If you want your boats or anything on your boats, go get them now. Otherwise, just leave them here.

Electric Current and Circuits

HEAT will flow if there is a difference in temperature

WATER will flow if there is a difference in pressure.

Is the pressure inside the jug (due to the weight of the water) the same as the pressure outside the jug (due to the weight of the atmosphere)?

If the pressure outside was the same as the pressure inside, would the water flow out?

At which location is there a larger difference between the pressure inside and outside, near the top of the water level or near the bottom of the water level?

What evidence can you see that demonstrates a larger difference in pressure at the two locations?

Electrons will flow if there is a difference in electric pressure.Electric pressure is called “Potential”, and is measured in Volts.If there is no difference in pressure from one location to another, the electrons will not flow.In other words, if there is no

“Potential Difference, V” from one location to another, there will be no electric “current”.

Current

CURRENT: a flow of charged particles (electrons) through a conductor

Current, I, is measured in amperes, A, or “amps”.

timecharge

I Andre Ampere

Example:

What is the current, I, if 8 C of charge passes through a wire in 2 seconds?

I = q / t

I = 4 amps

How long would it take for 3.2 x 1018 electrons to pass through the wire?

t = q / I

But… what is the charge, q?

q = (3.2 x 1018 )(1.6 x 10-19C) =

0.512 C

t = 0.512 C / 4 A = 0.128 seconds

The Damage Caused by Electric Shock

1 mA = 0.001 A

Mild shock can be felt

5 mA = 0.005 A

Shock is painful

15 mA = 0.015 A

Muscle control is lost

100 mA = 0.1 A

Death can occur

60W light bulb - 0.5 AStarter motor – 210 AClothes dryer – 18 AIron- 3 A

CircuitsELECTRIC CIRCUIT: Charges

moving in a closed loop • A circuit requires a both a

conductor, usually metal wires, and a “charge pump”.

• CHARGE PUMP: a device that provides a potential difference so that charges keep moving.

Alessandro Volta

The Potential Difference, V, provided by the charge pump is called its VOLTAGE.

If the voltage of a battery is 9 V, this means there is a difference of 9 V of potential (pressure) between the positive terminal and the negative terminal.

If the voltage of a battery is 1.5 V, this means there is a difference of 1.5 V of potential (pressure) between the positive terminal and the negative terminal.

The pressure DIFFERENCE, the VOLTAGE, is required for charges to flow!So, charge pumps such as batteries, are called voltage sources.

Circuits• This potential difference is

sometimes called the emf, (electromotive force)

• Examples of charge pumps: batteries, solar cells, generators, power supplies

The source of the electrons moving in the circuit is NOT the battery or the wall outlet! The free electrons are contained within the wire itself.An individual electron does not actually travel all the way around a circuit.One electron bumps into the next that bumps into the next that bumps into the next …..It is the ENERGY that gets transferred all the way around the circuit.You are not buying electrons from your electric company- you already have them! You are buying energy!

Resistance all conductors offer some

resistance to the flow of charges, even metal wires.

RESISTANCE =

The unit for resistance is the OHM, .

This equation is often called OHM’S LAW

IV

currentvoltage

R

George Ohm- first determined the math relationship now called Ohm’s Law

IV

currentvoltage

R

Example

What is the resistance of an appliance if 2 amps of current run through it when supplied with 120 V?

R = V / I

R = 120 V / 2 A

R = 60

WATER ANALOGYWater ElectricityFlow of water current flow of chargeWater pump keeps flow going charge pumppsi. pressure

voltagePipes of different resistance wires of

diameter different diameter

Small electrical components called “resistors” are inserted into circuits to control the amount of current flowing through that part of the circuit.

Certain metals offer less resistance to the flow of charges than others.

Example: Copper is a better conductor than iron

The resistance of a wire of length L and cross sectional area A is given by

RESISTANCE, R =

where is the resistivity of that

particular metal.

A

L

The resistivity increases as temperature increases. So, a hot wire resists the flow of charges more than a cooler wire.

RESISTANCE, R =

AL

Series and Parallel Wiring

SERIES CIRCUITSThe same current through each

device.

EQUIVALENT RESISTANCE: What is the net resistance? What one resistor could replace a group of resistors?

For resistors wired in series, the equivalent resistance is given by:

Req = R1 + R2 + R3 + …

PARALLEL CIRCUIT

Same voltage across each device.

For resistors wired in parallel, the equivalent resistance is given by:

...R

1

R

1

R

1

R

1

321eq

Example: What is the equivalent resistance of a 10 , 20 , and 30 resistor wired in series? In parallel?

Series: Req = 10 + 20 + 30 = 60 Parallel 1 / Req = 1/10 + 1/20 + 1/30

Req = 5.45

There’s much less resistance if resistors are wired in parallel than if they’re wired in series.

With less resistance, the charge pump will able to push much more current around the circuit.

Characteristics of Series and Parallel Wiring

Series: If one component goes out,They all go out!

As more resistors are added, the equivalent resistanceIncreases!

which means that the current in that part of the circuitDecreases!

Parallel: If one component goes out,The rest still work!

As more resistors are added, the equivalent resistanceDecreases!

which means that the current in that part of the circuitIncreases!

For maximum resistance- use series wiring.

For minimum resistance- use parallel wiring.

The flow of water is a very good analogy to the flow of charges in both series and parallel circuits.

