2/27 Do now
• Sphere A has a charge of -9.0 x 1024 e. Sphere B and sphere C
are neutral. Sphere A is used to charge sphere B by conduction.
Sphere A is used to charge sphere C by induction.
1. What is the order of magnitude of charges in Coulombs on
sphere A?
2. What is the type of charge on sphere B after charging?
3. What is the type of charge on sphere C after charging?
Due:
19.3 notes
Ω Work
1. Castle learning
2. Project – poster or power point on one topic in this
chapter - due 3/3
3. Chapter test on Tues. 3/4 – include some static electricity
questions from last chapter. Extra credit: make your own
Ω joke – due Mon 3/3
4. Unit test – 3/14: Questions from packets
5. Chapter 20-22 reading assignment
6. Project – choose one – due Fri 3/14
– build a simple motor
– build a series or a parallel circuit with mini lights bulbs,
insulated wires, switches and batteries.
Lesson 4: Electrical Power
Know:
• Definition and equation for electrical power.
Understand
• Power is directly proportional to both voltage and current.
Be able to
• Determine power in a system with a single resistor.
• Sketch/interpret graphs of relating voltage; current;
resistance and power with each other (assuming that all
other variables are fixed.)
Power: Putting Charges to Work
Electrical devices, generally referred to as loads, have power ratings.
A 1200 W hair dryer indicates it
transfers 1200 Joules of electrical
energy to heat, wind, sound energy
in 1 second.
Energy
Power 
time
The unit of power is watt.
1 watt = 1 joule / second
• A circuit with a battery and a wire leading from positive to negative
terminal without a load would lead to a high rate of charge flow.
Such a circuit is referred to as a short circuit. It would heat the wires
to a high temperature and drain the battery of its energy rather
quickly.
What About
Watts?
4.2.3B Electrical Power
Power Law
• Moving electrons (current) requires ENERGY
• How much energy gets used depends on:
• Strength of push – VOLTAGE
• Rate of flow – CURRENT
V2
PI VIV
2
P  IV  R  I R
R
Example #1
• A 12 volt battery is connected to a circuit which
allows 10 amperes of current to flow.
– What is the power output of this circuit?
P = IV
P = (12 V)(10 A)
P = 120 W
Example #2
• A 100 watt light bulb is connected to a 120 volt power
supply.
– What amount of current must pass through the light bulb?
P = IV
100 W = (120 V) I
I = 0.833 A
Example #3
• A 2.0 ampere current passes through a circuit with a
300 ohm resistance.
– What is the power generated in this circuit?
P = I2 R
P = (2.0 A)2 (300 Ω)
P = 1200 W or 1.2 kW
P = I2•R
P = V2/R
P = V·I
relate current and resistance to power, notice double
importance of current. Unit: A2∙Ω
relate potential difference and resistance to power,
notice double importance of potential difference.
Unit: V2/Ω
relate potential difference and current to power.
Notice that both have equal importance. Unit: V∙A
Warning:
While these three equations provide one with
convenient formulas for calculating unknown
quantities in physics problems, one must be careful
to not misuse them by ignoring conceptual
principles regarding circuits.
Check your understanding
1. If a 60-watt bulb in a household lamp was replaced
with a 120-watt bulb, then how many times greater
would the current be in that lamp circuit?
Check your understanding
2.
a.
b.
c.
d.
Which is a unit of electrical power?
volt/ampere
ampere/ohm
ampere2/ohm
volt2/ohm
Graphs of power vs. R, I, V
• P = VI = I2R = V2/R
• When V is constant: P = VI; P = V2/R – common house hold
appliances
P
P
Inverse, high R, low P
V is slope
R
I
• When R is constant: P = I2R; P = V2/R – same appliances
P
P
Direct squared
I
Direct squared
V
Check your understanding
3. As the resistance of a constant-voltage circuit
is increased, the power developed in the
circuit
a. decreases
b. increases
c. remains the same
Check your understanding
4. The potential difference applied to a circuit element remains
constant as the resistance of the element is varied. Graph power
(P) vs. resistance (R) for this circuit.
P
R
Check your understanding
5. Graph the relationship between the electrical
power and the current in a resistor that
obeys Ohm’s Law.
P
I
Check your understanding
6. An electric motor uses 15 amperes of current
in a 440-volt circuit to raise an elevator
weighing 11,000 Newtons. What is the
average speed attained by the elevator?
example
7. To increase the brightness of a desk lamp, a
student replaces a 60-watt light bulb with a
100-watt bulb. Compared to the 60-watt
bulb, the 100-watt bulb has
a. less resistance and draws more current
b. less resistance and draws less current
c. more resistance and draws more current
d. more resistance and draws less current
2/28 Do now
• A lithium ion has 3 protons and 4 neutrons. What is the
charge in coulombs of the lithium ion?
Ω Work
1. Castle learning
2. Project – poster or power point on one topic in this
chapter - due 3/3
3. Chapter test on Tues. 3/4 – include some static electricity
questions from last chapter. Extra credit: make your own
Ω joke – due Mon 3/3
4. Unit test – 3/14: Questions from packets
If there is no school Monday, additional castle learning
will be assigned. Chapter test will still be on Tuesday!
