Download Int. Sci. 9 - Electricity Powerpoint

Electricity
What is Electricity?
• a form of energy resulting from the
existence of charged particles (such as
electrons or protons)
• either static as an accumulation of charge or
dynamic as a current.
Electric Current
• Static Electricity: build up of charges that
pass QUICKLY to another object
• Electric Current: continuous flow of
charges through a conductor
Static Electricity
• Objects can acquire a static electric charge
through:
1) Friction (when an object whose electrons
are loosely held rubs against another
object)
2) Conduction (when an object with an excess
of electrons touches a neutral object)
3) Induction (a neutral object acquires a
charge from a charged object close by
without contact being made)
Friction
Induction
Charging by contact!
Conduction
Transferring electric charge
a. Conductors: e¯ move easily.
b. Insulators: hold e¯ tightly.
c. Contact charging is
done when two materials
are rubbed together (best
with insulators).
d. Charging by induction is
done when one charged object
induces a charge on another.
Lightning
a. Large static
discharge between the
earth and clouds.
b. Lightning was found
to be static electricity by
Ben Franklin.
Grounding
a. Conductive path to Earth.
b. Lightning rods & plumbing.
The electroscope
can be used to
detect electric
charge
B. Electric Current
1. The reason electric charge
flows from one place to another is
voltage. HIGH
LOW
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).
Electricity and Voltage
- Electricity is the flow of electrons (-)
Charges (-) flow from HIGH voltage areas to LOW
voltage areas
– Voltage is like electrical pressure that pushes and pulls
charges
– Voltage Difference: the push/pull that causes charges
to move and is measured in volts (V)
Voltage
• Voltage is created by
– a chemical cell (battery) when it changes
chemical energy to electrical energy
– by a generator when it changes
mechanical energy to electrical energy
– by a solar cell when it changes light
energy to electrical energy.
Voltage and Current
• When a wire connects the terminals of a battery or generators, then
the voltage will push and pull electrons through a conductor.
– One terminal has extra electrons thus a negative charge. The other terminal
has a deficit of electrons and thus a positive charge.
– Electrons in the wire are pushed by the negative terminal and pulled by the
positive terminal through the wire
Circuit: a closed, conducting path
• For changes to flow, the wire must always be connected in a circuit
• Electric Current: the flow of charges through a wire or any
conductor.
Measured in Amperes (A=Amps)
• Current is almost always the flow of electrons
• What happens if we break the circuit?
Current - The flow of electric
charge
a. measured in amperes, or
amps (A).
b. Voltage causes current.
The amount of electric charge is
measured in coulombs.
a. 1 coulomb is the charge
carried by 6.24 x 10^18 e¯.
b. 1 amp is 1 coulomb per sec.
Batteries are e¯ pumps.
a. They provide a voltage
difference to a circuit.
b. Types: wet-cells & dry-cells
Check for Understanding
• What is voltage?
• How is voltage generated? (3 ways)
• What is current?
Resistance
• Resistance: the tendency for a material to
oppose the flow of electrons
• Changes electrical energy into thermal energy
and light
• Ex: lightbulb filament
• Resistance is measured in Ohms (Ω)
Resistance
a. Opposition to the flow of e¯.
b. It changes
electrical energy
into thermal energy
and/or light.
c. Measured in
ohms.
d. Conductors
have less resistance
than insulators.
What Affects Resistance?
Wire resistance greater for:
1) Longer wires
2) Thinner wires
3) Higher temperatures
IV. Control the Flow
• A voltage difference causes the charges to
flow
• Flow of charges = current (Amps or A)
• Electrical resistance restricts the movement
of charges
Resistance =
current
Pressure =
current
(Voltage Difference)
Ohm’s Law
Current = voltage difference
Resistance
I = V/R or V=IR
I= current (units = A, amps)
V= voltage (units = V, volts)
R = resistance (units = ohms Ω)
6. Ohm’s law
Voltage Difference
Current =
Resistance
V
I
R
Electrical Circuits - A circuit is a
conducting path.
