Transcript n-P9-Electricity2PPTmra

Background
• Circuit- a connection including a power
source, wiring, and one or more resistors
• Resistor – something that uses electrical
energy (or “lowers the potential energy”) (ie:
bulb, motor, buzzer, appliance, etc.)
• Power Source – a battery or outlet
Schematics
Wire
Battery
Resistor
Light Bulb
Ammeter
Voltmeter
Switch
Types of Circuits
Series
Electricity has one
path to follow
Parallel
Combined
Electricity has more Parts of the circuit
than 1 path to follow are series; parts are
parallel
When one bulb goes When one bulb goes Depends on where
out, they all go out out, the others stay in the circuit the
lit
bulb goes out
Ohm’s Law : V = IR
Voltage (V)
Current (I)
Resistance (R)
Measured in Volts (V)
( J/C) (or N.m / C)
Measured in Amps (A)
which are C/s
Measured in Ohms (W)
which is a J.s / C2
“Electric Pressure”
Rate of Flow of e-
-power company’s
generator provides 120 V
to your home outlets;
-the two holes in an
outlet have a p.d. of 120
V between them (120 J of
energy is supplied to
each C of charge)
-named for Allesandro
Volta (Italian physicist)
-meas. by voltmeters
“the flow of e-‘s from
high potential to low
potential”
-Named for French
physicist Andre Ampere
- is maintained by e-‘s
pumping back (we say a
“circuit is complete”)
-ammeters measure
current;
-- galvanometers measure
v. weak currents
Opposition to e-flow
Power and Charge
Power (P)
Charge (q)
Measured in Watts (W)
Measured in Coulombs
(C)
Energy expended by a
current in 1 second
When a substance has
gained or lost electrons
1 W = 1 amp x 1 volt
1 C is the charge of
6.25 x 1018 e-’s. (lightning
~10C) and
1 e- = 1.60
x 10-19 C
The Water Analogy - CURRENT
(read, don’t copy)
• When a hose is attached to a faucet and the valve is
opened, water flows from the faucet through the
hose. If you used a stopwatch and a graduated
container, you could measure the rate of flow of
the water in, for example, liters per second.
• Similarly, we can measure the number of e-’s
flowing past a given point in a unit of time.
• Current is expressed as e-’s per second and is
measured in amps.
• 1 amp = 1 C of electrons per second
• 1C = 6.25 E 18 e-’s
Factoids About Amps
• “It’s not the volts that kill you, it’s the amps!”
• Plug: ground (removes excess charges from an a.c. object
Current in Amps
Effect
0.001
can be felt
0.005
painful
0.010
spasms
0.015
lose muscle control
0.070
fatal if >1 sec.
The Water Analogy - VOLTAGE
(read, don’t copy)
• Water flows through a hose because there is a
driving force behind the water. We can increase
the rate of flow of the water by increasing the
driving force. (Add pumps to the line)
• The driving force in a circuit is called the
“electromotive force” (emf) and is a “difference in
potential” that causes e-’s to move.
• EMF is also known as voltage and is measured in
volts
• Think of “potential” the way we say something has
a lot of “potential energy” when raised to a great
height
The Water Analogy – RESISTANCE
(read, don’t copy)
• Water molecules, moving through a pipe,
rub against the walls of the pipe and slow
down. The walls of the pipe oppose or offer
resistance to the flow of water. Longer and
narrower pipes offer more resistance to the
flow of water than shorter and wider pipes.
Factoids About Ohms
Typical Resistances:
Ohms
Common Wires
0.03
Household wiring (per m)
0.004
Body, soaked in salt water
Body, dry
100
500,000
What Impedes Electricity?
There are 4 factors that affect resistance in wires:
1. Length – longer conductors offer greater resistance
2. Temperature – R h as T h (for most metals)
However, R i as T h for C and semiconductors)
Superconductors are certain metals which become
excellent conductors at extremely low temperatures
3. Type of Material – every substance has its own
electrical resistance (ie: Cu conducts better than Al)
4. Diameter of Wires – thick wires have less resistance
than thin wires; resistance varies inversely with the
cross-sectional area of the conductor
The Water Analogy - SUMMARY
Voltage (V)
Current (I)
Resistance (R)
Measured in Volts
(V)
Measured in Amps
(A)
Measured in Ohms
(W)
“Electric Pressure”
Rate of Flow of e-
Opposition to Flow
of electrons
Increase the
pressure at the
pump increases
voltage
Amount of water
flowing through a
hose in 1s
Water has a harder
time going through
a long, thin hose
than a short, wide
hose
Why Does Electricity Flow?
Electricity flows whenever there is an electric “potential difference”
No flow – No
potential
difference
b/w bird’s
feet
Zap!
One wire with one V is connected
by balloon fabric to another wire
with another voltage
No flow – No
potential
difference b/w
person’s hands
Types of Current
• DC – “Direct Current”
– Batteries are a source of DC current
– Electricity travels in one direction (- to +)
– All energy is stored (chemical) and can be used
up quickly
• AC – “Alternating Current”
– Current coming from outlets
– First goes in one direction, then in the opposite
at a rate of 60 cycles per second (aka: 60 Hz)
Remember:
• Electrons don’t actually travel through the
wire, they wiggle and their energy transfers
• The power company sells you energy; you
supply the electrons
• Can be transmitted long distances with
voltage step-ups
• AC has a lower heat loss in wires
Safety Factoid
•Electricians work “with one hand in their
pocket” when there’s danger of a hot wire
since if they used 2 hands the current can
travel across their chest (the current doesn’t
necessarily want to travel through the
person; it just wants to get to the ground
since the ground has relatively few e-‘s)
Calculations Involving Circuits
Series
Parallel
Voltage
Vtot = V1+V2+V3…
Same in each
resistor
Current
Same in each resistor
Itot =I1 +I2 +I3…
Resistance Rtot = R1 +R2 +R3…
Example 1 - Series
If VT = V1 + V2…then
100V = V1 + 60V
V1 = 40 V
So…V1 = 40 V
If V = IR,……..then
40V = I1 ( 20W)
I1 = 2 A
So…I1 = 2 A
(and since it’s a series
circuit and I is the same
everywhere, then…)
To solve for R2,
V = IR, and
60V = 2A(R2)
So…R2 = 30W
40 V
2A
30 W
You have both R’s.
If 1/RT = 1/R1 + 1/R2
Then 1/RT = 1/3 + 1/6
Example 2 - Parallel
So RT = 6/3 or 2W
Now you have a V and a T.
If V = IR, then
6V = I(2W)
So I = 3A
Since, for a Parallel Circuit,
the voltage is the same in
each branch, V1 must be 6V
and V2 must be 6V
In a Parallel Circuit, the
current is not (necessarily)
the same in each branch.
V1 = I1R1; 6 = (I)3; I1 = 2A
V2 = I2R2; 6 = (I)6; I2 = 1A
I1+I2=IT
RT = 2W
IT = 3A V1 = 6 V
V2 = 6 V
6V
2A
1A
3A
6V
2W
Example 3 - Combined
You have two parallel resistors and
you know their values. Start there.
1/RT = 1/5 + 1/20
1/RT = 4/20 + 1/20 = 5/20 so
R2,3 = 4W
R2,3 = 4W
So it is as if…
And to find RT, simply add R1 and R2,3
So you get 9W
Since we know that the IT is 4A but we
know that in a parallel branch the
current splits up, it would only be true
that at I1 the current would be 4A.
9W
1
Kirchoff’s Rules
• For complex circuits