy … .
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Resistance is measure of how hard it is for
electrons to pass through … how much stuff will
they run into.
What if increase resistance (R) of filament… add
more stuff to hit…
a. Rate at which electrons pass through filament
stays the same
b. Rate at which electrons pass through filament
decreases
c. Rate at which electrons pass through filament
increases
Answer is b. More stuff to run into, average speed is slower, fewer
electrons per time.
What if increase voltage difference (V) across battery …
a. Rate at which electrons pass through filament stays the same
b. Rate at which electrons pass through filament decreases
c. Rate at which electrons pass through filament increases
Answer is c. More EPE released … stronger force pulling around
circuit … higher average speed, more electrons per time.
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Resistance is measure of how hard it is for
electrons to pass through … how much
stuff will they run into.
Voltage difference is measure of how much
EPE is released.
Current is measure of how many electrons
(charges) per second (Units are in coulombs per
second)
Current = I = (Voltage difference)/(Resistance)
I = V/R
OR
V = IR
Ohm’s Law
Resistance in Ohms; Voltage diff in Volts.
Current in Amperes (amps).
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What is the maximum voltage across a
perfect conductor (R=0)?
a)
b)
c)
d)
-1 volts
0 volts
1 volts
Infinity (∞) volts
b) V = IR. So with zero resistance
(R = 0), V has to be zero.
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A 12V car battery drives 50 amps through
a starter motor.
•
a)
b)
c)
d)
What is the resistance?
4.2 Ohms
2.4 Ohms
0.42 Ohms
0.24 Ohms
d) V = IR, so R = V/I = 12V/50 A = 0.24 ohms
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Power consumed by a circuit
• Same voltage, twice the current:
more jousting around, so P ∝ I
(proportional to)
• Same current, twice the voltage:
each electron loses more
energy, so P ∝ V
• Power of an electical circuit
P = IV
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case 1
a. 2 twice as much power as 1
b. same power but runs twice as long
c. much more than twice the power
d. 2 produces no light
case 2
Battery supplies voltage difference … voltage difference = 3 V
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c. much more than twice the power
case 1
case 2
Power = current x voltage
= (charges or electrons per sec) x
(energy released for each electron).
P = I V = (V/R) V = V2/R,
…. V bigger by 2, makes I bigger by 2,
…. so P bigger by 2 x 2 = 4.
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A 12V car battery drives 50 amps through
a starter motor.
•
a)
b)
c)
d)
What is the power?
600 watts
240 watts
0.42 watts
60 watts
a) P = VI, so P = 12V x 50 A = 600 watts
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Summary
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•
•
•
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What is voltage?
What do batteries do?
What do resistors do?
Ohm’s law I=V/R
Power consumption P=IV
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