U3e-L2

• Moving electrons have _______.• The _______ of an electric circuit uses

the electrons’ ability to do work.• The three basic units in electricity are

_________________________________.

February 28, 2011

DRILL

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

EnergyLoad

• 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

U3e-L2

TERMINOLOGY

U3e-L2

• Current (I) is the rate that charges flow during a given time– Current = Charge/Time– Current is measured in ampere (amps).

• Amp = Coulomb/second

TERMINOLOGYPower source: An artifact that provides a supply of “free

electrons to a circuit. A power source has a positive (+) and a negative (-) terminal.

The Power Source is the origin of the Voltage in a circuit.

Two types of Power Source:

1. Direct Current (DC) Power Source.Schematic symbol:

2. Alternating Current (AC) Power Source. Schematic symbol:

Realistic picture

Realistic picture

Power Source Symbols

DC Power Source: + (Positive Terminal - Long line)

- (Negative Terminal – Short line)

+

-

This is where the symbol

comes from

“Wet cell”- car battery

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

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

TERMINOLOGY

U3e-L2

U3e-L1

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

• Discovered ~1827• Voltage = Current x Resistance

The formula: V = I x R

What are the units of measure of this formula?

Volts(v) = Amperes(amps.) x Ohms(Ω)

Ohm’s Law

Voltage Current Resistance

Electrical Calculations – What is Ohm’s Law?

I =

3 V2 Ω

I = 1.5 amps

Schematics

A schematic is a diagram that represents the elements of a system using symbols instead of realistic pictures.

Remember: A complete circuit consists of

power source, conductor, and load.

Schematic vs. Realistic Diagrams

Which one is easier to draw?

+_

Power source

Conductor

Load

Power source

Conductor

Load

http://search.aol.com/aol/imageDetails?s_it=imageDetails&q=pedestal+fan&img=http%3A%2F%2Fwww.northerntool.com%2Fimages%2Fproduct%2Fimages%2F250723_lg.jpg&site=&host=http%3A%2F%2Fwww2.northerntool.com%2Ffans%2Fpedestal-fans%2Fitem-250723.htm&width=124&height=124&thumbUrl=http%3A%2F%2Fimages-partners-tbn.google.com%2Fimages%3Fq%3Dtbn%3AGxOVMweYdNv8YM%3Awww.northerntool.com%2Fimages%2Fproduct%2Fimages%2F250723_lg.jpg&b=image%3Fs_it%3Dtopsearchbox.image%26imgsz%3D%26q%3Dpedestal%2Bfan%26oreq%3De5d5e770588a4b759f7b77c26a94a3aa&imgHeight=400&imgWidth=400&imgTitle=%3Cb%3EPedestal%3C%2Fb%3E+%3Cb%3EFan%3C%2Fb%3E+%E2%80%94+30in.%2C&imgSize=10911&hostName=www2.northerntool.com

SCHEMATIC DIAGRAMSa.k.a. Schematics

(+)

(-)

POWER

SOURCE

CONDUCTOR

LOAD

CURRENT

(arrow shows the direction of electron flow)

(D.C.)

PROBLEM SOLVING

1. Draw the diagram.

2. List the table of knowns and unknowns.

3. Write the formula.

4. Substitute.

5. Calculate.

6. Box in answer.

U3e-L2

An Ipod uses a 0.5 volt battery. The current in the ear bud circuit is 0.2 amperes. What is the resistance of the ear bud?

Step #1 DiagramStep #2 List Information

Step #3 Formula

Step #4 Substitute

Step #5 Solve

V = 0.5v

I = 0.2amps.

R = ?

V = 0.5 V

I = 0.2amps.

R = ?

V = I x R

0.5 V = 0.2 A x R

2.5 Ω = R

U3e-L2

What voltage is needed to produce a 4 Amp circuit with a 5 Ohm resistor?

Step #1 DiagramStep #2 List Information

Step #3 Formula

Step #4 Substitute

Step #5 Solve

V = ?

I = 4 amp

R = 5 Ohm

V = ?

I = 4 Amp

R = 5 Ohm

V = I x R

V = 4 A x 5

V = 20 Volts

Solve the following circuit problems:

(USE ALL 6 STEPS!)

1. V = 6 v I = 2 amps. R = ?

2. V = 8.5 v I = ? R = 2.125 Ω

3. V = ? I = 9 amps. R = 10 Ω

4. The heater fan on the #33 bus is on a 7 amp. circuit.The bus has a 24 v battery. What is the resistanceof the fan motor?

U3e-L2

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 = ?

U3e-L2

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

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

U3e-L2

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.

U3e-L2

U3e-L1

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

U3e-L2

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

U3e-L2

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:

U3e-L2

Complete problem #5 on your worksheet

Problems – Joule’s Law

U3e-L2

U3e-L2

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

U3e-L2

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

U3e-L2

U3e-L2

Complete the worksheet on electrical problems using Joule’s

and Ohm’s Laws