#### Transcript Goal: To understand what Electric Fields are

Goal: To understand Electrical
Currents
3)
6)
Objectives:
1)
To understand The flow of charge
2)
To understand Voltage.
To understand Resistance and Ohm’s Law
4)
To learn about Shocking!
5)
To compare AC vs DC
To learn about the Motion of the electron ocean
7)
To understand POWER!
8)
9)
To understand Series Circuits
To understand Parallel Circuits
10) To learn about Fuses
11) To understand Transformers
Flow of charge
• Benjamin Franklin determined that current
was the flow of positive charge – even
though we know that it is the electrons that
really move.
• So, charge flows from high potential (or
high electric fields) to low potential (or low
electric fields).
• This is similar to how water goes from high
elevation to low elevations, and back into
the ocean.
Voltage
• Voltage is just a measure of how much
elevation you change.
• It is similar to measuring the height of a
mountain.
• So, and 8V battery takes charges and
pushes them 8 Volts uphill – electronically
that is.
• They are then free to flow back down.
Voltage - water
• Lets compare this to water.
• Imagine a fountain.
• What is the power source for the water to
go up high (what is the battery of the
fountain)?
• What is the power source for the
Hydrological cycle (the cycle which takes
ocean water, to clouds, to land, and
back)?
Why are resistors useful?
Resistors allow us to limit
the flow of current in a
circuit.
How calculated?
• Larger conduits offer less resistance.
• Some materials provide better flows than others.
• The further something has to flow the more
resistance it will encounter.
• So, R = ρ L / A (for those who really want it, but
we won’t be using it).
• Here ρ is not density but is a substances
resistivity (which can be looked up for any
substance).
• While there is a little dependence on
temperature the resistivity mostly depends on
the substance and only the substance.
Ohms Law
• There is a relation between resistance,
voltage, and current.
• Voltage = Current * Resistance
• Namely, how fast your water, or current
flows, depends on how high up you put it
and on the barriers you put into place
along its route.
Electrocution
• Two questions for you.
• 1) How can you tell that someone is being
electrocuted?
• 2) If you spot someone being electrocuted,
what can you do to save them without
putting yourself in harms way?
The human body
• Has a low tolerance to electrical current.
• Voltage doesn’t matter if there is no
current, but it doesn’t help.
• 0.07 Amps = fatal
• Using Omhs law, what Voltage will give
you a fatal current if your body’s
resistance is 1*105 Ohms
Grounded
• Most electric devices have a 3 pronged
cord.
• The 3rd prong goes to the ground.
• This way charges can’t build up on the
outside of the device.
• Why is this good, well if charges build up
and you touch it (say to turn on a lamp) –
ZAP!!!
AC vs DC
• There are two types of current.
• The first is Direct Current.
• Direct current is just a constant current (a
constant flow of charge).
AC
• The other type of current is alternating current.
• This current is the current we use in homes at it
is easier to transmit efficiently and effectively.
• The current does a sine wave in terms of
amount of current (from some max to zero, to
negative, to zero, to max, and so on).
• In the USA it does this 60 times per second
(frequency of 60 Hertz).
Electron motions
• You might think from what I have said that
a lot of electrons move down a wire like
cars going down the freeway.
• Well, this is not correct.
• All electrons go a random direction.
• BUT, when you apply an electric field,
there is a force, and that deflects the
electrons in a specific direction so that
more kinda go that way than the opposite.
However
• You don’t get a very high % of them going
in that direction net.
• Luckily for us though, there are a LOT of
electrons, and it does not take too many to
get a decent current.
In DC vs AC
• In Direct current you get some small direct flow
of charge.
• In AC though, the charges go back and forth –
sort of like a compression wave, or waves on an
ocean, or the springs we saw in class.
• This gives us our back and forth nature of the
current.
• So, when you pay for electric, you are paying for
energy, not electrons, and that is probably why
AC is more efficient at transferring energy.
POWER!
•
•
•
•
We have seen power before
(power = energy per time = work / time)
Circuits consume power.
Power = current * voltage
• A 60 W light bulb is attached to a 120 Volt
line. What is the current in that light bulb?
More power
• Suppose you have an object which has a
resistance of 100 Ohms that you attach to
a 120 Volt source.
• If you leave it on for 3 hours how many
watt hours of energy will it consume (this
will take a few parts)?
• Note – first find the current using Ohm’s
Law.
5 min break!
Resistors in series:
Resistors in series:
Reff = R1 + R2 + R3 + R4
Sample:
•
•
•
•
•
You have a circuit with 3 resisters:
R1 = 4 Ohms
R2 = 3 Ohms
R3 = 9 Ohms
What is the effective resistance of the
circuit?
