Download Chapter 20

Electric Charge
• Electric charge, like mass, is
also fundamental property
of matter.
• Inside atoms found in
matter, attraction between
positive and negative
charges holds the atoms
together.
Electric Charge
• Virtually all the matter
around you has electric
charge because atoms are
made of electrons and
protons (and neutrons).
• Because ordinary matter has
zero net (total) charge, most
matter acts as if there is no
electric charge at all.
Electrostatic Attraction and
Repulsion
• Opposites attract.
• Likes repel.
4
Electrical Forces
• The unit of charge is the coulomb (C).
• The name was chosen in honor of
Charles Augustin de Coulomb (17361806), the French physicist who
performed the first accurate
measurements of the force between
charges.
Electrical Forces
• Electric forces are
incredibly strong.
• A millimeter cube
of carbon the size
of a pencil point
contains about 77
coulombs of
positive and
negative charge.
Electrical Forces
Example:
• Lightning is caused by a giant
buildup of static charge.
• The cloud, air, and ground can act
like a giant circuit.
• All the accumulated negative
charges flow from the cloud to the
ground, heating the air along the
path (to as much as 20,000°C) so
that it glows like a bright streak of
light.
Electric Current
• Electric current is caused by moving
electric charge.
• Electric current comes from the motion of
electrons.
current
Electric Current
• Electric current is similar in
some ways to a current of
water.
• Like electric current, water
current can carry energy and do
work.
• Example: A waterwheel turns
when a current of water exerts
a force on it.
Electric Current
• Electricity can flow through conductors.
– Examples:
.
• Insulators resist the flow of electricity.
– Examples:
.
Insulator or Conductor?
Which are insulators & which are conductors?
• Wood
• Metal
• Water
• Air
• Plastic
Electric Circuits
• An electric circuit is a complete path through which
electric current travels.
• A good example of a circuit is the one found in an
electric toaster.
Electric Circuits
• Wires in electric circuits are similar in some ways to
pipes and hoses that carry water.
Electric Circuits
• When drawing a circuit diagram, symbols are used to
represent each part of the circuit.
Electric Circuits
• Electrical symbols are
quicker and easier to draw
than realistic pictures of
the components.
Resistors
• A resistor is an electrical device that uses the energy
carried by electric current in a specific way.
• Any electrical device that uses energy can be shown
with a resistor symbol.
• Example: Light bulb, hair dryer, or space heater
Current in a Circuit
• Current only flows
when there is a
complete and
unbroken path, or a
closed circuit.
• Flipping a switch to the
“off” position creates
an open circuit by
making a break in the
wire.
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.
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.
Voltage
• A difference in voltage provides the energy
that causes current to flow.
Voltage
• A useful meter is a multimeter, which can measure
voltage or current, and sometimes resistance.
• To measure voltage, the meter’s probes are touched to
two places in a circuit or across a battery.
Batteries
• A pump is like a battery because it brings
water from a position of low energy to
high energy.
Batteries
• A battery uses stored chemical
energy to create the voltage
difference.
• Example: Three 1.5-volt
batteries can be stacked to
make a total voltage of 4.5
volts in a flashlight.
Measuring Current
• If
you want to measure
current, you must force the
current to pass through the
meter.
• Multimeters can measure
two types of current:
alternating current (AC) and
direct current (DC).
Resistance
• Resistance is the measure
of how strongly an object
resists current flowing
through it.
• The relationship between
electric current and
resistance can be
compared with water
flowing from the open end
of a bottle.
Resistance
• The total amount of resistance in a circuit
determines the amount of current in the
circuit for a given voltage.
Resistance
• Electrical resistance is measured in units
called ohms.
• This unit is abbreviated with the Greek
letter omega (Ω).
Ohm’s Law
• The current in a circuit depends on
voltage and resistance.
• Ohm’s law relates current, voltage, and
resistance with one formula.
Resistance of common objects
• Every electrical device
is designed with a
resistor that causes
the right amount of
current to flow when
the device is
connected to voltage.
Resistance of common objects
• The resistance of many
electrical devices varies with
temperature and current.
• Example: A light bulb’s
resistance increases when
there is more current
because the bulb gets hotter
when more current passes
through it.
Ohm’s Law
• If you know two of the
three quantities, you can
use Ohm’s law to find the
third.
V
I
R
Calculating I from Ohm's Law
Ohm's Law:
I=V/R
I = (12 volts) / 3 ohms
= 4 amperes
37
Calculating V from Ohm's Law
Ohm's Law:
V=IR
V = (6 amperes)(3 ohms)
= 18 volts
38
Calculating R from Ohm's Law
Ohm's Law:
R=V/I
R=(36 volts)/(6 amps)
= 6 ohms
39
Solving Problems
• A toaster oven has a resistance of 12 ohms
and is plugged into a 120-volt outlet.
