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Chapter 7
Electricity
Electricity
• An atom is the basic unit of matter and is
made of protons, neutrons, & electrons
– protons: + charge
– electrons: - charge
– neutrons: no charge
Electricity
• An atom is the basic unit of matter and is
made of protons, neutrons, & electrons
– when atoms contain the same number of
protons & electrons they have a neutral
charge
– positive charge – occurs when electrons are
lost from atoms to other objects
– negative charge – occurs when electrons are
gained from atoms of other objects
Electricity
• positive charge – occurs when electrons are
lost from atoms to other objects
• negative charge – occurs when electrons are
gained from atoms of other objects
Electricity
• static electricity – the accumulation of
excess electric charges on an object
– example: as you walk on carpet, some
electrons that are loosely held by the carpet
are transferred to your shoes creating
opposite charges
Electricity
• law of conservation of charge – states that
charge can be transferred from object to
object, but it can’t be created or destroyed
– similar to the law of conservation of mass or
the law of conservation of momentum
Electricity
Like & Opposite Charges
• opposite charges attract
• like charge repel
Electricity
Conductors and Insulators
• Electricity is caused by the movement of
electrons, from a negative to a positive region
– conductors - materials in which electrons can
move through easily
• examples: metals & water
• responsible for shocks when an excess of
electrons are transferred from one surface to
another, such as from your hand to a
doorknob
Electricity
Conductors and Insulators
• Electricity is caused by the movement of
electrons, from a negative to a positive
region
– insulators - materials in which electrons
cannot move through easily
• examples: wood, plastic, rubber, & glass
Electricity
Transferring Electric Charge
• charge by contact – the process of transferring
charge by touching or rubbing
– causes a transfer of electrons from one object to
another, leaving one object with a positive charge
and the other one with an equal amount of negative
charge
– example: socks being attracted to each other
when coming out of the dryer because of rubbing
during drying
– touching a door handle and getting shocked
Warmup 12-1-11
• What is Electricity?
• What is a Positive Charge?
• What is Static Electricity?
• What is the Law of Conservation of Charge?
Electricity
Transferring Electric Charge
• charge by contact – the process of transferring
charge by touching or rubbing
Electricity
Transferring Electric Charge
2.charging by induction – rearranging of
electrons on a neutral object caused by a nearby
charged object
– example: bringing a negatively charged balloon
near clothing and having it cling to the clothing by
an electrical force
Electricity
Transferring Electric Charge
3. lightning - caused by a large static
discharge of electrons through the air that
meet with the positive charge that is built up
on the ground
– collisions of atoms and molecules in the air
during
– this discharge account for light being given
off
Electricity
• Lightning
Electricity
• Lightning
Electricity
Transferring Electric Charge
4. thunder - sound wave generated by lighting
– the heat from lightning causes air to expand
rapidly,
– producing sounds waves that you hear as
thunder
Electricity
• grounding – a way to transfer any excess
electric charge to the ground before it builds up
and creates lightning
– examples: lightning rods, pipes, plumbing
fixtures, metal faucets, etc.
Electricity
• The presence of electric charges can be
detected by a device called an electroscope
Electricity
7.2: Electric Current
• How is it that electrons can move from one
object to another or move through an object?
The answer lies with a difference in voltage
Electricity
• voltage difference – the push that causes
electrical charges to flow through a conductor
– measured in volts (v)
– Just how heat moves from a high temperature
area to a lower temperature area, charge flows
from a high voltage area to a low voltage area
Electricity
• circuit – closed, conducting loop through
which an electric current can flow
– If the circuit is not closed, the electrons would
have nowhere to go, just as if a water line
broke, the water would not be able to flow
through the pipe.
Electricity
• electric current – the flow of electric charge
through a wire or any conductor
– electric current is measured in amperes (A)
Electricity
Types of Batteries
• dry cell - involves a chemical reaction between
zinc and a chemical paste that cause the central
carbon rod to become positive and the
surrounding lower region to become negative
– once the two terminals are connected through a
circuit, electricity flows
– these types of batteries are the most common
type (in flashlights or calculators)
Electricity
• Dry Cell Battery
Electricity
Types of Batteries
• wet cell - involves two metal plates and
typically an acid solution with a “bridge”
– these types of batteries are used in cars
Electricity
• resistance – the tendency for a material to
oppose the flow of electrons, changing electrical
energy into thermal energy and light
– resistance means, how hard it is to get electricity
through
– resistance in measured in ohms ()
– all materials have some electrical resistance
– electrical conductors have much less resistance
than insulators
Electricity
• resistance – the tendency for a material to
oppose the flow of electrons, changing electrical
energy into thermal energy and light
– list some factors that affect resistance in a wire
• Material of the wire, length of the wire, and
thickness of the wire
Electricity
• Ohms’s Law – a formula that is used to
measure the amount of voltage or current in a
circuit
V=IR
– V: voltage (measured in V)
– I: current (measured in amps)
– R: resistance (measured in )
Electricity
• Ohms’s Law – a formula that is used to
measure the amount of voltage or current in a
circuit
V=IR
• Calculate the voltage difference across a 25-
resistor if a 0.3-amp current is flowing through
it.