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Unit 2
Electrical Quantities
and Ohm’s Law
Preview
• Electricity
– Standard set of values
• Values of electrical measurement
– Standardized
• Understood by everyone who uses them
• Must be the same for everyone
The Coulomb
• Quantity measurement for electrons
– One coulomb contains 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
The Ampere
• Amount of electricity flowing through a
circuit
– One ampere (A) is defined as one coulomb of
electricity flowing past a given point in one
second
• André Ampère
– Scientist in 1700s-1800s
– Worked in electromagnetism
FIGURE 2–1 One ampere equals one coulomb per second.
FIGURE 2–2 Current in an electric circuit can
be compared to flow rate in a water system.
Current Flow Theories
• Electron flow theory
– Current flows from negative to positive
• Conventional current flow theory
– Current flows from positive to negative
FIGURE 2–3 Conventional current flow theory and electron flow theory.
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.
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
– In a DC circuit, the impulse of electricity can
appear to be faster than the speed of light
FIGURE 2–6 When a ball is pushed into one end, another ball is forced out the
other end. This basic principle causes the instantaneous effect of electric impulses.
FIGURE 2–7 The impulse of electricity can appear to travel faster than light.
Basic Electric Circuits
• A complete path must exist before
electricity 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
FIGURE 2–8 Current flows only through a closed circuit.
FIGURE 2–9 A short circuit bypasses the
load and permits too much current to flow.
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
FIGURE 2–11 The grounding conductor provides a low-resistance path to ground.
The Volt
• Voltage: potential difference between two
points of a conducting wire
– Carrying a constant current of one ampere
when power dissipated between these points
is one watt
• Also referred to as potential difference or
electromotive force (EMF)
– Volt: amount of potential necessary to cause
one coulomb to produce one joule of work
FIGURE 2–12 Voltage in an electric circuit can
be compared to pressure in a water system.
.
The Ohm
• Unit of resistance to current flow
– Named after German scientist Georg S. Ohm
FIGURE 2–13 A resistor in an electric circuit can
be compared to a reducer in a water system
The Watt
• Amount of power being used in a circuit
– Named after English scientist James Watt
FIGURE 2–15 Force equals flow rate times pressure.
FIGURE 2–16 Amperes times volts equals watts.
Other Measures of Power
FIGURE 2–17 Common power units.
Ohm’s Law
• It takes one volt to push one ampere
through one ohm
– In a DC circuit, current is directly proportional
to voltage
• Inversely proportional to resistance
• Formula:
– E (volts) = I (amperage) x R (resistance)
FIGURE 2–18 Chart for finding values of voltage, current, and resistance.
FIGURE 2–19 Using the Ohm’s law chart.
FIGURE 2–20 Formula chart for finding values
of voltage, current, resistance, and power.
Metric Prefixes
FIGURE 2–24 Standard metric prefixes.
FIGURE 2–25 Standard prefixes of engineering notation.
The SI System
• Prefixes just discussed
– Referred to as metric units of measure
• Part of the SI (System Internationale) system
FIGURE 2–26 SI base and supplementary units.
FIGURE 2–27 Derived SI units.
Summary
• Coulomb: quantity measurement of
electrons
• Ampere (A): one 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
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
Summary (cont’d.)
• Before current can flow, there must be a
complete circuit
• A short circuit has little or no resistance