Download Section 2: Electric Current

Section 2: Electric
Current
• This section discusses electrical
pressure
• Charges flow from high voltage areas
to low voltage areas
• Voltage—like an electrical pressure
that pushes charge
(Section 2 cont’d)
• Just as water or air must have a pressure
difference to flow, voltage difference
must be present for electric charges to
flow
• Voltage difference—the push that causes
charges to move and is measured in volts
(V)
(fig. 11 pg. 202)
Closed Circuits
• The flow of charges through a wire
or any conductor is called electric
current
• The electric current in a circuit is
measured in amperes (A)
• Circuit—a closed, conducting path for
the flow of electrons
(Section 2 cont’d.)
• Current is almost always the flow of
electrons
• In order to keep current moving
continuously through a circuit a
power source (voltage source) is
needed
• One common source--battery
Dry-Cell Batteries
• Individual batteries used in flashlights,
etc.—dry cell batteries
• Made of a zinc container that surrounds a
moist chemical paste w/a solid carbon rod
suspended in the middle
• Provides a voltage difference between +/terminals
(Dry-cell batteries cont’d)
• When the two terminals of a dry cell
battery are connected in a circuit, a
reaction occurs.
• The voltage difference between
these 2 terminals causes current to
flow through a closed circuit
Wet-cell batteries
• Wet-cell battery—contains two
connected plates made of different
metals or metallic compounds in a
conducting solution
– Ex: car batteries (lead plates in sulfuric acid)
– The chemical reaction in ea/cell
provides a voltage difference
More on Batteries
• In addition to batteries a voltage
difference is provided at electrical
outlets
• Household devices are designed for
120 V (standard)
• Some wall sockets supply 240 V
– Ex: electric ranges, clothes dryers, etc.
Resistance
• Resistance—the tendency for a material
to oppose the flow of electrons, changing
electrical energy into thermal energy and
light
• All materials have some resistance
• Resistance is measured in ohms
*type of material & wire size effect resistance
Controlling the flow of
electricity
• So far: Voltage difference causes
charges to flow
• An electrical resistance restricts the
movement of charges
ex: water flowing in a pipe
Ohm’s Law:
• Current = Voltage difference /
resistance
• OR
• I(A) = V (V) / R (ohms)
• OR
• V=IR
• Ohm’s Law Diagram—pg. 207