CPO Science
Foundations of Physics
Unit 7, Chapter 19
Unit 7: Electricity and Magnetism
Chapter 19 Electricity
 19.1 Electric Circuits
 19.2 Current and Voltage
 19.3 Electrical Resistance and Ohm’s Law
Chapter 19 Objectives
1. Describe the difference between current and voltage.
2. Describe the connection between voltage, current,
energy, and power.
3. Describe the function of a battery in a circuit.
4. Calculate the current in a circuit using Ohm’s law.
5. Draw and interpret a circuit diagram with wires,
battery, bulb, and switch.
6. Measure current, voltage, and resistance with a
multimeter.
7. Give examples and applications of conductors,
insulators, and semiconductors.
Chapter 19 Vocabulary Terms
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electricity
electric current
voltage
resistance
Ohm’s law
battery
open circuit
closed circuit
short circuit
 switch
 circuit diagram
 electrical
conductivity
 potentiometer
 wire
 volt
 electrical symbols
 amperes (amps)
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multimeter
ohm
resistor
ammeter
electrical
insulator
 semiconductor
 conductor
 electric circuit
19.1 Electric Circuits
Key Question:
What is an electric circuit?
*Students read Section 19.1 AFTER Investigation 19.1
19.1 Electric Circuits
 Electricity refers to the
presence of electric current
in wires, motors, light bulbs,
and other devices.
 Electric current is similar to
a current of water, but
electric current flows in solid
metal wires so it is not
visible.
 Electric current can carry a
lot of power.
19.1 Electric Circuits
 An electric circuit is something that provides a
complete path through which electricity travels.
 Wires in electric circuits are similar in some ways to
pipes and hoses that carry water.
19.1 Electric Circuits
 When drawing a circuit diagram, symbols are used to
represent each part of the circuit.
 These electrical symbols are quicker and easier to draw
than realistic pictures of the components.
19.2 Current and Voltage
Key Question:
How does current move
through a circuit?
*Students read Section 19.2
AFTER Investigation 19.2
19.2 Current and voltage
 Electric current is measured in
units called amperes, or amps
(A) for short.
 One amp is a flow of a certain
quantity of electricity in one
second.
 The amount of electric current
entering a circuit always equals
the amount exiting the circuit.
19.2 Current and voltage
 Conventional current was
proposed by Ben Franklin in the
1700’s.
 Scientists later discovered that
the particles that carry electricity
in a wire actually travel from
negative to positive.
 Today, we still use Franklin’s
definition.
19.2 Voltage
 Voltage is a measure of electric
potential energy, just like height
is a measure of gravitational
potential energy.
 Voltage is measured in volts (V).
 A voltage difference of 1 volt
means 1 amp of current does 1
joule of work in 1 second.
19.2 Voltage
 Since 1 joule per second
is a watt (power), you
can interpret voltage as
measuring the available
electrical power per amp
of current that flows.
19.2 Voltage
 The positive end of a 1.5 volt
battery is 1.5 volts higher than
the negative end.
 If you connect batteries
positive-to-negative, each
battery adds 1.5 volts to the
total.
 Three batteries make 4.5 volts.
 Each unit of current coming
out of the positive end of the
three-battery stack has 4.5
joules of energy.
19.2 Measuring voltage of a cell
 Set the meter to DC volts.
 Touch the red (+) lead of
the meter to the (+) battery
terminal.
 Touch the black (-) lead of
the meter to the (-) battery
terminal.
 Adjust the meter dial as
necessary.
19.2 Measuring voltage in a circuit
 Measure the voltage
across the battery
exactly as before.
 DO NOT DISCONNECT
THE CIRCUIT.
NOTE: Since voltage is measured from
one point to another, we usually assign the
negative terminal of a battery to be zero
volts (0 V).
19.2 Current and voltage
 A battery uses chemical
energy to create a voltage
difference between its two
terminals.
 In a battery, chemical
reactions provide the
energy to pump the current
from low voltage to high
voltage.
 A fully charged battery adds
energy proportional to its
voltage.
19.2 What does a battery do?
 A battery uses chemical energy to move charges.
 If you connect a circuit with a battery the charges
flow out of the battery carrying energy.
19.2 How do these batteries differ?
 Some are smaller and don't store as much energy.
 Other batteries made with Ni and Cd can be recharged.
 Which battery above has the greatest voltage capacity?
19.2 Measuring Current
 In practical electricity,
we still label current
flowing from plus to
minus or HIGH voltage
to LOW voltage.
 Current can't be
measured unless the
charges flow through
the meter.
19.2 Current is a flow of charge
19.3 Electrical Resistance and Ohm’s
Law
Key Question:
How are voltage, current, and resistance related?
*Students read Section 19.3 AFTER Investigation 19.3
19.3 Electrical resistance
 Resistance measures how difficult it is for
current to flow.
19.3 Electrical Resistance
 The total amount of electrical resistance in a circuit
determines the amount of current that in the circuit for
a given voltage.
 The more resistance the circuit has, the less current
that flows.
19.3 Measuring resistance
 Set the meter to
measure resistance (W).
 Set the black and red
leads on opposite ends
of the objects.
19.3 The ohm
 Resistance is
measured in ohms
(W).
 One ohm is the
resistance when a
voltage of 1 volt is
applied with a
current of 1 amp.
19.3 Ohm's law
 German physicist Georg Ohm
experimented with circuits to
find an exact mathematical
relationship between voltage,
current and resistance.
 Ohm's law can be used to
predict any one of the three
variable if given the other
two.
19.3 Calculate current
 A light bulb with a resistance of 2 ohms is connected
in a circuit that has a single 1.5-volt battery.
 Calculate the current that flows in the circuit.
 Assume the wires have zero resistance.
19.3 The resistance of electrical devices
 The resistance of
electrical devices ranges
from very small (0.001 Ω)
to very large (10×106 Ω).
 Each device is designed
with a resistance that
allows the right amount
of current to flow when
connected to the voltage
the device was designed
for.
19.3 Changing resistance
 The resistance of many
materials, including those
in light bulbs, increases
as temperature increases.
 A graph of current versus
voltage for a light bulb
shows a curve.
 A device with constant
resistance would show a
straight line on this
graph.
19.3 Electrical Conductivity
 The electrical conductivity describes a material’s
ability to pass electric current.
19.3 Conductors and insulators
 A material such as copper is
called a conductor because it
can conduct, or carry, electric
current.
 Materials that insulate against
(or block) the flow of current
are classified as electrical
insulators.
 Some materials are neither
conductors nor insulators.
 These materials are named
semiconductors.
19.3 Resistors
 Electrical components called
resistors can be used to
control current.
 Resistors have striped color
codes to record their "values"
(writing on them is difficult).
19.3 Potentiometers
 Potentiometers are a type of "variable" resistor that
can change from low to high.
 They are wired so that as you turn the knob, it
changes the distance the current has to flow.
Application: Hybrid Gas/Electric Cars