Transcript Chapter 20 Electricity

Chapter 20
Electricity
20.1: Electric Charge
and Static Electricity
Electric Charge
• Question: What do static cling and lightening
have in common?
• Answer: They both have a similar cause – the
movement of charged particles
• Electric charge: a property that causes
subatomic particles such as protons and
electrons to attract or repel each other.
Types of Electric Charge
• There are two types of electric charge:
– Positive (protons)
– Negative (electrons)
• Remember that in an atom, the charges are
balanced…
– An excess or shortage of electrons produces a net
electric charge.
• The SI unit of electric charge is the coulomb (C)
Coulomb
• It takes about 6.2 x 1018 electrons to produce
one coulomb
• A lightning bolt is about 20 coulombs of
charge.
• A flash camera uses the energy from 0.025
coulombs of charge for each flash.
Electric Charges
• Why do two balloons repel off each other
after being rubbed on your hair?
– Like charges repel, opposite charges attract.
• Electric force- the attraction or repulsion
between electrically charged objects
Electric Force
• Electric force depends on two main factors:
– Charge
– Distance
• How does changing the size of the charge or
the distance between charges affect the
electric force? (Refer to figure 3, p. 601)
– Doubling the net charge on one object doubles
the electric force
– However, doubling the distance between the
objects will reduce the electric force to one fourth
as strong!
Reviewing the Atomic Structure
• Why are negative charges (electrons) more
mobile than positive charges (protons)?
• Think about where the subatomic particles are
located…
– Protons concentrated in the nucleus
– Electrons found outside the nucleus
Electric Fields
• Electric field: the effect an electric charge has
on other charges in the space around it
• The strength of an electric field depends on
the amount of charge that produces the field
and on the distance from the charge.
• Note the direction of the field of a positive versus
negative charge.
– The direction of each field line shows the direction of
the force on a positive charge (repels or attracts)
• The lines representing the field are close near the
charge, where the field is strongest!
• The more net charge an object has, the greater
the force is on it
Electric Fields
Static Electricity
• Static electricity: the study of the behavior of
electric charges, including how charge is
transferred between objects
• What are some ways that charges can build up
on an object or move from one object to
another?
Transferring Charge
• Charge can be transferred three ways:
– Friction
– Contact
– Induction
Conserving Charge
• Guess what happens to the total amount of
charge before and after the transfer of charge
occurs (regardless of the way the charge is
transferred)?
• Law of Conservation of Charge: the total
charge in an isolated system is constant.
Charging by Friction
• A charge builds up when one material has a
greater attraction for electrons than the other
and a build up of charge is created when they
are rubbed together (friction).
Charging by Friction
• Because electrons transfer from one material
to the other, an overall positive charge is
produced on the material that loses
electrons.
• An electric charge is produced when the
opposite charges come close together!
Transfer of Charge (friction)
Charging by Contact
• A Van de Graaff generator is an example of
how an electric charge can be obtained by
contact.
• When someone touches the metal sphere,
enough charge is acquired to make hair stand
on end.
Charging by Induction
• Induction- the transfer of charge without
contact between materials
Why do I get shocked by the
doorknob?
An Example of Charging by Induction
• This is what happens when you walk across
carpet and pick up extra electrons (so your
feet/hands are negatively charged).
– The net negative charge in your hand repels the
electrons in the doorknob.
– The electrons in the doorknob move to the base and
leave a net positive charge on the part closest to your
hand.
– The attraction between positive and negative charges
creates a spark!
– The name for this spark is called static discharge
Static Discharge
• Static discharge occurs when a pathway
through which charges can move forms
suddenly.
• Lightning is a dramatic example of static
discharge
Lightening & Static Discharge
• Friction between moving air masses causes a
charge to build up in the clouds
• The lower part of the cloud has an overall
negative charge, which induces an overall
positive charge on the ground below it.
• As the amount of charge on the cloud increases,
the force of attraction increases between the
charges in the ground and the cloud
• Eventually, the air becomes charged, forming a
pathway for the electrons to travel to the
ground.
Lightening  Static Discharge
20.2: Electric Current
and Ohm’s Law
Electric Current
• For a flashlight to work properly, it must form
a continuous path through which charges can
flow, in other words, make an electric current.
