Transcript electric current

Electric Current
Chapter 20
Electricity and Energy Song Video
Solids: Conductors, Insulators
and Semiconductors
 Conductors:
 Insulators:
mostly metals
mostly nonmetal
materials
 Semiconductors:
metalloids
Bonding in Metals
 The
electron-sea model is a
simple depiction of a metal as an
array of positive ions surrounded by
delocalized valence electrons.
– Metals are good conductors of
electricity because of the mobility of
these delocalized valence electrons.
– A metal also conducts heat well because
3 electrons can carry additional
the mobile
kinetic energy.
Bonding in Metals
4
Semiconductors
 Metalloids:
semiconducting
elements
 Low electrical conductivity at room
temperature
 Electrical conductivity increases
with temp.
Semiconductors
 Semiconducting
elements form
the basis of solid state electronic
devices.

Metalloids (such as silicon or germanium)
are semiconducting elements whose
electrical conductivity increases as
temperature increases.
Shocking!

If you reach for a
metal doorknob after
walking across a
carpet, you might see
a spark.

The spark is caused
by electrons moving
from your hand to the
doorknob.
Conductor!
A
material in which electrons are
able to move easily is a conductor.
 The
best electrical conductors are
metals.
Not a Conductor!
A
material in which electrons are
not able to move easily is an
insulator.
 Most
plastics are insulators
Charging Objects
 Rubbing
two materials together can
result in a transfer of electrons.
 Then
one material is left with a
positive charge and the other with
an equal amount of negative
charge.
 The
process of transferring charge
by touching or rubbing is called
charging by contact.
INDUCTION!
 Because
electrical forces act at a
distance, charged objects brought
near a neutral object will cause
electrons to rearrange their
positions on the neutral object.
 The
rearrangement of electrons on
a neutral object caused by a nearby
charged object is called charging
by induction.
Induction

The balloon on the
left is neutral. The
balloon on the right is
negatively charged. It
produces a positively
charged area on the
sleeve by repelling
electrons.
Current and Potential
 The
net movement of electric
charges in a single direction is an
electric current.
 In
a metal wire, or any material,
electrons are in constant motion in
all directions. As a result, there is
no net movement of electrons in
one direction.
Current and Voltage
 When
an electric current flows in
the wire, electrons continue their
random movement, but they also
drift in the direction that the current
flows.
 Electric
current is measured in
amperes.
Voltage difference

In a similar way, electric charge flows from higher
voltage to lower voltage.

A voltage difference is related to the force that causes
electric charges to flow. Voltage difference is measured
in volts.
Electric Circuits

This figure shows an
electric current doing
work by lighting a
lightbulb.

A closed path that electric
current follows is a
circuit.

If the circuit is broken by
removing the battery, or
the lightbulb, or one of
the wires, current will not
flow.
Resistance

As the electrons flow through
the filament in a lightbulb, they
bump into the metal atoms that
make up the filament.

In these collisions, some of the
electrical energy of the
electrons is converted into
thermal energy.

Eventually, the metal filament
becomes hot enough to glow,
producing radiant energy that
can light up a dark room.
Electric Current
 Electrons
in motion.
 Current: The number of electrons
that pass a specific point in a circuit
in one second
I = Q/t
 Circuit:
electric current flows through
a closed, continuous path.
Electric Current
 The
reason electric charge flows
from one place to another is voltage.
a. Voltage is the difference in
electrical potential between two
places where e¯ are flowing.
b. Voltage is the “push” that makes
electric charges move.
c. Measured in volts (V).
HIGH
LOW
Batteries are e¯ pumps.
a. They provide a voltage
difference to a circuit.
b. Types: wet-cells & dry-cells
Generating Electric Current
Electrochemical cell: (battery) changes
chemical energy into electric energy.
Two types wet cell and dry cell.
 Thermocouples: a tool that uses
differences in temperature to generate
electric currents.
 Generator- next chapter but make
alternating current

Types of current
Direct current: electrons that flow in the
same direction in a wire. (DC)
 From batteries
 Alternating current: electrons that flow
in different directions in a wire. (AC)
 From Generators
 Used in your home
 Transformers change AC to DC

Measuring Electricity
Current: Measured in Amperes or amps
(A)
 Voltage: Measured in volts (V)
 Higher voltage, the more work the
electrons can do.

Measuring Electricity
Resistance: the force opposing the flow
of electrons.
 Measured in ohms
 Symbol is Greek letter omega 
 Thicker wire- less resistance
 Longer wire- more resistance
 Conductors- low resistance
 Insulators- high resistance

Ohm’s Law
The relationship among
current, voltage, and
resistance.
 Ohm’s law states that
the current in a circuit is
equal to the voltage
divided by the resistance
I=V
R

V
I R
V
Do the Math
I R
A
car has a 12 volt system. The
headlights are on a 10 amp circuit.
How much resistance do they have?
V
Do the Math
I R
A
car has a 12 volt system. The
headlights are on a 10 amp circuit.
How much resistance do they have?
 R =V / I
 R = 12 V / 10 amp
 R =1.2 
V
Do the Math
I R
 Your
house uses 120 volts. What
amount of current would flow through
a 20 ohm resistor?
V
Do the Math
I R
 Your
house uses 120 volts. What
amount of current would flow through
a 20 ohm resistor?
I = V / R
 I = 120 V / 20 ohm
 I = 6 amperes (A)
Electric power and energy
Power: The rate at which electricity
does work or provides energy The
amount of electric power a device uses
to do work is determined by its
resistance.
 P= VxI
 (P) power = (V) voltage x (I) current in
the circuit.

Energy
Formula for energy
E=Pxt
 (E) energy used = (P) power x (t) time
 The SI unit for energy is a joule.
 Kilowatt-hour meters measure the
electricity used in your home.
 (kWh)

Electricity and Energy Song Video
Write a Paragraph: (Ch 20)
 Investigate
and explain the
relationships among:
current
 Voltage
 Resistance
 power

Grade the paragraph




Current –
 number of electrons that pass a specific point in
a circuit in one second.
 Reason it flows is due to voltage.
Voltage –
 is the push that makes the electric charge move
Resistance –
 force opposing the flow of electrons (or
current)
Power –
 rate electricity does work. The amount of work
is determined by its resistance.