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1. Pass over last night’s homework.

2. Moving electrons have _______.

3. The _______ of an electric circuit uses the electrons’ ability to do work.

4. The three basic units in electricity are _________________________________.

March 2, 2011

DRILL

Voltage (V), Current (I), Resistance (R)

EnergyLoad

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1. Identify 5 electrical technologies not mentioned in class today

• (Remember, an electrical technology produces, stores, controls, transmits or gets work from electrical energy)

2. Identify the problem that the technology system solves

3. List the technological subsystems that are used in the technology

4. Identify the technology that preceded it (i.e. what was used before the technology was invented?)

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LOAD

NEGATIVEPOSITIVE

INSULATORS

CONDUCTORS

TERMINOLOGY

• Current (I) – the motion of electrons from one atom to the next in a material. – In our circuit, electrons flow from ___________ terminal

to the ___________terminal.– This motion of electrons through our conductor (wire) is

considered current (like water through a pipe).

• Materials that resist current are called ___________.

• Materials that provide a path for current are called ___________.

• Current is measured in AMPERES (Amps)

• Voltage (V) – the electric potential between two points of opposite charge.

• Voltage is the potential energy of the electrical power source.

• Voltage is measured in Volts (V).

TERMINOLOGY

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• Resistance (R) –a measure of the degree to which an object opposes an electric current through it .

• Resisting an electric current often generates heat.– This is the principle behind how light bulbs work.

• Resistance is measured in Ohms (W – the Greek letter Omega)

TERMINOLOGY

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• George Ohm (1789 – 1854) – a high school and university mathematics instructor

• Discovered ~1827• Voltage = Current x Resistance

V = I R

What are the units of this formula?

Volts = Amperes x Ohms

Ohm’s Law

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Copy the steps to solve this problem on your worksheet:

1. A cell phone charger converts the wall outlet voltage to 5 Volts to charge the cell phone. The amount of current that the charger outputs is .7 Amperes. Sketch a schematic diagram of this circuit. What is the resistance of the cell phone battery?

1) Write the given information:V = 5 volts

I = .7 ampsR = ?

Problems – Ohm’s Law

A schematic is a diagram that represents the elements of a system without using realistic pictures

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1) Write the given information:V = 5 volts

I = .7 ampsR = ?

2) Sketch a schematic diagram of the circuit:

a) Start with power sourceb) Add conductorsc) Add resistance d) Add current

Problems – Ohm’s Law

V = 5V

I = .7A

-

+R = ?

1) Write the given information:

V = 5 volts I = .7 ampsR = ?

3) Rearrange, Substitute, and Solve

Problems – Ohm’s Law

V = 5V

I = .7A

-

+R = ?

2) Sketch a schematic diagram of the circuit:

V = I x R

R = 5V / .7A

R = 7.14 W

R = V / I

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Electrical Power

• Electric Power – the rate at which electrical energy is transferred by an electric circuit.

• Units of power are the Watt (W)

• A human climbing a flight of stairs is doing work at a rate of about 200 watts.

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Joule’s Law

• James Joule (1818 – 1889) – showed that heat and electrical energy are interchangeable.

• Joule’s Law:

Power = Voltage x Current

P = V I

Watts = Volts x Amperes

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Let’s look at problem #4, which is very similar to problem #1:

4. A cell phone charger converts the wall outlet voltage to 5 Volts to charge the cell phone. The amount of current that the charger outputs is .7 Amperes. Sketch a schematic diagram of this circuit. What is the power used by the cell phone in recharging its battery?

1) Write the given information:V = 5 volts

I = .7 ampsP = ?

Problems – Joule’s Law

A schematic is a diagram that represents the elements of a system without using realistic pictures

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1) Write the given information:V = 5 volts

I = .7 ampsP = ?

2) Sketch a schematic diagram of the circuit:

a) Start with power sourceb) Add conductorsc) Add resistance d) Add current

Problems – Joule’s Law

V = 5V

I = .7A

-

+P = ?

P = V I P = 5V x .7A

P = 3.5 W

1) Write the given information:

V = 5 volts I = .7 ampsP = ?

3) Write Formula, Substitute, and Solve

Problems – Joule’s Law

V = 5V

I = .7A

-

+P = ?

2) Sketch a schematic diagram of the circuit:

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Complete problem #5 on your worksheet

Problems – Joule’s Law

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5. A Honda Insight (hybrid) has a 144 Volt battery system in its trunk. The electric motor that the batteries power can generate 20,000 Watts of power. Sketch a schematic diagram of this circuit. What is the current traveling through this high voltage circuit?

1) Write the given information:V = 144 volts

P = 20,000 watts I = ?

Problems – Joule’s Law

A schematic is a diagram that represents the elements of a system without using realistic pictures

1) Write the given information:V = 144 Volts

P = 20,000 Watts I = ?

2) Sketch a schematic diagram of the circuit:

Start with power sourceAdd conductorsAdd load Add current

Problems – Joule’s Law

V = 144V

I = ?

-

+P = 20,000 W

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1) Write the given information:

V = 144 VoltsP = 20,000 Watts

I = ?

3) Rearrange, Substitute, and Solve

Problems – Joule’s Law

V = 5V

I = .7A

-

+R = ?

2) Sketch a schematic diagram of the circuit:

P = V x I

I = 20,000W / 144V

R = 138.9 A

I = P / V

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Complete the worksheet on electrical problems using Joule’s

and Ohm’s Laws