Download Flashlights and circuits

Flashlights and circuits.
Before we had static electricity
Now we will talk about dynamic electricity
 Moving charges
o Current
Think about a flashlight circuit
Electric circuit – electricity (electrons flowing) in a complete circular
path
What does the switch do?
Open circuit – no current (no charges flowing)
Closed circuit – current flows
Use analogy to water flowing in pipes.
Another type of circuit – short circuit
 When two sides of the battery come into direct contact without
bulb in between
 Electrons then have an alternative path to flow
 Normally the bulb is there to impede the flow of electrons
o Converts the electrons energy to thermal energy & light
o Called resistance
 Limits the current flowing through the circuit
 In a short circuit there is very little resistance
o Current get very high
 Melt the wires
 That’s why we have fuses & circuit breakers
 Explain this to class
Current:
The charged particles that flow though the circuit are negatively
charged electrons.
 Each electron carries a certain amount of energy
 Power
o How many electrons per second (current)
o How much energy each has (volt – coulomb = Joule)
Current
 Ampere (A) = 1 Coulomb per second = 1C/s
 1C = 6.25 x 1018 electrons
DEMO: K5-13: ELECTRIC CURRENT – MODEL
Each electron carries a tiny amount of energy.
But there are so many
1 Coulomb of 1 Volt electrons carries 1 Joule of Energy
So 1 A at 1 volt = 1C/s at 1 volt = 1 J/s = what? 1 watt
Power = energy/s
Convention is that current flows from + to -, even though charges flow
– to +. They are negative so we pretend there are + charges flowing
+ to –
Battery:
A device that pumps + charges from the negative terminal to the
positive terminal.
 Raises the potential energy of the electrons
DEMO: K5-11: BATTERY MODEL
Performs work on the charge using chemical potential energy to
make electrostatic potential energy.
Typical battery
1.5 Volt = 1.5 J/C
DEMO: K5-14: ELECTRIC CELL
Can connect them in a chain to add voltages.
Show batteries in series adding & subtracting.
The Bulb is the opposite of the battery.
 Allows the charges to flow from the + terminal to the –
 But, not a perfect conductor
o Impedes the flow of the charges
o The ordered potential energy cannot be converted to
kinetic energy.
 Changes it to thermal energy
 Analogy of friction on the moving electrons
 Analogy of water flow with restriction
 Charges don’t get lost
 They just don’t flow as fast.
DEMO: K6-21: HEATING IN CURRENT-CARRYING WIRE
DEMO: K6-22: ENERGY CONVERSION - IMMERSION HEATER
POWER:
Consider the flashlight circuit
 Assume 3 Volts & 1 Amp
 The bulb consumes power because the current passing through
it experiences a voltage drop (energy loss)
 The power consumes is given by the current multiplied by this
voltage drop (or energy loss)
Power consumed = voltage drop x current
P =VI
3volts at 1 amp = 3 watts consumed
The batter chain produces the electric power because the current
through it gets a voltage rise
Power provided = voltage rise x current
P =VI
3 volts at 1 amp = 3 watts provided
The bulb is what controls the amount of current that goes through
it given a certain voltage (or electrostatic potential)
 Again use water analogy with pressure, flow rate, resistance
DEMO: K5-32: RESISTANCE VS DIAMETER AND LENGTH
POWER DISTRIBUTION:
Batteries are fine for flashlights, but not for houses. Run out of energy
and have to be replaced etc…
More convenient to produce power in a central spot and distribute it.
Use coal or oil powered electric generators.
Thomas Edison 1882 - began to electrify New York
Thickness of wire is important for long runs
 Thicker to lower resistance
Use copper
 Only silver is better
 But still long runs made for significant resistance
o Get a voltage drop
 Power consumed by wire!!!
Overall the current in a wire (for a given voltage) is inversely
proportional to the resistance.
Voltage drop = current x resistance
Ohm’s Law
Resistance called an Ohm use the symbol 
DEMO: K5-31: OHM'S LAW
Now we can figure out just how much power is lost in those
transmission lines!
Power consumed = voltage drop x
current
= (current x resistance) x current
= current2 x resistance
wasted power in wire goes like current squared!
Edison tried to lower resistance
 Copper
 Very thick
 Short runs
Can deliver more power by using high voltage and small current.
 Dangerous
 Hard to use high voltage in the home
Enter AC and transformers!
Real problem is that with DC no easy way to transfer power from one
circuit to another.
 It all has to be one circuit
 Light bulb and high voltage source all one
o Can’t use high voltages
 Low voltages have to much loss by they need high currents to
deliver enough power.
AC makes it easy to transfer power from one circuit to another so that
different parts of the system can work at different voltages.
In AC the direction of current flow reverses periodically.
Reversal occurs every 120th of second. Full cycle is 60 Hz
Draw on board a sine wave.
AC championed by Tesla & Westinghouse.
Power can be transferred & transformed via the electromagnetic
action (induction remember) by a device called a transformer.
Transformer uses 2 principles encountered in the previous material.
 A current creates a magnetic field
 A changing magnetic field creates an electric field
 Together these allow electricity to produce electricity.
Show figure 12.2.3
DEMO: K3-04: DEMOUNTABLE TRANSFORMER - V VS N –
OSCILLOSCOPE
More on power distribution and transformers
Review induction
DEMO: K2-02: INDUCTION IN A SINGLE WIRE
DEMO: K2-22: INDUCTION COIL WITH LIGHT BULB
Show picture of transformer 12.2.8 power distribution
And figures 12.2.6 & 7 for step up and step down
Equations
Secondary voltage = prim voltage x sec turns / prim turns
Sec current = prim current x prim turns / sec turns
Voltage x Current = Power is the same! Assuming 100% efficiency
DEMO: K3-04: DEMOUNTABLE TRANSFORMER - V VS N –
OSCILLOSCOPE
DEMO: K3-05: DEMOUNTABLE TRANSFORMER – WELDER
DEMO: K2-28: DEMOUNTABLE TRANSFORMER - 10 KV ARC
Electric Power Generation:
Use induction just like in transformer. But, now the changing
magnetic field is produced by rotating magnet.
Lenz’s law means the induced current will opposes motion of magnet
 need to perform work to rotate the magnet.
Do demo of induction again and show figure 12.3.1 & 2 from book.
The more current you draw (flows in the coil) the more the field
resists the motion and the more work you have to do.
Even with transformer in between house and generator
DEMO: K3-06: TRANSFORMER - PRIMARY CURRENT VS LOAD
DEMO: K4-07: BICYCLE GENERATOR
DEMO: K4-41: MOTOR-GENERATOR PAIR
Need work (energy) to turn generator.
Steam produced by heat (nuclear, coal, oil) or hydro (gravity) to turn a
generator. Use a turbine.
Like a fan blade in reverse. Force steam or water or air through
it to make it turn.
Other forms.
Solar electric panels.