E.THEORY UNIT 2-OHMS LAW

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Safety, Basic Electricity, and Ohm’s Law

Section 1

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website, in whole or in part.

Electrical Quantities and Ohm’s
Law

Unit 2

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website, in whole or in part.

LEARNING TARGET

• Students will be able to:

• Define a coulomb

• Define an ampere

• Define a volt

• Define an ohm

• Define a watt

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website, in whole or in part.

ESSENTIAL QUESTION

• Why is it important to know the different parts of

electricity?

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website, in whole or in part.

SUCCESS CRITERIA

• SURFACE: I.W.B.A.T calculate different electrical values

using Ohm’s law

• DEEP: I.W.B.A.T discuss different types of electrical circuits

• TRANSFER: I.W.B.A.T select the proper Ohm’s law formula

from a chart

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website, in whole or in part.

SIDE NOTE

• Electricity

• Standard set of values

• Values of electrical measurement

• Standardized

• Understood by everyone who uses them

• Must be the same for everyone

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website, in whole or in part.

The Coulomb

• Quantity measurement for electrons

• One coulomb requires 6.25 x 1018 electrons

• 6,250,000,000,000,000,000 electrons

• Charles Augustin de Coulomb

• French scientist in 1700s

• Experimented with:

• Electrostatic charges

• Coulomb’s law of electrostatic charges

• Attraction and repulsion of forces

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website, in whole or in part.

The Ampere

• André Ampère

• Scientist in 1700s-1800s

• Worked in electromagnetism

• Amount of electricity flowing through a circuit

• One ampere (A) is defined as 1 coulomb of electricity

flowing past a point in one second

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website, in whole or in part.

FIGURE 2–1 One ampere equals one coulomb per second.

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website, in whole or in part.

FIGURE 2–2 Current in an electric circuit can
be compared to flow rate in a water system.

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website, in whole or in part.

Current Flow Theories

• Electron flow theory

• Current flows from negative to positive

• Conventional current flow theory

• Current flows from positive to negative

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website, in whole or in part.

FIGURE 2–3 Conventional current flow theory and electron flow theory.

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website, in whole or in part.

Speed of Current

• Establish exactly what is being measured

• Current: flow of electrons through a conductive substance

FIGURE 2–5 Electrons moving from atom to atom.

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website, in whole or in part.

Speed of Current (Cont’d)

• Consider whether the circuit is DC, AC, or radio waves

• Radio waves move at approximately the speed of light

• Velocity of AC through a conductor is less than the speed of

light

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website, in whole or in part.

Basic Electric Circuits

• A complete path must exist before current can flow

through a circuit

• A complete circuit is often referred to as a closed circuit

• If the switch is opened, there is no longer a closed loop and

no current can flow

• Often referred to as an incomplete, or open, circuit

• A short circuit has very little or no resistance

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website, in whole or in part.

FIGURE 2–6 Current flows only through a closed circuit.

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website, in whole or in part.

FIGURE 2–7 A short circuit bypasses the
load and permits too much current to flow.

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website, in whole or in part.

Basic Electric Circuits (Cont’d)

• Grounded circuits

• Occur when a path other than the one intended is

established to ground

• Grounding conductor

• Extra conductor

• Provides the return path and completes the circuit back to the

power source

• Used to help prevent a shock hazard

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website, in whole or in part.

FIGURE 2–9 The grounding conductor provides a low-resistance path to ground.

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website, in whole or in part.

The Volt

• Voltage: potential difference between two points of a

conducting wire

• Carrying a constant current of 1 ampere when power

dissipated between these points is 1 watt

• Also referred to as potential difference or electromotive force (EMF)

• Volt: amount of potential necessary to cause 1 coulomb to

produce 1 joule of work

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website, in whole or in part.

FIGURE 2–10 Voltage in an electric circuit can

be compared to pressure in a water system.

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website, in whole or in part.

The Ohm

• Unit of resistance to current flow

• Named after German scientist Georg S. Ohm

FIGURE 2–11 A resistor in an
electric circuit can be compared
to a reducer in a water system

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website, in whole or in part.

The Watt

• Amount of power being used in a circuit

• Named after English scientist James Watt

FIGURE 2–13 Force equals flow rate times pressure.

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website, in whole or in part.

FIGURE 2–14 Amperes times volts equals watts.

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website, in whole or in part.

Other Measures of Power

FIGURE 2–15 Common power units.

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website, in whole or in part.

Ohm’s Law

• It takes 1 volt to push 1 ampere through 1 ohm

• In a DC circuit, current is directly proportional to voltage

• Inversely proportional to resistance

• Formula:

• E (volts) = I (amperage) x R (resistance)

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website, in whole or in part.

FIGURE 2–16 Chart for finding values of voltage, current, and resistance.

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website, in whole or in part.

FIGURE 2–17 Using the Ohm’s law chart.

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website, in whole or in part.

FIGURE 2–18 Formula chart for finding values

of voltage, current, resistance, and power.

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website, in whole or in part.

Metric Prefixes

FIGURE 2–22 Standard metric prefixes.

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website, in whole or in part.

FIGURE 2–23 Standard prefixes of engineering notation.

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website, in whole or in part.

The SI System

• Prefixes just discussed

• Referred to as metric units of measure

• Part of the SI (System Internationale) system

FIGURE 2–24 SI base

and supplementary units.

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website, in whole or in part.

FIGURE 2–25 Derived SI units.

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website, in whole or in part.

Summary

• Coulomb: measure of charge

• Ampere (A): 1 coulomb per second

• Letter I: stands for intensity of current flow

• Normally used in Ohm’s law formulas

• Voltage: referred to as electric pressure, potential

difference, or electromotive

• Represented with an E or a V

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website, in whole or in part.

Summary (Cont’d)

• Ohm (Ω): measurement of resistance (R) in an electric

circuit

• Watt (W): measurement of power in an electrical circuit

• Represented by a W or a P (power)

• Electric measurements: generally expressed in

engineering notation

• Uses steps of 1000

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website, in whole or in part.

Summary (Cont’d)

• Before current can flow, there must be a complete

circuit

• A short circuit has little or no resistance

• An open circuit has infinite resistance