• How much current does it draw?
Solving Problems
1. Looking for: current in amps
2. Given: R = 12 ; V = 120 V
3. Relationships: I = V
R
4. Solution:
I = (120 V) / (12 )
I = 10 A
• EOC page 497-498 Concepts 9, 10, 11,
15, 16, 17, 18, and Problems 1, 2, 4, 5, 6.
1. How much current is in a circuit that includes a 9volt battery and a bulb with a resistance of 3 ohms?
• 9 v/ 3  = 3 amps
2. How much current is in a circuit that includes a 9volt battery and a bulb with a resistance of 12
ohms?
• 9 v/ 12  = 0.75 amps
3. A circuit contains a 1.5 volt battery and a bulb with
a resistance of 3 ohms. Calculate the current.
• 1.5 v / 3  = 0.5 amps
4. A circuit contains two 1.5 volt batteries and a
bulb with a resistance of 3 ohms. Calculate the
current.
• (1.5 v + 1.5 v) / 3  = 1 amps
5. What is the voltage of a circuit with 15 amps of
current and toaster with 8 ohms of resistance?
• 15 amps x 8  = 120 volts
6. A light bulb has a resistance of 4 ohms and a current
of 2 A. What is the voltage across the bulb?
• 2 amps x 4  = 8 volts
7. How much voltage would be necessary to generate 10
amps of current in a circuit that has 5 ohms of resistance?
• 10 amps x 5  = 50 volts
8. How many ohms of resistance must be present in a
circuit that has 120 volts and a current of 10 amps?
• 120 volts / 10 amps = 12 
9. An alarm clock draws 0.5 A of current when connected
to a 120 volt circuit. Calculate its resistance.
• 120 volts / 0.5 amps = 240 
10. A portable CD player uses two 1.5 V batteries. If the
current in the CD player is 2 A, what is its resistance?
• (1.5 volts +1.5 volts) / 2 amps = 1.5 
11. You have a large flashlight that takes 4 D-cell batteries.
If the current in the flashlight is 2 amps, what is the
resistance of the light bulb? (Hint: A D-cell battery has 1.5
v.)
• (1.5 volts +1.5 volts + 1.5 volts +1.5 volts) =
6v
• 6 volts / 2 amps = 3 
12. Use the diagram below to answer the following
problems.
• a. What is the total voltage in each
circuit?
– A = 6 volts
– B = 6 volts + 6 volts = 12 volts
12. Use the diagram below to answer the following
problems.
• b. How much current would be measured in each circuit if
the light bulb has a resistance of 6 ohms?
– A = 6 volts / 6  = 1 amps
– B = 12 volts / 6  = 2 amps
12. Use the diagram below to answer the following
problems.
• c. How much current would be measured in each
circuit if the light bulb has a resistance of 12 ohms?
– A = 6 volts / 12  = 0.5 amps
– B = 12 volts / 12  = 1 amps
12. Use the diagram below to answer the following
problems.
• d. Is the bulb brighter in circuit A or circuit B? Why?
• It is brighter in B because it has a greater voltage
which causes more current and more power.
13. What happens to the current in a circuit if a 1.5-volt
battery is removed and is replaced by a 9-volt battery?
• The current becomes 6 times greater.
14. In your own words, state the relationship between
resistance and current in a circuit.
• If resistance increases, the current
decreases. The two are inversely
proportional.
15. In your own words, state the relationship between
voltage and current in a circuit.
• If voltage increases, current increases. The
two are directly proportional.
16. What could you do to a closed circuit consisting of 2
batteries, 2 light bulbs, and a switch to increase the
current? Explain your answer.
• Remove one of the light bulbs. This
decreases the resistance and increases the
current.
17. What could you do to a closed circuit consisting of 2
batteries, 2 light bulbs, and a switch to decrease the
current? Explain your answer.
• Remove one of the batteries. This
decreases the voltage and decreases the
current.
18. You have four 1.5 V batteries, a 1 Ω bulb, a 2 Ω bulb,
and a 3 Ω bulb. Draw a circuit you could build to create
each of the following currents. There may be more than
one possible answer for each.
• a. 1 ampere
– a. 2 batteries and a 3 ohm bulb
• (or 4 batteries and all 3 bulbs)
3v
2 3
6
v
1
3Ω
1
18. You have four 1.5 V batteries, a 1 Ω bulb, a 2 Ω bulb,
and a 3 Ω bulb. Draw a circuit you could build to create
each of the following currents. There may be more than
one possible answer for each.
• b. 2 ampere
– b. 4 batteries and a 3 ohm bulb
6v
3Ω
2