• Electric current- a continuous flow of electric
charge
– The SI unit of electric current is the ampere (A), or
amp
– 1 amp = 1 coulomb/second
Types of Current
• There are two types of current:
– Direct Current (DC)
– Alternating Current (AC)
Direct Current (DC)
• Direct current: charge flows in ONE direction
(DC)
– Example: battery-operated devices like a flashlight
Alternating Current (AC)
• Alternating current- a flow of electric charge
that regularly reverses its direction (AC)
– Example: most homes and schools use this type of
current
Important to Notice
• Notice Figure 7 on p. 604—
• In direct currents, like a flashlight, the
electrons flow from the negative terminal of
one battery to the positive terminal of
another battery
Important to Notice
• However, look at the direction of the flow of the
current…
– Why does the current seem to be moving in the
opposite direction?
– Current moves in the opposite direction because
scientists define current as the direction in which
positive charges would flow.
Conductors & Insulators
• What is the material required for running
electrical currents usually made of and why?
– Metals such as copper and silver are good
electrical conductors.
– Wood, plastic, rubber, and air are good electrical
insulators.
Resistance
• What do you think of when you hear the term
resistance?
• Resistance: is opposition to the flow of charges
in a material
– The SI unit of resistance is the ohm.
Resistance
• As electrons move through the wire, they
bump into each other and convert some
kinetic energy into thermal energy.
• Because of this, less energy is available to
move the electrons through the wire, so the
current is reduced.
Various Ways to Increase Resistance
• There are various ways to increase resistance
in a current…
– A material’s thickness, length, and temperature
affect its resistance.
Thickness & Resistance
• Using a thicker wire is like drinking a milkshake
through a thicker straw, it becomes easier!
Length & Resistance
• If you have a longer wire, the resistance is
greater because it has to travel further—so
length increases it!
Temperature & Resistance
• As temperature increases, collisions increase.
When there are more collisions, there is more
energy therefore more resistance
Superconductors
• Could the temperature ever be lowered
enough to achieve a resistance of zero???
– This is the idea behind superconductors.
Voltage
• In order for charge to flow in a conducting
wire, the wire must be connected in a
complete loop that includes a source of
electrical energy
• What would the electrical energy source be in
a flashlight?
Ohm’s Law
• German scientist Georg Ohm found a
mathematical formula that relates voltage,
resistance, and current.
• He said that as resistance increases, voltage
decreases.
Ohm’s Law
• Ohm’s law is stated in the following formula:
V = I x R or I = V/R
– V = voltage
– R = resistance
– I = current
• Increasing the voltage increases the current.
• Keeping the same voltage and increasing the
resistance decreases the current.
20.3: Electric Circuits
Electric Circuits
• Electric circuit: a complete path through
which charges can flow
– There are circuit diagrams designed to read the
path of a current.
• REMEMBER!!! The direction of current is
defined as the direction in which positive
charges will flow. Electrons flow in the
opposite direction.
Types of Circuits
• There are two types of circuits:
– Series Circuit
– Parallel Circuit
Series Circuit
• Series circuits: charge has ONE path through
which it can flow.
• There are some problems to this system that
keeps people from wiring series circuits in
their homes…
– If one element stops functioning in a series circuit,
none of the elements can operate!
– Basically, if a light bulb were to burn out, the TV
would shut off!
Series Circuit
• Also, adding bulbs to a series circuit increases
the resistance, therefore, decreasing the
current and brightness of the bulb!
Parallel Circuit
• Parallel circuit: an electrical circuit with two or
more paths through which charges can flow.
• If one bulb burns out, the charge can still flow
so the others stay lit.
– If one element stops functioning in a parallel
circuit, the rest of the elements still can operate.
Electrical Power
• To find the electrical power of the system, use
the following equation:
– P (watts) = I (amps) x V (volts)
• Electric power can be calculated by
multiplying voltage by current.
Power Ratings
• An appliance’s power rating tells how much
power it uses under normal conditions.
• To find the electrical energy used by an
appliance, use the following equation:
• E=PxT
– E = energy
– P = power
– T = time
Safety Measures for Circuits
• Fuses:
– Prevents current overload- wire melts if too much
current and stops flow  “Blowing a fuse”
Safety Measures for Circuits
• Circuit breakers
– A switch that opens when current in the circuit is
too high
– Must be reset before using again
Safety Measures for Circuits
• Insulation  Don’t touch bare wires!
Safety Measures for Circuits
• Grounded plugs (Ground
Fault Circuit Interrupter
 GFCI)
– Transfers excess charge
through a conductor to
the Earth
– Example: the third prong
on electrical plugs