Goal: To understand Electrical Currents

Objectives:1) To understand The flow of charge

2) To understand Voltage.3) To understand Resistance and Ohm’s Law

4) To learn about Shocking!

5) To compare AC vs DC6) To learn about the Motion of the electron ocean

7) To understand POWER!

8) To understand Series Circuits9) To understand Parallel Circuits

10) To learn about Fuses11) 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

+ R2With 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:

So, R = 10 * 5 / (10 + 5)

R = 3.3 Ohms

Note that the resistance is lowered.

If just two resistors this simplifies to: R1 * R2

Reff =

R1 + R2

Resistors in parallel:

If the Voltage across E is 8V then what is the voltage across R2?

Voltage:

Resistors in parallel:

In parallel the voltage across each path is the same.

Think about it, they both fall down the same height.

Voltage:

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 receives?

• 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