Transcript ELECTRIC CIRCUITS Chapter Twenty: Electric Circuits 20.1 Charge

ELECTRIC CIRCUITS
Chapter Twenty: Electric Circuits
20.1 Charge
20.2 Electric Circuits
20.3 Current and Voltage
20.4 Resistance and Ohm’s Law
Chapter 20.1 Learning Goals
Define static electricity and discuss
its causes.
Explain what it means when an object
is electrically charged.
Discuss the relationship between like
and unlike charges.
Investigation 20C
Electric Charge
Key Question:
What is static electricity?
20.1 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.
20.1 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.
20.1 Electric and magnetic forces
Whether two charges
attract or repel depends
on whether they have the
same or opposite sign.
A positive charge
attracts a negative
charge and vice versa.
Two similar charges
repel each other.
20.1 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.
20.1 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.
20.1 Electrical forces
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.
Chapter Twenty: Electric Circuits
20.1 Charge
20.2 Electric Circuits
20.3 Current and Voltage
20.4 Resistance and Ohm’s Law
Chapter 20.2 Learning Goals
Define electricity.
Describe the components of an
electric circuit.
Explain the difference between a
closed circuit and an open circuit.
20.2 Electric current
Electric current is caused by moving
electric charge.
Electric current comes from the motion
of electrons.
current
20.2 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.
A waterwheel turns when a
current of water exerts a
force on it.
20.2 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.
20.2 Electric Circuits
Wires in electric circuits are similar in some
ways to pipes and hoses that carry water.
20.2 Electric Circuits
When drawing a circuit diagram, symbols
are used to represent each part of the
circuit.
20.2 Electric
Circuits
Electrical symbols are
quicker and easier to
draw than realistic
pictures of the
components.
20.2 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.
20.2 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.
Chapter Twenty: Electric Circuits
20.1 Charge
20.2 Electric Circuits
20.3 Current and Voltage
20.4 Resistance and Ohm’s Law
Chapter 20.3 Learning Goals
Explain how current flows in an
electric circuit.
Define voltage and describe how
it is measured.
Discuss the function of a battery
in an electric circuit.
Investigation 20A
Electricity
Key Question:
How do you measure voltage and current in
electric circuits?
20.3 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.
20.3 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.
20.3 Voltage
A difference in voltage provides the
energy that causes current to flow.
20.3 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.
20.3 Batteries
A battery uses stored
chemical energy to create
the voltage difference.
Three 1.5-volt batteries
can be stacked to make a
total voltage of 4.5 volts
in a flashlight.
20.3 Batteries
A pump is like a battery because it
brings water from a position of low
energy to high energy.
20.3 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).
20.3 Measuring current
 Circuit breakers and fuses are two
kinds of devices that protect circuits
from too much current by making a
break that stops the current.
Chapter Twenty: Electric Circuits
20.1 Charge
20.2 Electric Circuits
20.3 Current and Voltage
20.4 Resistance and Ohm’s Law
Chapter 20.4 Learning Goals
Use Ohm’s law to relate current,
voltage and resistance.
Apply Ohm’s law to solve
problems.
Classify materials as conductors,
insulators, and semiconductors.
Investigation 20B
Resistance and Ohm’s Law
Key Question:
What is the relationship between current and
voltage in a circuit?
20.4 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.
20.4 Resistance
The total amount of resistance in a
circuit determines the amount of
current in the circuit for a given
voltage.
20.4 Resistance
Electrical resistance is measured in units
called ohms.
This unit is abbreviated with the Greek
letter omega (Ω).
20.4 Ohm’s Law
The current in a circuit depends on
voltage and resistance.
Ohm’s law relates current, voltage,
and resistance with one formula.
If you know two of the three
quantities, you can use Ohm’s law to
find the third.
Solving Problems
A toaster oven has a resistance of
12 ohms and is plugged into a 120volt 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 
= 10 A
20.4 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.
20.4 Resistance of common
objects
The resistance of many
electrical devices varies
with temperature and
current.
A light bulb’s resistance
increases when there is
more current because
the bulb gets hotter
when more current
passes through it.
You are Wired!
Your nervous system uses specialized cells
called neurons to transfer electrical signals
from one part of your body to another. A
neuron has three basic parts: the cell body; a
long, thin portion called the axon; and
fingerlike projections called dendrites.