Transcript Electric Current

Electric Current
Current - rate at which charge
passes a given point.
Current
• The higher the current is, the more charge
passes the point each second.
• Unit for current is ampere (A) or amp
• In equations, the symbol for current is I
• 2 Different types of electric current:
– Direct current (DC) - charges flow in same
direction
– Alternating current (AC) - charges
continually switch from flowing in one
direction to flowing in the reverse direction.
Currents Cont’d
• DC - Electric current produced by batteries
and cells
• AC - used in outlets in your home
Voltage
• The current in a wire is determined by voltage.
• Voltage - the difference in energy per unit charge
as a charge moves between 2 points in the path of
a current.
• Another word for potential difference.
• Expressed in Volts (V).
• The amount of energy released as a charge moves
between 2 points in the path of a current.
• The higher the voltage = the more energy is
released per charge
• The greater the voltage = the greater the current
• In the US, electrical outlets usually supply a
voltage of 120 V.
• Most electrical devices (TV’s, toasters,
lamps, alarm clocks) are designed to run on
120 V.
• Devices that run on batteries or cells usually
need a lower voltage
Resistance
• Also determines the current in a wire.
• Resistance is the opposition to the flow of
electric charge.
• Expressed in ohms (Ω)
• In equations the symbol for resistance is R
• Think of resistance as “electrical friction”
• The higher the resistance, the lower the current
• As resistance increases, current decreases
• Varies depending on the objects material,
thickness, length and temperature.
• Good conductors have low resistance.
• Poor conductors have higher resistance.
• The resistance of insulators is so high that
electric charges do not flow in them.
• Materials with low resistance are used to
make wires and other objects that are used
to transfer electrical energy from place to
place.
Resistance vs. Temperature
• Resistance of metals increases as temperature
increases
• This happens because atoms move faster at
higher temperatures and get in the way of the
flowing electric charges.
• If you cool certain materials to an extremely low
temperature, resistance will drop to nearly 0
ohms.
• These materials are called superconductors.
• Little energy is wasted when electric charges
travel in them, but it takes a lot of energy to cool
them so they aren’t very practical.
Ohm’s Law
Amperes (A) = volts (V)/ohms (Ω)
OR
I = V/R
Try These!
• Find the resistance of an object if a voltage
of 10 V produces a current of 0.5 A.
• Find the current produced if a voltage of 36
V is applied to a resistance of 4 Ω.
Electric Power
• The rate at which electrical energy is
used to do work.
• Unit for power is the watt (W)
• The symbol for power is P
• Power = voltage x current or P = V x I
• Watts = volts (V) x amperes (A)
Measuring Electrical Energy
• Electrical Energy = power x time
• E = Pt