Check Your Understanding
8. Which would be thicker (wider) - the filament of a 60-Watt
light bulb or the filament of a 100-W light bulb? Explain.
9. Calculate the resistance and the current of a 7.5-Watt night
light bulb plugged into a US household outlet (120 V).
Electrical energy
• E = P∙t = V∙I∙t = I2∙R∙t = (V2/R)∙t
• The SI unit for energy is Joule.
• 1 joule = (1 Newton)(1 meter)
= (1 kg∙m/s2)(1 meter)
= 1 kg∙m2/s2
The kilowatt-hour
• Electrical utility companies provide energy for homes
charge those homes for the electrical energy they
used. A typical bill will contain a charge for the
number of kilowatt-hours of electricity which were
consumed.
• How many Joules is in one kWh?
Check your understanding
1. Your 60-watt light bulb is plugged into a 110-volt household
outlet and left on for 10 hours. The utility company charges
you $0.20 per kWh. What is the cost?
2. A current of 0.40 ampere is measured in a 150 ohm resistor,
how much energy is expended by the resistor in 20. seconds?
3. An electric dryer consumes 6.0 × 106 joules of energy when
operating at 220 volts for 30. minutes. During operation, how
much current does the dryer draws approximately?
Energy can be transformed, but is conserved
• The purpose of every circuit is to supply the energy
to operate various electrical devices. These devices
are constructed to convert the energy of flowing
charge into other forms of energy (e.g., light,
thermal, sound, mechanical, etc.). Use complete
sentences to describe the energy conversions that
occur in the following devices.
4. Windshield wipers on a car
5. Defrosting circuit on a car
6. Hair dryer
Rechargeable Batteries
• Rechargeable batteries has nothing to do with charges.
• Rechargeable batteries rely upon a reversible reaction, turning the
chemical products back into chemical reactants within the cell.
Alert: Statement True or False?
7. When an electrochemical cell no longer works, it is out of charge
and must be recharged before it can be used again.
8. An electrochemical cell can be a source of charge in a circuit. The
charge which flows through the circuit originates in the cell.
9. Charge becomes used up as it flows through a circuit. The
amount of charge which exits a light bulb is less than the amount
which enters the light bulb.
10. Charge flows through circuits at very high speeds. This explains
why the light bulb turns on immediately after the wall switch is
flipped.
11. The local electrical utility company supplies millions and millions
of electrons to our homes everyday.
Example 12
• A 12.0-meter length of copper wire has a
resistance of 1.50 ohms. How long must an
aluminum wire with the same cross-sectional
area be to have the same resistance?
Example 13
• Calculate the resistance of a 1.00-kilometer
length of nichrome wire with a cross-sectional
area of 3.50 × 10-6 meter2 at 20°C.
3/3 do now
• A clothes dryer connected to a 240-volt line
draws 30. amperes of current for 20. minutes
(1,200 seconds). Approximately how much
electrical energy is consumed by the
dryer?[show work]
Due:
• Project
• Ω joke
Ω work
• Packet 4.2.1-4.2.3
Test Tomorrow
objectives
Presentation of project
Regents Review book pp. 112-113 #29-48
• Due today
Finish lab 15
3/4 objectives
Presentation of project
Regents Review book pp. 112-113 #29-48
• Due today
Finish lab 15
3/5 objectives
• Due: packet 4.2.1-4.2.3
• Test
• Homework – 20.1 reading questions due
tomorrow
• Finish Regents Review book pp. 112-113 #29-48
• Finish practice in note packet
• Finish lab 15
Quantities, Symbols, Equations and Units!
• The tendency to give attention to units is an
essential trait of any good physics student.
• Many of the difficulties associated with
solving problems may be traced back to the
failure to give attention to units. As more and
more electrical quantities and their respective
metric units are introduced, it will become
increasingly important to organize the
information in your head.
Quantity
Symbol
Equations
Standard
Metric
Unit
Potential Difference
(a.k.a. voltage)
V
V= W / Q
V=I•R
Volt (V)
J/C
Current
I
I=Q/t
I=V/R
Amperes (A)
C/s
V/Ω
Power
P
P=W/t
P = V∙I
P = V2/R
P = I2 R
Watt (W)
J/s
V∙A
V/ Ω2
A2∙Ω
Resistance
R
R = ρ•L / A
R=V/I
Ohm (Ω )
V/A
Energy
W
W=V•Q
W=P•t
Joule (J)
V•C
W•s
Other
Units
Lab 15 – Resistance
PURPOSE:
1. Determine the relationship between Resistance and the length of the wire
2. Determine the relationship between Resistance and the area of the wire
3. Determine resistivity of the wire
MATERIAL:
• Nichrome wire boards, multipurpose meter, ruler, graph paper
DATA:
diameter _________ m
Area __________m2
Length (m)
R (Ω)
L (m)
Resistance ∙Area
(Ω∙m2)
Length _________ m
R (Ω)
Area (m2)