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Bellringer

What are some ways that you use electricity in your daily life?

Objectives

We will investigate electricity as you are used to seeing it, and using it at home

Learn what electric current is and what an electric circuit it

Electricity as you know it!

- Electric Current is a flow of charged particles.
- Conventional Current is the direction in which a positive test charge moves.
- The flow of electrons and conventional current are in opposite directions

Electric Current

Electric Current is measured in Amperes (A, or amps)

1 Ampere is equal to 1Coulomb moved past a single point in 1 second

Electric Current

Electric current is the measure of electrons moving past a certain point. Much like the current of a river is measure by the amount of water moving past a certain point.

Electric Current

I is current measured in Amperes (A)

Delta q is charge measured in Coulombs(C)

t is time measured in seconds (s)

Amps

0.001-0.01 Amps = Little to no shock

0.01-0.02 Amps = Painful shock

0.02-0.075 Amps= Serious shock, muscle control lost

0.075-0.1 Amps = Heart misses beats

0.1-0.2 Amps= Changes heart’s rhythm, possible death

Amps

Over 0.2 Amps = Severe burns and muscle contraction. Heart stops from chest muscle pressure. Internal organs are damaged. Can survive if you are removed from the circuit fast enough. Very painful recovery, but heart very likely to go back to its regular beat

Amps

If a wire has 1 amp of current running through it should you touch it?

http://www.youtube.com/watch?v=umDr0mPuyQc

Examples

If 2.5 Coulombs move past a point in 3 seconds, what is the current?

Practice

What is the current of a 6C charge that moves past a point in 2.5 seconds?

I=2.4 Amperes

How long does it take a 5 amp current to move 3 Coulombs of charge past a single point?

t=0.6 seconds

Potential Difference

CHARGE PUMP

Electric Circuits

- Electric Circuit: Any closed loop or conducting path allowing electric charges to flow.
- All circuits must have two things:
- Charge pump (to increase electric potential)
- Something to reduce the electric potential

Moving Electrons

- Light Bulb Demo
- Difference between plugged in and out

Infinite Energy Idea

Could a machine like this produce an infinite cycle of energy? Why?

Where is the energy lost?

Objectives

Recreate Ohm’s discover of Ohm’s Law, and practice its application.

Learn how to draw schematic diagrams of electrical circuits.

Learn about “electrical friction” in long wires and other objects

Energy Lost in Wires

Just like friction in mechanics, electric circuits have friction that cause energy to be lost. It is called “Resistance”

Ohm’s Law

Around 1825 Ohm was experimenting with electricity when he noticed an important trend between the current in a circuit and the voltage.

Investigating Ohm’s Law

For the sake of science will recreate Ohm’s experiment to see if we can come up with Ohm’s Law on our own!

Discovering Ohm’s Law

- Record this data!
- Supplies:
- Voltmeter
- Ammeter
- Wires
- Batteries

Ohm’s Law

R is the resistance measured in Ohms (Ω)

V is the potential difference measured in Volts (V)

I is the current measured in Amperes (A)

Resistance

The resistance of an object in a circuit can be thought of like a frictional force because it makes it harder for the electrons to move.

Resistance Demos

How can we tell that electrical energy is being “lost” due the resistance of these objects?

Hotdog Demo

Pickle Demo

Example

What is the resistance of a light bulb that has a current of 2A running through it and is connected to a 120 volt power source?

R=60Ω

Practice

What is the current running through a 100 Ohm motor that is connected to a 120 V source?

1.2A

What is the voltage across a hotdog that has a resistance of 10 Ohms and has 0.2 amps running through it?

2V

Schematics

http://phet.colorado.edu/en/simulation/circuit-construction-kit-dc

Schematics

The easiest way to describe the different parts of a circuit is with a Schematic

A schematic is a diagram that symbolically represents and simplifies a circuit

Schematics

Different symbols are used for different parts of a circuit

The symbols you need to know are found on your reference table

Schematics

The positive side of the battery is the longer end.

To show the current through the circuit you draw arrows showing which ways positive charges would flow.

Schematic Practice

If the diagram to the right has a 1.5V battery hooked up to it, why won’t the light turn on?

Bellringer

DO WORK

STOP

What is the resistance in a wire that is connecting both ends of a 9 volt battery and has a current of 1.5 A running through it?

Objectives

Review your schematics from last night

Learn about how different materials have different resistances.

Variable Resistance Demos

Picture Practice

On page 603 answer questions 8 through 11

Physics Trip

Darien Lake May 19th?

Is Darien Lake nice?

Is it worth e-mailing them for more information?

Resistance in the Wires

In schematics we will ignore the resistance in the wires. (Much like we ignore air resistance).