Schematic diagramssymbols to represent circuit components.

wires:

Charge pumps:

Resistors:

Switches:

Ground:

All devices connected to a circuit (light bulbs, TV’s, toasters, etc.) resist the flow of charges and are sometimes drawn as a resistor in the circuit (if you’re considering the unit as a whole).

For resistors wired “in series” the same current flows through each one, however the potential difference, voltage, is additive

Voltage gain through battery

=

Sum of Voltage drop through resistors

For resistors wired “in parallel”, the potential difference, voltage, is the same for each of them, however the current is additive.

Total Current pushed by battery

=

Sum of Current going through all the resistors

Electric Power

Power is the rate that work is done or energy is transferred, that is

Power =

Power is measured in Watts, W

)onds(secTime)Joules(Energy

Electric power delivered to a circuit by a power supply is given by

Power = Current x Voltage

P = IV

This equation can be combined with Ohm’s Law, R = V / I in its different forms:

V = IR, I = V / R

Electrons do NOT leave the circuit-

Energy leaves the circuit through the different “resistors” in the form of light, heat, and any kind of work done by the appliance the current is running through.

The rate that the energy leaves the circuit is the power output.

ExamplesHow much energy does a 75 W light bulb give off in

five minutes?Power = Energy / timeEnergy = Power x timeEnergy = 75 W x 5 x 60 seconds Energy = 22500 JWhat is the power output of a 3 A motor running on

regular house voltage?P = IVP = 3 A x 120 VP = 360 W

“Power” lines• The higher the current, the more

the wires in the circuit heat up, thereby “wasting” energy. This is a big problem when electric companies must provide electricity at great distances away from the power plants.

• The solution: Electric lines that carry current great distances are at very high voltage, so the current can be relatively small.

P = IV

P = IV

High Voltage / Low Voltage

TRANSFORMERS: devices that “step-up” the voltage at the power plant and then “step-down” the voltage at the customers’ location.

Even with very high voltage, there is still some current running through those wires. Therefore, some electrical power (dissipated through heat) is lost. If the entire length of wire has a total resistance R, the power lost along the way is given by

Power lost (dissipated) = I2R

Electrical Safety

Open circuit Closed circuit

Req = R1 + R2 + R3 + …...R

1

R

1

R

1

R

1

321eq

Each bulb has the same current running through it! That current is the TOTAL current pushed by the battery, IT = Vbattery / Req

Each bulb has the same voltage across it! That voltage is the voltage of the battery.However, the current splits up so that each bulb only receives a portion of the total current pushed by the battery

Series CircuitParallel Circuit

Electrical Safety

• Fuse: a short piece of metal that melts if current exceeds a set value (to protect device)

• Circuit breaker: an automatic switch that opens the circuit when current exceeds a set value. (uses a bimetallic strip)

• Ground-fault circuit interrupter: opens a circuit if the current going into a device is not the same as the current coming out of the device.

• gfci

• Short Circuit- a circuit that is formed when wires touch. This effectively shortens the path of the circuit because the current no longer passes through the resistor. The extremely low resistance in the circuit produces very high current, which could cause melt down and fire.

NOT a good idea!!

• Ground wire: provides a path for high current to go in case of a short circuit-

• Voltmeter- device to measure voltage

• The voltmeter is placed “in parallel” with the component whose voltage is being measured.

• Ammeter- device to measure amps (current)

• An ammeter is placed “in series” with the component whose current is being measured.

• Ohmmeter- device to measure resistance

• Resistance is measured with the power OFF!

2.3

• Multimeter- can measure current, resistance, and voltage!

• Diode- allows only one direction of current to pass through it.

• Light emitting diode, LEDAs electrons pass through these

“semiconductors”, light is emitted

• Transistors are devices that are often used to amplify small signals into a larger one.

Alternating Current, AC

The electricity in your houses is alternating current, AC, not direct current, DC – as in batteries.

The main reason for this is that transformers can only step up or step down the voltage if it is alternating current.

Without transformers, there would be great loss of electrical power between the power stations and your houses. For electricity to be very efficient, there would have to be a power generating plant in EVERY community!

Alternating current means that the voltage pushes back and forth, not just one direction.

The electrons just wiggle back and forth.

In the US, they wiggle back and forth 60 times every second.

In other words, the frequency is 60 Hz.

The first use of alternating current in the US was at the Chicago World’s Fair, in 1893, where Nicholas Tesla’s alternating current circuits were utilized instead of direct current, which was backed by Thomas Edison and his General Electric Company.

• Tesla Coil• Alternating current• Systems for wireless communication (radios, remote

controls)• robotics• electrotherapy• wireless transfer of electricity• x-ray tubes• arc lights• concepts for electric vehicles• devices for lightning protection• concepts for vertical takeoff aircraft

Ferris designed and built the first 264 foot (80 meter) wheel for the World's Columbian Exposition in Chicago, Illinois in 1893. This first wheel was 26 stories tall and could carry 2,160 persons. There were 36 cars accommodating 60 people each (40 seated, 20 standing). It took 20 minutes for the wheel to make two revolutions - the first to make six stops to allow passengers to exit and enter; the 2nd a single non-stop revolution - and for that, the ticket holder paid 50 cents. The wheel was moved twice after the 1893 Fair and was eventually disassembled in 1906.