1. Series circuit
a. One path
b. Any break &
all devices go out
c. Current is the same
throughout the circuit
3. Parallel circuit
a. Multiple paths
b. A break in one
branch & the other
branches stay on
c. Voltage is the same in each
branch, but current and
resistance may be different
Electrical Energy
I. Series Circuit: the current has only one loop to
flow through
–
–
–
things are wired one right after the other
If one thing (bulb) goes out every thing goes out
If the circuit is broken the entire flow of current stops
Series Circuit Continued….
• Current is the same at each point in the circuit
• When another resistor (light bulb) is added in series, the
total resistance increases.
• When resistance increases, current will decrease.
• Decreased current means dimmer light.
Series Circuit Concepts
(assuming 3 resistors)
RTOTAL = R1 + R2 + R3
V drop for each R is VX = I • RX
so, VTOTAL = I • R1 + I • R2 + I • R3
or,
VTOTAL = I • (R1 + R2 + R3)
since
I=V÷R
then… ICIRCUIT = V ÷ (R1 + R2 + R3)
II. Parallel Circuit: contains two or more
branches for current to move through
–
–
–
current splits up to flow through the
different branches
because all branches connect the same two
points of the circuit – the voltage difference
is the same in each branch
more current flows through the branches that
have the lower resistance
Parallel Circuit Concepts
(assuming 3 paths)
V is the same everywhere in the circuit
ITOTAL = I1 + I2 + I3
since ITOTAL = V ÷ R and I1 = V ÷ R1
then V ÷ R = V ÷ R1 + V ÷ R2 + V ÷ R3
1
1 1 1
RTOTAL R1 R2 R3
Household Electrical Safety
•
In a house, many appliances draw current from
the same circuit
–
–
–
•
If more appliances are connected to a circuit, more
current will flow through the wires
More current in wires = more heating in the wires
More heat causes insulation on wires to melt, which
increases chances of fire
To protect a house from this, all household
circuits have a:
1. Fuse, or
2. Circuit breaker
Household Circuits:
Fuse: a small piece of metal that melts if the
current becomes too high
Circuit Breaker: contains a small piece of
metal that bends when it gets hot
bending causes a switch to flip and
opens the circuit
5. Electrical safety devices
a. Fuses - one time use.
b. Circuit breakers - can be
reset and reused.
4. Household circuits
a. Mostly parallel.
b. 120 V in the U.S.
c. More devices plugged in a
circuit mean more current.
d. More current means more
heat in the wiring.
Let’s Compare Series and Parallel Circuits
Series Circuits
Parallel Circuits
• _______ path(s) for current
• _______ path(s) for current
• Current ________________
• Current ________________
• Voltage ________________
• Voltage ________________
• Break in circuit
_______________________
• Break in circuit
_______________________
• Adding resistance in series
______________________
• Adding resistance in parallel
_______________________
Let’s Compare Series and Parallel Circuits
Series Circuits
Parallel Circuits
• 1 path(s) for current
•
multiple path(s) for current
• Current is the same at every point
•
Current can be different in each
branch
• Voltage drops at each resistor
•
Voltage same across each resistance
• Break in circuit stops all current
•
Break in circuit does not affect other
bulbs
• Adding resistance in series
decreases total current (dimmer
light bulbs)
•
Adding resistance in parallel
increases total current
Comparison of series and parallel
circuits
The same
voltage battery
Notice the
brightness of
the bulbs
Electric Power and Energy
1. Electrical power is the rate at
which electricity is converted into
another form of energy.
a. Power = current x voltage
b. Unit is the watt or kilowatt.
Electrical energy
a. Energy = power x time
b. Unit is the kilowatt-hour.
(1000 watts for 1 hour)
c. This is what we buy from the
electric company.
Light Bulbs
Incandescent
a. Have a tiny
filament that resists
the flow of electrons.
b. The filament
gets hot and glows to
produce light.
c. Very hot and
inefficient.
Fluorescent
a. Filled with gas
and coated with
phosphor.
b. Electricity
excites the gas
making it give off U-V rays.
c. Phosphor absorbs U-V rays
and glows to make light.
d. Very cool, efficient, and last
a long time.