Transcript Physics First Ch 20 ppt

CPO Science
Foundations of Physics
Unit 7, Chapter 20
Unit 7: Electricity and Magnetism
Chapter 20 Electric Circuits and Power
 20.1 Series and Parallel Circuits
 20.2 Analysis of Circuits
 20.3 Electric Power, AC, and DC Electricity
Chapter 20 Objectives
1. Recognize and sketch examples of series and parallel
circuits.
2. Describe a short circuit and why a short circuit may
be a dangerous hazard.
3. Calculate the current in a series or parallel circuit
containing up to three resistances.
4. Calculate the total resistance of a circuit by
combining series or parallel resistances.
5. Describe the differences between AC and DC
electricity.
6. Calculate the power used in an AC or DC circuit from
the current and voltage.
Chapter 20 Vocabulary Terms
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series circuit
parallel circuit
short circuit
network circuit
circuit analysis
power
Kirchhoff’s voltage law
voltage drop direct
current (DC)
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alternating current (AC)
kilowatt
Kirchhoff’s current law
horsepower
power factor
circuit breaker
watt
kilowatt-hour
20.1 Series and Parallel Circuits
Key Question:
How do series and parallel circuits work?
*Students read Section 20.1 AFTER Investigation 20.1
20.1 Series and Parallel Circuits
 In series circuits, current can only take one path.
 The amount of current is the same at all points in
a series circuit.
20.1 Adding resistances in series
 Each resistance in a series
circuit adds to the total
resistance of the circuit.
Rtotal = R1 + R2 + R3...
Total resistance
(ohms)
Individual resistances (W)
20.1 Total resistance in a series circuit
 Light bulbs, resistors, motors, and heaters usually
have much greater resistance than wires and
batteries.
20.1 Calculate current
 How much current flows in a circuit with a 1.5-volt
battery and three 1 ohm resistances (bulbs) in series?
20.1 Voltage in a series circuit
 Each separate resistance creates
a voltage drop as the current
passes through.
 As current flows along a series
circuit, each type of resistor
transforms some of the electrical
energy into another form of
energy
 Ohm’s law is used to calculate
the voltage drop across each
resistor.
20.1 Series and Parallel Circuits
 In parallel circuits the current can take more than one
path.
 Because there are multiple branches, the current is not
the same at all points in a parallel circuit.
20.1 Series and Parallel Circuits
 Sometimes these paths are called branches.
 The current through a branch is also called the branch
current.
 When analyzing a parallel circuit, remember that the
current always has to go somewhere.
 The total current in the circuit is the sum of the currents
in all the branches.
 At every branch point the current flowing out must equal
the current flowing in.
 This rule is known as Kirchhoff’s current law.
20.1 Voltage and current
in a parallel circuit
 In a parallel circuit the voltage is the same across
each branch because each branch has a low
resistance path back to the battery.
 The amount of current in each branch in a parallel
circuit is not necessarily the same.
 The resistance in each branch determines the
current in that branch.
20.1 Advantages of parallel circuits
Parallel circuits have two big advantages over
series circuits:
1. Each device in the circuit sees the full battery
voltage.
2. Each device in the circuit may be turned off
independently without stopping the current flowing
to other devices in the circuit.
20.1 Short circuit
 A short circuit is a parallel path in a circuit with zero or
very low resistance.
 Short circuits can be made accidentally by connecting
a wire between two other wires at different voltages.
 Short circuits are dangerous because they can draw
huge amounts of current.
20.1 Calculate current
 Two bulbs with different resistances are connected in
parallel to batteries with a total voltage of 3 volts.
 Calculate the total current supplied by the battery.
20.1 Resistance in parallel circuits
 Adding resistance in parallel provides another
path for current, and more current flows.
 When more current flows for the same voltage,
the total resistance of the circuit decreases.
 This happens because every new path in a
parallel circuit allows more current to flow for the
same voltage.
20.1 Adding resistance in parallel
circuits
 A circuit contains a 2 ohm resistor and a 4
ohm resistor in parallel.
 Calculate the total resistance of the circuit.
20.2 Analysis of Circuits
Key Question:
How do we analyze
network circuits?
*Students read Section 20.2
AFTER Investigation 20.2
20.2 Analysis of Circuits
 All circuits work by manipulating currents and
voltages.
 The process of circuit analysis means figuring out
what the currents and voltages in a circuit are,
and also how they are affected by each other.
 Three basic laws are the foundation of circuit
analysis.
20.2 Three circuit laws
20.2 Voltage divider circuit
20.2 Voltage divider
 A circuit divides any supplied voltage by a ratio of
the resistors.
