Download Physics 1010: The Physics of Everyday Life

Physics 1010:
The Physics of Everyday Life
TODAY
• Circuits-2
Circuits in parallel and in Series
1
Homework Hint
• Many calculators will do arithmetic in
unexpected sequence
• Use a simple calculator that does arithmetic in
the sequence you think it does
• If your calculator does not have scientific
notation, do the arithmetic of the exponents
by hand:
(1.3x1012)x(2.8x10-18) = (1.3 x 2.8) x (1012 x 10-18)=
3.6x10-6
2
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)
3
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
4
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
-
-
+
-
+
+
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 (more later) 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!
-
-
+
Demo resistor applet
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Electrons carry charge around in a wire
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
CURRENT is the amount of charge going through a
cross-section of the wire per unit time
Ampere is the unit of current: 1A = 1C/s
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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
-
+
8
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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- - - - -- - - - - - -
-
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-
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-
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-
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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 (IN A CIRCUIT). Otherwise will pile up
and push back (Coulomb's law) preventing any more from
flowing.
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How long before the bulb lights up?
A) Instantly (time delay = 0)
B) When the electrons that left the battery
get to the bulb, so they can deposit their
EPE
C) Some other time
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How long before the bulb lights up?
A) Instantly (time delay = 0)
B) When the electrons that left the battery
get to the bulb, so they can deposit their
EPE
C) Some other time
Answer: C) The time it takes the electric field
to get to the bulb; the electric field signal
travels with the speed of light in the wire
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- - - - -- - - - - - -
-
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-
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-
-
-
-
-
-
Electrons flowing then there is Current (I)
Not flowing, then no current (I=0 amps)
Like sound:
SIGNAL travels MUCH FASTER than actual electrons
Remember TIP O’NEIL: WHEN PUSH COMES TO SHOVE,
ALL PHYSICS IS LOCAL.
Bulb lights up when the SIGNAL (the “pushing”) gets there,
NOT when the electrons get there
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Voltage Difference is a measure of how much energy is added or
released when a charge moves across that 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
------
battery
Stage 1
++
e ++
++
++
++
Stage 2
resistor
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Voltage Difference is a measure of how much energy is added or
released when a charge moves across that 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 electrons’
EPE as they flow?
a) EPE is always the same.
b) EPE increases during 1;
EPE decreases during 2.
c) EPE decreases during 1;
EPE increases during 2.
Answer is b.
Takes energy to move
electron (negative charge)
Release energy as moves
back to positive.
19
Resistance is measure of how hard it is for
electrons to pass through … how much
stuff will they run into.
Voltage difference is a 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).
20
Resistance is measure of how hard it is for
electrons to pass through … how much
stuff will they run into.
VOLTAGE DIFFERENCE IS A 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).
21
What is the maximum voltage across a
perfect conductor (R=0)?
a)
b)
c)
d)
-1 volts
0 volts
1 volts
Infinity (∞) volts
22
What is the maximum voltage across a
perfect conductor (R=0)?
a)
b)
c)
d)
-1 volts
0 volts
1 volts
Infinity (∞) volts
b) R=0 means no energy can be
extracted, so no voltage.
Ohm’s law: V = IR
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Ohm’s law: V = IR
A 120 V battery is connected to 240 Ohm of
resistance. How much current flows?
a) 120 V
b) 120 A
c) 240 A
d) 0.5 A
d) 0.5 A, because I = V/R = 120V/240 Ohm = 0.5 A
24
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
25
Power consumed by a circuit
• Same voltage, twice the current: more
electrons depositing energy, so P ∝ I
(proportional to)
• Same current, twice the voltage: each
electron loses more energy, so P ∝ V
• Power of an electrical 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.
