Unit 2 Electrical Quantities
and Ohm’s Law
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Objectives:
Define a coulomb.
Define an ampere.
Define a volt.
Define an ohm.
Define a watt.
Unit 2 Electrical Quantities
and Ohm’s Law
Objectives:
• Compute electrical values using Ohm’s
law.
• Discuss basic types of circuits.
• Use the Ohm’s wheel charts.
Unit 2 Electrical Quantities
and Ohm’s Law
A coulomb is a quantity measurement for
electrons.
One coulomb contains 6.25 x 1018
electrons, or 6,250,000,000,000,000,000
electrons.
Unit 2 Electrical Quantities
and Ohm’s Law
The ampere is a measurement of the amount of
electricity that is flowing through a circuit.
One ampere (A) is defined as one coulomb of
electricity flowing past a given point in one
second.
Unit 2 Electrical Quantities
and Ohm’s Law
Compare and contrast these two systems.
Unit 2 Electrical Quantities
and Ohm’s Law
There are two theories about current flow.
Electron flow theory describes current flow from
negative to positive.
Conventional current flow theory states
electrical current flows from positive to
negative.
Unit 2 Electrical Quantities
and Ohm’s Law
Conventional current flow theory and electron flow
theory.
Unit 2 Electrical Quantities
and Ohm’s Law
Electrons moving from atom to atom.
Unit 2 Electrical Quantities
and Ohm’s Law
Electrical sources are divided into two basic
types:
– Direct Current (DC) which is unidirectional
(one way).
   
– Alternating Current (AC) which is
bidirectional (two way, or back and forth).
   
Unit 2 Electrical Quantities
and Ohm’s Law
A complete path must exist before electricity can
flow through a circuit.
A circuit with a complete path for electrical flow is
called a closed circuit.
If the circuit path is incomplete or broken, this is
called an open circuit.
Unit 2 Electrical Quantities
and Ohm’s Law
A simple switch
closes and
opens an
electrical
circuit.
Unit 2 Electrical Quantities
and Ohm’s Law
A short circuit has an unintended shorter pathway.
Unit 2 Electrical Quantities
and Ohm’s Law
The basic principle of the instantaneous effect of
electric impulses.
Unit 2 Electrical Quantities
and Ohm’s Law
The impulse of electricity can travel faster than light. It
would take light 1.3 seconds to travel around the
earth 10 times. If a wire were wrapped around the
earth 10 times, when the switch was closed the light
would come on almost instantly.
Unit 2 Electrical Quantities
and Ohm’s Law
A volt or voltage is electrical pressure.
Unit 2 Electrical Quantities
and Ohm’s Law
An ohm is the unit of resistance or
opposition to the flow of electricity.
Unit 2 Electrical Quantities
and Ohm’s Law
The watt is the unit of electrical power.
Unit 2 Electrical Quantities
and Ohm’s Law
Force equals flow rate times pressure.
Unit 2 Electrical Quantities
and Ohm’s Law
Amperes times volts equals watts.
Unit 2 Electrical Quantities
and Ohm’s Law
Common power units.
Unit 2 Electrical Quantities
and Ohm’s Law
Ohm’s law states that in a DC (Direct Current)
circuit, the current is directly proportional
to the voltage and inversely proportional
to the resistance.
E(volts) = I(amps) x R(ohms)
Unit 2 Electrical Quantities
and Ohm’s Law
Ohm’s Law Symbols:
P is the symbol for Watts.
E is the symbol for Volts.
I is the symbol for Amperes.
R is the symbol for Resistance or Ohms.
Unit 2 Electrical Quantities
and Ohm’s Law
Ohm’s Law states that it takes one volt to push
one ampere through one ohm.
E=IxR
I = Amps
E = Volts
R = Resistance
Unit 2 Electrical Quantities
and Ohm’s Law
Using the Ohm’s law chart.
E=IxR
I=E/R
R=E/I
Unit 2 Electrical Quantities
and Ohm’s Law
Adding P (watts) to the Ohm’s law chart.
P = E2 / R
P=ExI
P = I2 x R
I= PR
I=P/E
I=E/R
R=E/I
R = P / I2
R = E2 / P
E = PR
E=IxR
E=P/I
Unit 2 Electrical Quantities
and Ohm’s Law
Remember:
E = EMF, or voltage
I = intensity of current, or amperage
R = resistance in ohms
P = power in watts
E(volts) = I(amperes) x R(ohms)
P(watts) = I(amperes) x E(volts)
Unit 2 Electrical Quantities
and Ohm’s Law
Unit 2 Electrical Quantities
and Ohm’s Law
Metric Units
Unit 2 Electrical Quantities
and Ohm’s Law
Review:
1. A coulomb is a quantity measurement of
electrons.
2. An ampere (A) is one coulomb per second.
3. Either the letter I, which stands for intensity
of current flow, or the letter A, which stands
for amperes, can be used in Ohm’s law
formulas.
Unit 2 Electrical Quantities
and Ohm’s Law
Review:
4. Voltage is referred to as electric pressure,
potential difference, or electromotive force.
An E or a V can be used to represent
voltage in Ohm’s law formulas.
5. An ohm (Ω) is a measurement of resistance
(R) in an electric circuit.
Unit 2 Electrical Quantities
and Ohm’s Law
Review:
6. The watt (W) is a measurement of power in
an electrical circuit. It is represented by
either a W or a P (power) in Ohm’s law
formulas.
7. Electric measurements are generally
expressed in engineering notation.
Unit 2 Electrical Quantities
and Ohm’s Law
Review:
8. Engineering notation differs from the
standard metric system in that it uses steps
of 1000 instead of steps of 10.
9. Before current can flow, there must be a
complete circuit.
10. A short circuit has little or no resistance.