Download Electric current

Potential difference : charge flows
when there is a difference in voltage
across the ends of a conductor
Electric current is simply the flow of
electric charge or electrons.
A voltage source provides a sustained
potential difference to allow electrons
to flow.
• Dry cells, wet cells, and generators
are good voltage sources.
• Dry cells and wet cells are the result
of a chemical reaction.
• Generators convert mechanical
energy to electrical energy.
Voltage – is the electric potential per
charge moving between terminals.
This is like the electric pressure pushing the
electrons.
Example: The potential difference
between two slots in a household
outlet is ~120 volts or 120
joules/coulomb of charge.
Voltage is the potential energy /unit
of charge that pushes the electrons.
Electric current is measured in
amperes.
5 amps = 5 coulombs of charge in
one second or about 31.25 billion
billion electrons each second.
The net charge of any wire is zero
because just as many electrons
leave the wire that enter it.
Electric resistance can slow down the
flow of electrons.
Resistance depends on the
conductivity of the wire, and also the
thickness and length of the wire.
Thick wires have less resistance.
Short wires have less resistance.
Resistance is measured in ohms (Ω)
Ohm’s law:
V = IR
Resistors are used in electrical
devices to control the resistance.
Resistance in the body can be from
about 100 ohms if you’re soaked in
salt water to about 500,000 ohms
for dry skin.
Touching a 120 volt electric circuit
normally would do no harm because
of high resistance
If you were standing in water, the
resistance is lowered so that the
current travels through faster.
Bibliographic Entry
Result
(w/surrounding text)
Standardized
Result
Cutnell, John D., Johnson, Kenneth W. Physics.
4th ed. New York, NY: Wiley, 1998.
"Currents of approximately 0.2 A are potentially
fatal, because they can make the heart
0.2 A
fibrillate, or beat in an uncontrolled manner."
"In general, for limb-contact electrical shocks,
accepted rules of thumb are: 1-5 mA is the level
Carr, Joseph J. Safety for electronic hobbyists.
of perception; 10 mA is the level where pain is
Popular Electronics. October 1997. as found in
0.1–0.3 A
sensed; at 100 mA severe muscular contraction
Britannica.com.
occurs, and at 100-300 mA electrocution
occurs."
"Electrical Injuries." The Merck Manual of
Medical Information: Home Edition.
Pennsylvania: Merck, 1997.
"At currents as low as 60 to 100 milliamperes,
low-voltage (110-220 volts), 60-hertz
0.06–0.1 A
alternating current traveling through the chest (AC)
for a split second can cause life-threatening
irregular heart rhythms. About 300-500
0.3–0.5 A
milliamperes of direct current is needed to have (DC)
the same effect."
Zitzewitz, Paul W., Neff, Robert F. Merrill
Physics, Principles and Problems. New York:
Glencoe McGraw-Hill, 1995.
"The damage caused by electric shock depends
on the current flowing through the body -1 mA can be felt; 5 mA is painful. Above 15 mA, 0.07 A
a person loses muscle control, and 70 mA can
be fatal."
"0.10 death due to fibrillation
Watson, George. SCEN 103 Class 12. University
> 0.20 no fibrillation, but severe burning, no
of Delaware. March 8, 1999.
breathing"
Miller, Rex. Industrial Electricity Handbook.
Peoria, IL: Chas. A. Bennet, 1993.
0.1–0.2 A
"Currents between 100 and 200 mA are lethal." 0.1–0.2 A
Your heart has an electrical
component as well.
Birds standing on an electric wire
are not harmed because there is no
potential difference.
Direct current (dc) – is when the
current flows in only one direction.
Ex: battery
Alternating current (ac) – the
current flows in one direction then
the other.
Ex: outlets
In the U.S. We alternate the electric
current at about 60 cycles per
second or 60 hertz.
Diodes can be used to change ac to
dc current.
Diodes allow only half the electrons
to flow through so that they are all
flowing in the same direction.
When electric current in a
material is proportional to
the voltage across it, the
material is said to be
"ohmic", or to obey Ohm's
law. A microscopic view
suggests that this
proportionality comes from
the fact that an applied
electric field superimposes a
small drift velocity on the
free electrons in a metal. For
ordinary currents, this drift
velocity is on the order of
millimeters per second in
contrast to the speeds of
the electrons themselves
which are on the order of a
million meters per second.
Even the electron speeds are
themselves small compared
to the speed of transmission
of an electrical signal down
a wire, which is on the order
of the speed of light, 300
million meters per second.
At room temp. electrons have a
speed of a few million kilometers
per hour.
The electric field around the
electrons moves at nearly the speed
of light.
Inside wire, the electric field is
directed along the wire.
In dc current the electrons drift
about .01cm/s
In ac current the electrons move a
fraction of a cm in one direction
then move back the same amount
equaling no net movement.
Electrons themselves do not flow.
Electrons already in a material
vibrate causing energy to flow by
the electric field.
Therefore power plants do not sell
electrons, they sell energy and you
supply the electrons.
Electric power is the rate in which
electricity is changed to mechanical
energy, heat, or light.
Electric power = current x voltage
Or
1 watt (J/S)= 1 amp (C/S) x 1 volt
(J/C)
P=IxV
Use Ohm’s Law to derive other expressions for power
A kilowatt is 1000 volts.
A kilowatt hour is the amount of
energy consumed in one hour at the
rate of 1 kilowatt.