28
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
29
The imaginary flowing positive charge
• Current = flow of
charge
• Negative charges
going to right has
same effect as
positive charges to
left
• Current =
pos charge/sec
- neg. charge/sec
+ + -
+
+
-
-
+
+
+
+
-
+ + -
Done
Usually “think” of positive charges
flowing
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Power = VI
• Ends up dissipated as heat
• More power with more voltage, as each electron
loses more electrostatic potential energy
• More power with more current, as more charges
losing potential energy
• Other formulas (Ohm’s Law: V=IR)
 P = VI
 P = V(V/R) = V2/R
 P = (IR) I = I2R
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Parallel: voltages the same, but more total
current
• Same voltage
• Each resistor has
same current as
before
• Twice the power,
but in two devices
-
-
-
-
-
-
-
Electrons going
through both resistors
combine
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Series: voltages add
• Battery: electrons
gain energy with
each unit
• Resistor:
electrons lose
energy with each
unit
-
+
-
-
Electron gains energy
twice, so twice the
voltage
-
-
-
+
-
Chemical to electrostatic energy
-
-
-
-
-
Electrostatic energy to heat or light
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Series: current the same
• Battery: electrons
out one end of
battery go into
other end of
other battery
• Same with
resistors
-
+
-
-
Electron gains energy
twice, so twice the
voltage
-
-
-
+
-
Chemical to electrostatic energy
-
-
-
-
-
Electrostatic energy to heat or light
34
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
-
+
+
+
0
0
+
+
+
-
+1.5
-
0
+1.5
+
+
+
+
+ +
+1.5
+
+
+
+3
0
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Shorting a circuit
• Circuit operates safely with
given resistance
• Shorting, causes resistance
to go way down - just that
of wires
• Current through wires goes
way up
36
Electrocution dangers
• What kills
• What does it take
37
Water as a conductor
• Pure water not conductor
• Ions (salts) in water act as free charges
that move around
• Acid battery (car) ions migrate to
terminals, chemical reaction releases
electrons
38
Electrical resistance of human body
depends on wetness
• Dry skin: 500,000 Ohms
• Wet skin: 1,000 Ohms, sometimes less
• Under the skin, through blood (salty
water): 100 Ohms
• With dry hands I grab the ends of 1.5 V
battery. How much current flows through
me?
V=IR => I=V/R=1.5/500000 A=3x10-6 A
39
Electricity effects increase with current
•
•
•
•
a)
b)
c)
0.001 A = 1 milliamp: feel
0.01 A = 10 milliamps: muscles contract
0.1 A = 100 milliamps: stop heart
I push two probes through my skin so that the
resistance between them (internal body
resistance) is 100 Ohms. How much voltage is
required to kill me?
1V
10 V
100 V
b) 10 V. V = IR =
0.1 A x 100 Ohms = 10 V
40
DO NOT DO THIS: A human places two
wires into a wall socket (120V), holding
one in each hand
•
His hands are dry (500,000 ohms). Does
he (typically) die?
a) Yes
b) No
b) No. Current is 120
V/500,000 ohms = 0.0002
amps.
41
DO NOT DO THIS: A human places two
wires into a wall socket (120V), holding
one in each hand
•
His hands are wet (1000 ohms). Does he
(typically) die?
a) Yes
b) No
a) Yes. Current is 120 V/1,000
ohms = 0.12 amps.
42
Power to a house at high and low voltage
House uses 1,000 Watts average
1,000 ft or 10 Ga. Copper wire is 1 ohm
• US
120V from wall
I = P/V = 8.3 A
Power lost in wire is
I2R = 8.3x8.3x1 = 69
W (or 7%)
• Europe
220V from wall
I = P/V =4.5 A
Power lost in wire is
I2R = 4.5x4.5x1 = 20.5
W (or 2%)
With higher voltage,
less power lost in
wires
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Summary
•
•
•
•
•
•
What is voltage?
What do batteries do?
What do resistors do?
Ohm’s law I=V/R
Power consumption P=IV
Circuit in parallel: same voltage, currents
add up
• Circuit in series: same current, voltages
add up
44