Transcript Electric Current and Ohm`s Law

Electric Current
and Ohm’s Law
Sec 20.2
Electric Current
electric current – continuous flow of
electric charge
 measured in ampere (A) – 1 coulomb per
second
 two types


direct current – charge flows one way
 Found

in devices like flashlights and calculators
alternating current – charges vibrate back and
forth around a fixed position
 Type
of current in households
Conductors and Insulators

conductor – material through which charge
can easily flow
examples: metals, salt water, gases in
fluorescent light
 conductors transmit charge because they
have free electrons in their structure


insulator – material through which charge
cannot easily flow

Examples: plastic, foam, rubber, wood
Challenge Question
Q: If a lightening storm occurs, you are
relatively safe in a car. Is this because of
the car’s conducting properties or
insulating properties?
A: You are safe because of the car’s
conducting properties. If lightening strikes
the car, the metal conducts the electricity
around the outside of the car and to the
ground. Since you are inside the car, the
electric current never reaches you.
Resistance
opposition to the flow of electrons
 measured in ohms (Ω)
 occurs because electrons moving through
a conductor collide with other electrons,
ions and atoms
 depends on material’s length, thickness
and temperature

Longer wire increases resistance
 Thicker wire decreases resistance
 Increasing temperature increases resistance

Quick Check
Determine which of the following in each
pair would have the lower resistance:
a.
b.
c.
A wire 10 mm thick or 5 mm thick
A spoon at 25°C or at 30°C
A fluorescent bulb 20 cm long or 120 cm
long.
Voltage

need source of electrical energy in order
for charges to flow


batteries, solar cells and generators are all
sources of electrical energy
charges flow from areas of high potential
to areas of low potential in much the same
way as objects fall from areas of high
gravitational potential energy to low
gravitational potential energy
Voltage (continued)
electrical potential energy of a charge
depends on its position in an electric field
just like the gravitational potential energy
of an object depends on its height above
the earth
 potential difference = the difference in
electrical potential energy of two places in
an electric field

measured in volts (V)
 also called voltage

Ohm’s Law

relationship between current, resistance
and voltage in a circuit


first discovered by Georg Ohm (1826)
Voltage = current x resistance or V = I x R
Ohm's Law Simulation
Summary of Simulation
1.
2.
If you increase the voltage and keep the
resistance constant, what happens to the
current?
If you increase the resistance and keep
the voltage constant, what happens to
the current?
Challenge Question
Q: A simple lie detector consists of an electric
circuit, one part of which is part of your body. A
sensitive meter shows the current that flows
when a small voltage is applied. How does this
technique indicate that a person is lying?
A: When people lie their skin tends to get moist
from increased sweating. Moist skin has a lower
resistance than dry skin. As the resistance of the
skin goes down, the current will increase which
is detected by the meter.
Challenge Question
Q: If power lines have high voltages and are so
dangerous, why don’t birds get electrocuted
when they land on the lines?
A: In order for the current to flow through the bird,
their needs to be a potential difference. When
both feet are on the same line there is no
potential difference between their feet and no
current flows. You also would be safe hanging
on the power line as long as you were not
touching anything else. If you touch the line and
touch another object or the ground there would
then be a potential difference and you would be
electrocuted.