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Chapter 4
The Simple Circuit and Ohm’s Law
Conductors • Switches • Switch Characteristics • Loads •
Overcurrent • Overcurrent Protection Devices • Voltage
and Current Measurements • DC Voltage Measurements
• DC Current Measurements • Ohm’s Law • Determining
Current • Determining Voltage • Determining Resistance •
Determining Power
Chapter 4 — The Simple Circuit and Ohm’s Law
An electrical circuit
consists of a voltage
source, insulated
conductors, a load, a
switch, and a fuse.
Chapter 4 — The Simple Circuit and Ohm’s Law
In a schematic or
wiring diagram,
conductors are shown
as lines. Conductors
that are connected
often use a dot to
indicate the
connection.
Chapter 4 — The Simple Circuit and Ohm’s Law
Switches are control
devices and are used to
close and open circuits
safely.
Chapter 4 — The Simple Circuit and Ohm’s Law
The position of the contacts, number of poles,
number of throws, and type of break are used to
describe switch contacts.
Chapter 4 — The Simple Circuit and Ohm’s Law
Switches are available in
many shapes and are
often designated
according to their use.
Chapter 4 — The Simple Circuit and Ohm’s Law
Common lighting circuit
switches include twoway, three-way, and fourway switches.
Chapter 4 — The Simple Circuit and Ohm’s Law
Rotary switches are used to connect multiple
positions to a single pole.
Chapter 4 — The Simple Circuit and Ohm’s Law
Loads convert
electrical energy to
another form of
energy such as
motion, light, heat, or
sound.
Chapter 4 — The Simple Circuit and Ohm’s Law
A short circuit has a
resistance that is lower
than the normal circuit
resistance.
Chapter 4 — The Simple Circuit and Ohm’s Law
Overcurrent protection devices such as fuses are
used to protect a circuit from a short circuit or
overcurrent that can cause circuit damage.
Chapter 4 — The Simple Circuit and Ohm’s Law
Cartridge and plug fuses
may be surrounded with
glass or encased in a
composite material to
suppress an arc or flame.
Chapter 4 — The Simple Circuit and Ohm’s Law
A circuit breaker is an overcurrent protective device
that does not need to be replaced each time the
circuit current rating is exceeded. Circuit breakers
may be thermally or magnetically operated.
Chapter 4 — The Simple Circuit and Ohm’s Law
Thermal circuit breakers use a bimetallic strip
attached to a latch mechanism to open the circuit
when a short circuit or overload occurs.
Chapter 4 — The Simple Circuit and Ohm’s Law
Magnetic circuit breakers use an electromagnet coil
and armature to open the circuit when a short circuit
or overload occurs.
Chapter 4 — The Simple Circuit and Ohm’s Law
Thermal overload relay contacts open when the
current level is exceeded for a given period of time.
The temperature rise in the metal frame of the motor
is used to heat the bimetallic strip.
Chapter 4 — The Simple Circuit and Ohm’s Law
In a standard motor control circuit, a relay coil
controls a set of normally open contacts and a set of
normally closed overload relay contacts.
Chapter 4 — The Simple Circuit and Ohm’s Law
DC voltage measurements using a digital multimeter
are taken by connecting the black test lead to the
negative polarity test point and the red test lead to the
positive polarity test point.
Chapter 4 — The Simple Circuit and Ohm’s Law
DC voltage is measured with an analog meter using
standard procedures.
Chapter 4 — The Simple Circuit and Ohm’s Law
To measure current flow through a component, a
meter must be connected so that the total electron
flow is through the meter circuit.
Chapter 4 — The Simple Circuit and Ohm’s Law
DC current is measured with an analog multimeter
using standard procedures.
Chapter 4 — The Simple Circuit and Ohm’s Law
Clamp-on ammeters measure current by measuring
the strength of the magnetic field around a single
conductor.
Chapter 4 — The Simple Circuit and Ohm’s Law
Ohm’s law is the relationship between voltage,
current, and resistance in an electrical circuit.
Chapter 4 — The Simple Circuit and Ohm’s Law
Current in a circuit
increases with an
increase in voltage and
decreases with an
increase in resistance.
Chapter 4 — The Simple Circuit and Ohm’s Law
Voltage in a circuit
increases with an
increase in current and
increases with an
increase in resistance.
Chapter 4 — The Simple Circuit and Ohm’s Law
Resistance in a circuit
increases with an
increase in voltage and
decreases with an
increase in current.
Chapter 4 — The Simple Circuit and Ohm’s Law
The power formula is the relationship between power,
voltage, and current in an electrical circuit.
Chapter 4 — The Simple Circuit and Ohm’s Law
Power in an electrical circuit is calculated by
multiplying current by voltage.
Chapter 4 — The Simple Circuit and Ohm’s Law
Power in an electrical circuit calculated by multiplying
current squared by resistance.
Chapter 4 — The Simple Circuit and Ohm’s Law
Power in an electrical circuit can be calculated by
dividing voltage squared by resistance.