u3e-l3 1.why are the + and – symbols unnecessary when drawing the power source in an electric...

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1. Why are the + and – symbols unnecessary when drawing the power source in an electric schematic?

2. A ______ circuit occurs when the positive terminal is connected directly to the negative terminal by a conductor, with no resistance.

February 25, 2010

DRILL

Short

1. The length of the lines tell you which is positive and which is negative

What is the difference between an open circuit and a closed circuit?

A closed circuit is one in which the pathway of the electrical current is complete and unbroken.

An open circuit is one in which the pathway of the electrical current is broken. A switch is a device in the circuit in which the circuit can be closed (turned on) or open (turned off).

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1. What is a schematic?

2. Which of the following is a schematic?

Electric Schematics

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

Circuit Schematics

• Series connection– Connected along a single path– Same current

• Parallel connection– Split and merge– Same voltage

Resistors connected in series

Resistors connected in parallel

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 U3e-L3

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

Problem #5 – 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

Problem #5 – Joule’s Law

5V

.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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Homework Problems #2 and 3 Review

Problems – Joule’s Law

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1) Write the given information:

V = 9 Volts I = .02 AmpsR = ?

3) Rearrange, Substitute, and Solve

Homework Problem #2

9V

.02A

R = ?

2) Sketch a schematic diagram of the circuit:

V = I x R

R = 9V / .02A

R = 450 W

R = V / I

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This is how alternating current (AC) power sources are represented schematically

1) Write the given information:

V = 120 Volts I = 5 AmpsR = ?

3) Rearrange, Substitute, and Solve

Homework Problem #3

120V

5A

R = ?

2) Sketch a schematic diagram of the circuit:

V = I x R

R = 120V / 5A

R = 20 W

R = V / I

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Electric CircuitsWhat is a circuit?

A closed path through which an electric current may flow

Closed loop network of electrical elements

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• The unidirectional flow of electrons.

• Direct current is current travelling in 1 direction – from the negative terminal to the positive terminal.

• Direct current is used in almost all electronic systems as the power supply.

• Batteries are a sign of direct current.

Direct Current

one

I

1.5 V

• Measuring the voltage of a direct current power supply:

• The voltage is constant.

Direct Current

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• If voltage is constantly 1.5 V, what does its graph over time look like?

Direct Current

Time (seconds)

Volt

ag

e (

volt

s)

0 2 4 6 8 10 12 14

1

2

3

1.5 V

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• Electrons flow back and forth through an electric circuit.

• Most artifacts you plug in.

Alternating Current

120VR = ?

120 V+-

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• If voltage is alternating 120 V to -120 V, what does its graph over time look like?

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Alternating Current

Time (seconds)

Volt

ag

e (

volt

s)

0 2 4 6 8 10 12 14

-1

20

0

1

20

+/-120 V

What are batteries?Batteries are composed of a chemical substance which can generate voltage which can be used in a circuit.

There are two kinds of batteries: dry cell and wet cell batteries. Below is an example of a dry cell.

The zinc container of the dry cell contains a moist chemical paste surrounding a carbon rod suspended in the middle.

Wet cell batteries are most commonly associated with automobile batteries.

A wet cell contains two connected plates made of different metals or metal compounds in a conducting solution. Most car batteries have a series of six cells, each containing lead and lead oxide in a sulfuric acid solution.

What are electric circuits?Circuits typically contain a voltage source, a wire conductor, and one or more devices which use the electrical energy.

What is a series circuit? A series circuit is one which provides a single pathway for the current to flow. If the circuit breaks, all devices using the circuit will fail.

• Resistors in series are added:

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Resistance Calculations

R1 R2 R3

R = R1 + R2 + R3

T

Total Resistance:

What is a parallel circuit?A parallel circuit has multiple pathways for the current to flow. If the circuit is broken the current may pass through other pathways and other devices will continue to work.

• Resistors in parallel: add their reciprocals

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Resistance Calculations

R1 R2 R3

R R1 + R2 + R3T

Total Resistance:

1 1 1 1=

• Calculate the resistance in the following circuit:

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Resistance Calculations

R = R1 + R2 + R3

T

Total Resistance:100 W 100 W 100 W

R = 100 W + 100 W + 100 W

T

R = 300 W

T

• Calculate the resistance in the following circuit:

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Resistance Calculations

100 W

R R1 + R2 + R3T

Total Resistance: 1 1 1 1=

100 W 100 W

.03 .03

1 1 1

=100 100 100

+ +

.01 + .01 + .01

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Resistance Calculations

R R1 + R2 + R3T

1 1 1 1=

RT

1 =

RT

1.03= (Multiply both sides by

Rt)

1 = .03 x Rt

Rt = 33.3 W

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1. Complete the worksheet on electrical problems using Joule’s and Ohm’s Laws.

2. Solve for V, I, R, and P for each of the six problems.