Download Electricity

Electricity
• Electricity did not become an integral part
of our lives until scientists learned to
control the movement of electric charge,
known as current.
• Luigi Galvani (1737-1798) was conducting
electrical experiments near a recently
dissected frog. Sparks caused the frogs legs
to twitch.
• We now know that electric currents transmit
signals from the brain to our muscles to
cause movement.
• Current is a rate of
charge movement.
Q  C 
I
 
t  s 
C
1  1 Ampere
s
• Conventional current is defined in terms of
positive charge movement.
• In metal conductors, negative charges move.
• In particle accelerators, protons may be set in
motion.
• In gases and dissolved salts current is the result
of (+) charges moving in one direction and (-)
charges moving in the other.
• Charge carriers are either positive or
negative charges that are in motion.
• Electric fields may set charges in motion.
• When a circuit is closed, the electric field
travels at nearly the speed of light. The
actual charges travel more slowly.
Drift Velocity
• Drift velocity is the net velocity of charge
carriers.
• When a conductor is in electrostatic
equilibrium, the electrons move randomly.
• When a potential difference is applied
across the conductor, an electric field is set
up inside the conductor.
• The force due to that field sets the electrons
in motion creating a current.
• Temperatures increase in the conductor due
to collisions.
• The electrons move slowly in a direction
opposite the electric field.
Strong
E Field
The E Field weakens
with distance.
• In a copper wire with a current of 10 Amps,
the drift velocity of electrons is 2.46 EE -4
m/s.
• It would take an electron 68 minutes to
travel one meter.
• Differences in potential maintain the current
in a circuit.
• There are two different types of current:
direct (DC) and alternating (AC).
Alternating Current
• In alternating current, the terminals of the
source of potential difference are constantly
changing sign.
• There is no net motion of charge carriers in
AC.
• In the US, AC alternates 60 times each
second. f = 60 Hz.
• The current in a circuit is dependent upon
potential difference (V) and the resistance
that a conductor has.
• Some conductors allow charges to move
through them more easily than others.
Resistance
• Resistance is defined
as the ratio of potential
difference to current.
V
R
I
R ()
Volt
1  1
Amp
• Resistance is constant
over a range of
potential differences.
• This was discovered
by Georg Simon Ohm.
V
 constant
I
Ohm’s Law
• The law is usually written as
V  IR
• R is understood to be independent of
• NASA Link
.
V
• Ohm’s Law is not a fundamental law.
Materials that have a constant resistance
over a wide range of potential differences
are said to be ohmic.
• Walter Fendt’s website
• Walter Fendt’s website
• A graph of current versus potential
difference for an ohmic material is linear.
I
R
V
• Materials that do not function according to
Ohm’s Law are said to be nonohmic.
Diodes are nonohmic. The resistance is
small for currents in one direction and large
for currents in the reverse direction.
I
R
V
• Resistance depends on length, crosssectional area, material, and temperature.
• The smaller the length of wire the less the
resistance.
• The greater the cross-sectional area of wire
the less the resistance.
• Copper is less resistant than aluminum.
• The lower the temperature the less the
resistance.
• When V remains constant, current
decreases when resistance increases.
• V = IR
• When potential difference increases, current
increases but resistance is not affected.
Changes in potential difference do not affect
the resistance of a resistor.
The Wheatstone Bridge
• A Wheatstone bridge is a measuring instrument
invented by Samuel Hunter Christie in 1833 and
improved and popularized by Sir Charles
Wheatstone in 1843. It is used to measure an
unknown electrical resistance by balancing two
legs of a bridge circuit, one leg of which includes
the unknown component. Its operation is similar to
the original potentiometer except that in
potentiometer circuits the meter used is a sensitive
galvanometer.
• Salt water and perspiration lower the body’s
resistance.
• When the skin is dry, the human body’s
resistance to current is around 500,000 .
• Resistance decreases when skin is wet.
• When the body is soaked with salt water or
lots of ions, the resistance can be as low as
100.
• Low resistance can be dangerous because
it’s the flow of electrons that is lethal.
• Perspiration contains ions that conduct
electric charge. In a galvonic skin response
test (GSR), a small potential difference is
set up across the body. Perspiration
increases when a person is nervous or
stressed. This decreases the resistance of
the body.