Transcript Chapter 16

Chapter 16
Electricity
The Electric Motor
• Electric motors convert electricity into
motion!
How a Motor works!
• Electric motors operate according to the
law of magnetic poles
• Unlike poles attract!
• Like poles repel!
• A simple motor has an electromagnet
mounted on a shaft to freely rotate!
How an Electric Motor Works!
• The electromagnet and shaft are placed
between poles of a field magnet used to
provide a stationary magnetic field!
• The force of attraction and repulsion cause
the armature (electromagnet) to rotate and
spin!
Electric Current
• Electrons are in motion!
• When a lightning strike hits the earth, a
massive current of electricity flows to the
earth!
• The flow of electrons is electricity!
• If the electrons are at rest, it is static
electricity!
DC and AC Current
• Current electricity can be subdivided into
either of two types depending on how it
flows!
• If it flows in one direction, it is direct
current (DC).
• If it flows in one direction and then
another, it is alternating current (AC).
• AC is used in our homes because it can
be transmitted over long distances.
Causes of Electric Flow
• Electrons move from one place to another!
• This is a result of attraction and repulsion.
• They will flow from negative to
positive…like with a battery!
Voltage
• The greater the difference between two charges
– the more negative and positive they are – the
stronger the force that drives the flow!
• The difference between charges determines
voltage.
• Voltage is also called “electric potential”.
• The SI unit of voltage is Volt (V)
• A device that measures voltage is a voltmeter!
Amperage
• Amperage refers to the volume of flow of
current!
• The unit of electric current is ampere (A)
• It is one coulomb per second!
• Remember – it is a RATE of flow!
• A device that measures current is an
ammeter!
Power
• Power is the amount of
voltage times the amount
of amperage.
Power = voltage X
amperage
P=VI
“I” represents amperage
• The unit for power is the
Watt (W).
Resistance
Conductors and Insulators
Electric current flows through some
materials much more easily than it flows
through others.
Materials through which current flows easily
are called conductors.
Resistance
Conductors and Insulators
Materials through which current does not
easily flow are called insulators.
They are called insulators because they can
be used to insulate objects from electric
current.
Resistance
• The amount by which a material hinders
the flow of electrons is called resistance.
• The unit for resistance is the ohm!
factors which increase resistance
1. Type of material
2. Diameter of the conductor – thick wire
has less resistance
3. Length of the conductor – the longer the
more resistance
4. Temperature – higher temperature
creates more resistance
Ohm’s Law
• There is a mathematical
relationship between
current and resistance.
• This relationship is called
Ohm’s law.
• Current is directly
proportional to voltage
and inversely proportional
to resistance.
V=IXR
Ohm’s Law
•
•
•
•
V is the voltage in volts
I is the current in amperes or amps
R is the resistance in ohms
IR is I times R
• The unit for resistance is the Ohm (Ω)
Ohm’s Law
Problem #1
A small electrical pump
is labeled with a
rating of 3 amps and
a resistance of 40
ohms. What voltage
was it meant to run
at?
Ohm’s Law
Problem #2
A nine volt battery is
hooked up to a light
bulb with a rating of
three ohms. How
much current passes
through the light?
Ohm’s Law
Problem #3
A lamp is plugged into
the wall outlet, which
is providing 110 volts.
An ammeter attached
to the lamp shows 2
amps flowing through
the circuit. How many
ohms of resistance is
the lamp providing?
Ohm’s Law
Current and voltage are directly related.
↓↓ ↑↑
Current and resistance are inversely related.
↓↑
Limiting Current
• A resistor is a device used to limit the flow
of electricity.
• Resistors are used to protect delicate
electrical components that cannot handle
large currents!
Electric Circuits
• The path current follows
is the electric circuit.
• A simple electric circuit
contains three parts:
1. battery or
generator
2. conductor (wire)
3. load (uses the
current – ex:
bulb)
Closed and Open Circuits
• Electric current
requires a complete
circuit in order to flow!
• A circuit is a closed
circuit if the electrons
can flow in a
complete path back to
the source.
Closed and Open Circuits
• An open circuit has a
gap and so the
current cannot flow!
• Often times the gap is
created with the use
of a switch!
Circuit Arrangements
• Simple circuits contain only one load (one
Bulb).
• More complex circuits have multiple loads.
• For this reason, a circuit can be arranged
in two basic ways:
– In series
– In parallel
Series Circuit
• In series circuits, the loads are
arranged so that the electric
current flows through each
load one after the other.
• The current must pass through
each bulb to get to the next.
• If one bulb (load) blows out,
the circuit is now open and the
current cannot flow.
• Also, due to the resistance
being cumulative, the current is
reduced from one bulb to the
next and so they do not burn
as brightly.
Series Circuit
• If switch 1 and 2 are
closed, the bulbs will
light up.
• If switch one is open.
Both bulbs will go out.
• If switch 2 is open,
both bulbs will go out.
Parallel Circuits
• In parallel circuits, loads
are arranged in separate
branches of the circuit
and the current is divided
among them.
• Parallel circuits have the
same amount of current
flowing through each
load!
• If one load blows out the
rest of the loads will
continue to receive
current and remain
active!
Parallel Circuits
• If all three switches are closed
then the bulbs will light up.
• If switch 2 is open then only
bulb a will go out. Current is
still flowing through bulb b so it
will stay on.
• If switch 3 is open then only
bulb b will go out. Current is
still flowing through bulb a so it
will stay on.
• If switch 1 is open, all the bulbs
will go out.
Drawing Circuits
• When drawing
circuits, symbols are
used to signify
various components.
The End