Transcript 17 Notes

ELECTRICITY
CHAPTER 17
ELECTRIC CHARGE
• Charges Exert Force
• Atoms are composed of particles with charges. The
law of electric charges states that like charges repel
and opposite charges attract.
THE FORCE BETWEEN PROTONS AND
ELECTRONS
• Protons and electrons have opposite charges, they
are attracted to each other
THE ELECTRIC FORCE AND THE
ELECTRIC FIELD
• Electric force- the force of attraction or repulsion on
a charged particle that is due to an electric field
• Electric field – the space around a charged object
in which another charged object experiences an
electric force
CHARGE IT!
Friction
• Friction- occurs when
electrons are “wiped”
from one object to
another
Conduction
• Conduction – electrons
move from one object to
another by direct
contact.
CHARGE IT!
• Induction – charges in an uncharged metal object
are rearranged without direct contact with a
charged object
CONSERVATION OF CHARGE
• When you charge something by any method, no
charges are created or destroyed. The numbers of
electrons and protons stay the same.
• Detecting Charge
• You can use a device called an electroscope to
see if something is charged
MOVING CHARGES
• Conductors- a material in which charges can move
easily
• Examples: copper, aluminum
• Insulators- a material in which charges
cannot move freely
• Examples: plastic, rubber, wood
STATIC ELECTRICITY
• Static electricity- electric charge at rest; produced
from friction or induction
• Electric Discharge – the release of electricity stored
in a source
17.2 ELECTRIC CURRENT & ENERGY
• Electric Current (I) – rate at which charges pass a
given point
• When you flip a switch, an electric field is set up in
the wire at the speed of light. The electric field
causes the electrons in the wire to move.
• This electric field is created so quickly that all
electrons start moving through the wire instantly.
• Think of the electric field as a command to the
electrons to charge ahead.
TYPES ELECTRIC CURRENT
Direct Current (DC)
• Direct Current (DC) –
charges always flow in
one direction.
• Ex: batteries used in
appliances
Alternating Current (Ac)
• Alternating Current
(AC) – charges shift
from flowing in one
direction to flowing in
the reverse direction
• EX: Wiring in house
VOLTAGE AND ENERGY
• Voltage – the potential difference between points
• SI unit : Volts (V)
• You can think of voltage as the amount of energy released as
a charge moves between two points in the path of a current.
• As long as there is a voltage between 2 points, charges will
flow in the wire. The size of the current depends on the
voltage.
Varying Nature of Voltage
• Different devices need different levels of voltage.
1.5 V
9V
12 V
Resistance
• Resistance (R) – opposition to the flow of electric
charge
• SI unit: ohm’s (Ω)
• Good conductors, such as copper, have low resistance
• Poor conductors, such as iron, have high resistance
• Resistance depends on thickness, length, &
temperature
• Thick, short wires have less resistance than long, thin wires.
• The resistance of metals increases as temperature increases
Resistance
• Resistance (R) – opposition to the flow of electric
charge
• SI unit: ohm’s (Ω)
• Good conductors, such as copper, have low resistance
• Poor conductors, such as iron, have high resistance
• Resistance depends on thickness, length, &
temperature
• Thick, short wires have less resistance than long, thin wires.
• The resistance of metals increases as temperature increases
Resistance
Generating Electrical Energy
• Remember energy is not created or destroyed – it
only changes forms
• Parts of a Cell
• A cell contains a mixture of chemicals called an electrolyte.
Every cell also has a pair of electrodes made from
conducting materials
Types of Cells
• There are 2 types of cells:
• Wet – have an electrolyte
• Example: batteries – sulfuric acid is the electrolyte
• Dry – have a dry electrolyte
• Example: small radios or flashlights
Generating Electrical Energy
• Cells change chemical or radiant energy into
electrical energy. Batteries are made of one or
more cells.
• Thermocouple can take thermal energy and
transfer it to electrical energy.
• Photocell convert light energy into electrical
energy.
17.3 ELECTRICAL CALCULATIONS
• How fast is a nanosecond? A nanosecond (ns) is
one-billionth of a second. Electrical signals travel at
30 cm/ns. Calculate how far electrical signals travel
in 1 second.
•
CONNECTING CURRENT, VOLTAGE,
AND RESISTANCE
• Georg Ohm Ohm (1789 –1854) studied the
resistance of materials. He measured the current
that resulted from different voltages applied to a
piece of metal wire.
Electric Power
• Ohm’s Law – the ratio or voltage (V) to current (I) is
the resistance (R) of a material
• V=IxR
Unit
SI Unit
Voltage (V)
V - volt
Current (I)
A- Amps
Resistance
(R)
Ohm - Ω
OHM’S LAW CALCULATIONS
• Ex: 1 What is the voltage if the current is 2 A and the
resistance is 12 Ω ?
• Ex: 2 Find the voltage if the current is 0.2 A and the
resistance is 2 Ω
OHM’S LAW CALCULATIONS
• Ex 3: The resistance of an object is 4 Ω. If the current
in the object is 9 A, what is the voltage used?
• Ex 4: An object has a resistance of 20 Ω. Calculate
the voltage needed to produce a current of 0.5 A.
ELECTRIC POWER
• Electrical Power (P) – the rate at which electrical
energy is converted into other forms of energy
• Si Unit: Watts (W)
• Power = voltage x current
• P=VxI
Unit
SI Unit
Voltage (V)
V - volt
Current (I)
A- Amps
Power (P)
W - Watts
ELECTRICAL POWER CALCULATIONS
• Ex 1: A toaster draws approximately 10 A of current.
A home receives 120 V at each electrical outlet.
What is the power of the toaster?
• Ex 2: A car uses a 12 V battery. One headlight draws
3.0 A. What is the power of the headlight?
ELECTRICAL POWER CALCULATIONS
• EX 3: A light bulb draws a 0.5 A current at a voltage
of 120 V. What is the power rating of the light bulb?
ELECTRICAL ENERGY
• Measuring Household Energy Use
• Different amounts of electrical energy are used
each day in a home. Electric companies usually
calculate electrical energy by multiplying the
power in kilowatts by the time in hours.
17.4 ELECTRIC CIRCUIT
• Just like a roller coaster, an electric circuit always forms a
loop— it starts and ends at the same place
• Parts of an Electric Circuit
• Energy source (Battery)
• Wires
• Load (light bulb, appliance)
SWITCHES
• Sometimes, a circuit also contains a switch
• Switch – opens or closes a circuit
• Closed – electrons can flow
• Open – electrons cannot flow
TYPES OF CIRCUITS
• Series Circuit – a circuit in which the parts are joined one
after another such that the current in each part is the
same.
• One path for charges to follow – charges must flow through
each part of the circuit
• The voltage across each load is different
Series circuits are useful
in wiring burglar alarms. If
any part of the
circuit fails, there will be
no current in the system
and the alarm will sound.
TYPES OF CIRCUITS
• Parallel Circuits - a circuit in which the parts are
joined in branches so that the voltage (potential
difference) across each part is the same
• Loads do not have the same current
• Charges have more than one path to follow
Parallel Circuits are used
in almost all appliances
and decorative string
lights
HOUSEHOLD CIRCUIT SAFETY
• Circuits can fail if they are overloaded or a wire
breaks or has water damage.
• To prevent fire:
• Fuses
• A thin strip of metal in circuit that the charges flow through.
If the current is too high, the metal will melt and the circuit
is broken
• Circuit Breakers
• A switch that automatically opens if the current is too high. A metal
strip heats up, bends, and opens the switch stopping the flow of
current