But this will not always be the case.

Resistance in a Wire

In real life it is often important to know how much voltage you lose due to resistance in a wire.

It is easy to calculate!

Resistance in a Wire

R is the resistance in the wire measured in Ohms (Ω)

is the resistivity of the wire measured in Ohms*meter (Ω*m)

L is the length of the wire measured in meters (m)

A is the cross-sectional area of the wire measured in meters squared ()

Resistivity

Resistivity is a property of each material.

It is a constant number that is different for each material.

Example

What is the resistance in a copper wire that is 2km long, and has a cross sectional area of ()?

Practice

What is the resistance is a nichrome wire that is 12 meters long and has a cross sectional area of ?

3.6 ohms

What is the resistance of a gold wire that has a length of 1 meter and a diameter of 1mm?

0.031ohms

Bellringer

DO WORK

STOP

What is the resistance in a wire that is connecting both ends of a 9 volt battery and has a current of 1.5 A running through it?

Objectives

Recap Resistivity

Variable Resistor Demos

Learn how to measure the current and the voltage at different points in a circuit

Variable Resistor Demo

A variable resistor or a slide resistor is a very handy device.

It allows you to change the resistance of an object to any resistance in a set range.

Examples: Dim light switches, volume knobs, fan speed, heaters, etc.

Measuring Current

You measure current with an “Ammeter”

Current is how much charge is going through a loop of a circuit so you must connect the ammeter in the loop of the circuit.

Measuring Potential Difference (Voltage)

You measure potential difference (voltage) with a “Voltmeter”.

When you measure voltage you measure between two points or “across” an object in the circuit.

Bellringer

DO WORK

STOP

Draw a circuit with a battery, a light bulb, an ammeter, and a voltmeter that is measuring the potential difference across the light bulb.

Objectives

Begin to practice your skills as an electrical engineer by working on the next lab.

Objectives

Collect labs and review homework

Learn how electrical power is calculated and use it to solve problems and relate it to everyday items.

Labs

Please pass up your labs if you did not turn it in yesterday.

Grades

Answer the following questions in complete sentences:

Did you follow through with what you wanted to change last time?

Are you currently happy with your grade?

What will you do to improve your grade?

Power

Power, which is still measured in Watts, is the rate at which energy is transferred.

Most appliances, such as a hair dryer are rated in watts.

Power

If a generator transfers 1 Joule of kinetic energy to electrical energy each second, it is transferring energy at the rate of 1 Joule/second.

Power

It is more common to use P=VI when working with electricity, so we tend to use this equation when solving for the power of a part of a circuit.

Power

Pis the power, in Watts

V is the potential difference, in Volts

I is the current, in Amps

R is the resistance, in Ohms

This is useful because you only need two variables to calculate the power!

Example

How much power does an oven draw from a circuit breaker if the oven has a potential difference of 220V and draws 40 amps of current?

P=VI

P=220V*40A

P=8,800Watts

Practice

How much electrical current does a clothing iron use that draws 1,600W and has a potential difference of 120V?

P=VI

1,600W=120V*I

I=13.33Amps

Circuit Breakers

Circuit breakers are an engineering marvel that prevent billions of house fires

When an appliance is trying to draw a dangerous amount of current from a circuit breaker it “trips”, or disconnects itself from the circuit, causing all current to stop.

Tripping Breakers

A breaker is easily reset by just moving the switch.

If a breaker continually trips it means you must go and unplug something from that circuit, or that the circuit is corrupted (improperly grounded, or shorted)

Homework

Find a household appliance (hair dryer, oven, washing machine, hair straightener or curler, Xbox, TV, laptop, etc.) and figure out its power, current, voltage, and resistance.

Go look at your circuit breaker, and find out what is the largest and smallest breaker in your home. Calculate the maximum number of amps you can have coming into your house at one time.

Practice

On page 601 in the textbook read through “Example problem 1” alone.

Then answer questions 1-7 with a partner.

Objectives

Go over your homework and the textbook problems from the past weekend.

Learn about electrical energy and how we pay for it.

Learn the difference between types of circuits.

Electricity in your home.

What appliance did you analyze?

How powerful was it?

What was your weakest circuit breaker?

What was your strongest circuit breaker?

What is the maximum current you can have coming into your house?

Practice

On page 601 in the textbook read through “Example problem 1” alone.

Then answer questions 1-7 with a partner.

Work

As always work is the change in energy.

Now we’ll focus on the change in electrical work instead of the change in mechanical work.

Work

W is work, in Joules

P is power, in Watts

t is time, in seconds

V is potential difference, in Volts

I is current, in Amps

R is resistance, in Ohms

Example

How much electrical energy is used by a 300 Watt blow-dryer that is used for 2 minutes?