Sample:
•
•
•
•
•
You have a circuit with 3 resisters:
R1 = 4 Ohms
R2 = 3 Ohms
R3 = 9 Ohms
What is the effective resistance of the
circuit?
• R = R1 + R2 + R3 = 4 + 3 + 9 = 16 Ohms
Resistors in parallel:
Resistors in parallel:
1
1
1_
=
+
Reff
R1 R2
With a math trick this would go to:
R = R1 * R2 / (R1 + R2)
Resistors in parallel:
You have 2 resistors in
parallel. R1 = 5 Ohms, R2 =
10 Ohms. What is the
effective resistance?
Resistors in parallel:
If just two resistors this simplifies to:
R1 * R2
Reff =
R1 + R2
So, R = 10 * 5 / (10 + 5)
R = 3.3 Ohms
Note that the resistance is lowered.
Resistors in parallel:
Voltage:
If the Voltage across E is 8V then
what is the voltage across R2?
Resistors in parallel:
Voltage:
In parallel the voltage across each
path is the same.
Think about it, they both fall down
the same height.
Calculating the current:
V = i R or i = V / R
So, in series:
i = V / Rtotal
In parallel:
For each branch, ibranch = V / Rbranch
And the total current is still i = V / Rtotal
Too much current
• What can happen if you have too much
current, and what can you do to prevent it?
Too much current
• What can happen if you have too much
current, and what can you do to prevent it?
• You can fry your electronics, or burn down
your house (please do not attempt at
home…)!
• How to prevent?
• There are 2 ways
Fuse
• A fuse is a material that will break once you
reach a certain current.
• So, a 20 Amp Fuse will break if the current gets
above 20 Amps.
• This will kill the circuit, and the current.
• In the days of old houses used fuses.
• If something went wrong, you went into the
basement with a flashlight and replaced the fuse
in the fuse box.
Modern day: circuit breaker
• As we will learn, currents generate
magnetic fields.
• You can design a device such that if the
magnetic field gets high enough, it closes
off.
• That is a circuit breaker.
• You don’t need to replace these, just turn it
back on.
• HOWEVER – be sure to fix the problem
first!
Transformers
• Transformers transform 1 voltage to another.
• For example high voltage power lines carry large
amounts of energy long distances.
• They do this to save on losses.
• Then, they transform that from 100k volts to the
120 volts you use in your home using a series of
transformers.
Current in transformer
• You cannot gain energy – so the energy you get
in one is the same as the other.
• Therefore the powers are also the same!
• So, P1 = P2
• Since P = IV therefore
• I1V1 = I2V2 (or I1/I2 = V2/V1)
• Since V2/V1 = N2/N1, therefore, I1/I2 = N2 / N1
• When you shrink the voltage you increase the
current and visa versa.
Sample
• A 120k voltage line carries 0.1 A of current.
• This line is passed through a transformer.
• The high voltage end has 3000 loops and the
low end has 3 loops. After the 3 loops is a
simple light bulb.
• A) What is the power that is passed through the
receiver to the light bulb?
• B) what is the current which the light bulb
• C) What is the voltage across the 3 loop
transistor?
Sample
• A 120k voltage line carries 0.1 A of current.
• This line is passed through a transformer.
• The high voltage end has 3000 loops and the low end
has 3 loops. After the 3 loops is a simple light bulb.
• A) What is the power that is passed through the receiver
to the light bulb?
• P1 = P2 = IV and you can use I1V1 or I2V2
• I1V1 = 120k V * 0.1A = 12000 W or 12 kW
• B) what is the current which the light bulb receives?
• I2/I1 = N1/N2, so I2 = 0.1A * 3000 / 3 = 100 A
• C) What is the voltage across the 3 loop transistor?
• V2/V1 = N2 / N1, so V2 = 120k V * 3/3000 = 120V
Transformer types
• There are 2 types of transformers:
• Step up transformers – these step up the
voltage (and decrease the current).
• These start with a small # of loops and the
other end has a large # of loops.
• If you reverse the step up transformer then
you have a step down transformer.
• These start with a large # of loops and end
up with a small # of loops – and decrease
the voltage while increasing current.
Conclusion
• We have learned a lot about the flow of circuits.
• We have learned that the flow (current) depends
on voltage (height) and resistance (barriers).
• We have seem that choosing resistance and
how we set up the resistance allows us to design
a circuit such that we get to choose the current
in the circuit.
• And if we don’t like a voltage, we can transform
it!
• Oh and something about power = current *
voltage