V0 =
Output
voltage
(volts)
R1
Vi
R1 + R2
resistor ratio
(W)
Input
voltage
(volts)
20.2 Solving circuit problems
1. Identify what the problem is asking you to find.
Assign variables to the unknown quantities.
2. Make a large clear diagram of the circuit. Label all
of the known resistances, currents, and voltages.
Use the variables you defined to label the
unknowns.
3. You may need to combine resistances to find the
total circuit resistance. Use multiple steps to
combine series and parallel resistors.
20.2 Solving circuit problems
4. If you know the total resistance and current, use
Ohm’s law as V = IR to calculate voltages or
voltage drops. If you know the resistance and
voltage, use Ohm’s law as I = V ÷ R to calculate
the current.
5. An unknown resistance can be found using
Ohm’s law as R = V ÷ I, if you know the current
and the voltage drop through the resistor.
6. Use Kirchhoff’s current and voltage laws as
necessary.
20.2 Solving circuit problems
 A bulb with a resistance of 1Ω is to be
used in a circuit with a 6-volt battery.
 The bulb requires 1 amp of current.
 If the bulb were connected directly to
the battery, it would draw 6 amps and
burn out instantly.
 To limit the current, a resistor is
added in series with the bulb.
 What size resistor is needed to make
the current 1 amp?
20.2 Network circuits
 In many circuits, resistors are connected both in
series and in parallel.
 Such a circuit is called a network circuit.
 There is no single formula for adding resistors in a
network circuit.
 For very complex circuits, electrical engineers use
computer programs that can rapidly solve
equations for the circuit using Kirchhoff’s laws.
20.2 Calculate using network circuits
 Three bulbs, each with a
resistance of 3Ω, are combined
in the circuit in the diagram
 Three volts are applied to the
circuit.
 Calculate the current in each of
the bulbs.
 From your calculations, do you
think all three bulbs will be
equally bright?
20.3 Electric Power, AC, and DC
Electricity
Key Question:
How much does
electricity cost and what
do you pay for?
*Students read Section 20.3
AFTER Investigation 20.3
20.3 Electric Power, AC, and DC
Electricity
 The watt (W) is a unit of power.
 Power is the rate at which
energy moves or is used.
 Since energy is measured in
joules, power is measured in
joules per second.
 One joule per second is equal
to one watt.
20.3 Reviewing terms
20.3 Power in electric circuits
 One watt is a pretty small amount of power.
 In everyday use, larger units are more convenient
to use.
 A kilowatt (kW) is equal to 1,000 watts.
 The other common unit of power often seen on
electric motors is the horsepower.
 One horsepower is 746 watts.
20.3 Power
Voltage (volts)
Power (watts)
P = VI
Current (amps)
20.3 Calculate power
 A light bulb with a
resistance of 1.5Ω is
connected to a 1.5-volt
battery in the circuit
shown at right.
 Calculate the power used
by the light bulb.
20.3 Paying for electricity
 Electric companies charge for
the number of kilowatt-hours
used during a set period of time,
often a month.
 One kilowatt-hour (kWh) means
that a kilowatt of power has been
used for one hour.
 Since power multiplied by time is
energy, a kilowatt-hour is a unit
of energy.
 One kilowatt-hour is 3.6 x 106
joules.
20.3 Calculate power
 Your electric company charges 14 cents per
kilowatt-hour. Your coffee maker has a power rating
of 1,050 watts.
 How much does it cost to use the coffee maker one
hour per day for a month?
20.3 Alternating and direct current
 The current from a battery is
always in the same direction.
 One end of the battery is
positive and the other end is
negative.
 The direction of current
flows from positive to
negative.
 This is called direct current,
or DC.
20.3 Alternating and direct current
 If voltage alternates, so does
current.
 When the voltage is positive,
the current in the circuit is
clockwise.
 When the voltage is negative
the current is the opposite
direction.
 This type of current is called
alternating current, or AC.
20.3 Alternating and direct current
 AC current is used for almost all high-power
applications because it is easier to generate and
to transmit over long distances.
 The 120 volt AC (VAC) electricity used in homes
and businesses alternates between peak values
of +170 V and -170 V at a frequency of 60 Hz.
 AC electricity is usually identified by the average
voltage, (120 VAC) not the peak voltage.
20.3 Power in AC circuits
 For a circuit containing a
motor, the power calculation
is a little different from that
for a simple resistance like a
light bulb.
 Because motors store
energy and act like
generators, the current and
voltage are not in phase
with each other.
 The current is always a little
behind the voltage.
20.3 Power for AC circuits
 Electrical engineers use a power factor (pf) to
calculate power for AC circuits with motors
Avg. voltage
(volts)
Power (watts)
Avg. current (amps)
P = VI x pf
power factor
0-100%
Application: Wiring in Homes and
Buildings
Application: Wiring in Homes and
Buildings