W=Pt

W=300W*120s

W=36,000 Joules

Kilowatt hours

The SI unit of energy is Joule, but in RL we use “Kilowatt Hours” to buy energy from the energy company.

W=Pt, or Joule=Watt second

1,000 Watts are in 1 Kilowatt

3,600 seconds in 1 hour

3,600,000 Joules = 1 Kilowatt Hour

Answer page 611

a. 8.0A b. 29,000J c. 29,000J

a. 1.2A b. 16,000J

a. 1,300J b. 4,700J

a. 20A b. 130,000J

1.1 hours

Types of Circuits

- There are two types of circuits
- Series Circuits – when the current only has one way to flow
- Parallel Circuits – when the current has two or more ways to flow

Measuring

When you measure the current you wire the ammeter in series

When you measure voltage you wire the voltmeter in parallel

Bellringer

How would you wire an ammeter into a circuit?

How would you wire a voltmeter into a circuit?

Lab – “Running Some Wires”

You and your partners will be responsible for investigating which is the most efficient way to wire a house.

Series Circuit

Why do both light bulbs glow equally?

Charge cannot be created destroyed and since the charge only has one path to flow, all of it must flow through both light bulbs.

Series Circuit

If current does not decrease when it passes through a lamp, where does the energy to light the lamps come from?

P=VI

The current doesn’t change so there must be a potential difference across each lamp.

Series Circuit

The resistance of each light bulb must be constant, if the potential difference, and the current are both constant.

Series Circuit

From the river model, you know that the height from sea level to the top of the mountain equals the height that the water drops.

The potential difference across the battery equals the sum of the potential differences across lamp 1 and lamp 2.

Series Circuit

So you can write:

Where is the voltage of the battery, generator, etc.

are the potential differences of all the different objects in the whole series circuit

Series Circuit

Where Req is the total resistance in a series circuit

are the resistances of each object in the circuit

Series Circuit

The current in a series circuit is the same everywhere you measure it so

Series Circuits

These are the three equations you’ll need to be able to use to find the total current, voltage, and resistance in a series circuit.

Example

What is the potential difference across each resistor?

V1=2 volts

V2=6 volts

V3=1 volts

V=2V+6V+1V=9 volts

Practice Problems

Page 626 #’s 1-4

What is the total voltage of the batteries?

What is the equivalent resistance in this circuit?

What is the current?

What is the voltage across each resistor?

Answers

66Ω, 2.9A

32Ω, 2.3A

200Ω

a. It will increase b. It will decrease c. No; it does not depend on the resistance

Parallel Circuits

Picture a mountain that has multiple rivers running down it all with different amounts of water.

Same drop in height over each path, but can have different amounts of water

Parallel Circuits

Similar to the mountain, in parallel circuits the total current is the sum of the currents through each path, and the potential difference across each path is the same.

Parallel Circuits

Draw and label this diagram (make it big)

What is the voltage across each resistor?

What is the current in each part of the circuit?

What is the total current?

Parallel Circuits

I is the total current in the circuit

and the currents across each segment of the parallel circuit

V is the total potential difference in the circuit

are the voltages across each resistor in the parallel circuit

Parallel Circuits

So we know how to find the total current and the total potential difference in a parallel circuit. What about the total resistance in a parallel circuit?

Parallel Circuits

But 90+45+180 doesn’t equal 25.7!

That’s because placing 2 or more resistors in parallel always decreases the equivalent resistance of a circuit.

The resistance decreases because each new resistor provides an additional path for current, thereby increasing the total current while the potential difference remains unchanged.

Parallel Circuits

Req is the total resistance in a parallel circuit.

R1,R2,R3 are the resistances of each resistor in the parallel circuit

Example Part 2

What is the potential difference across each resistor if the battery is 12V?

What is the equivalent resistance of the circuit?

What is the current across each branch of the parallel circuit?

What is the total current moving through the parallel circuit?

Practice

On page 633 answer questions 14-16

Answers to page 633

a. 5.00Ω b. 6.00A c. 2.00A

a. It gets smaller b. It gets larger c. It remains the same. Currents are independent.

a. 20.0 b. 0.600A c. 0.100A, 0.200A, 0.300A

The Next Level Circuit

What the heck is going on over there?

Series-Parallel Circuits

To solve a series-parallel circuit you must “convert” it into a simple series circuit.

Example

If V is 12 volts and all the resistors are 100 ohms what is the total resistance?

Req =

Now it’s a series circuit!

So the total resistance is 150 Ohms

Example

What is the total current?

Series-Parallel Circuit Lab

The best way to practice this is with some hands on circuit working!

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