Transcript Electric Current and Ohm`s Law

Electric Current
and Ohm’s Law
Sec 20.2
Electric Current
electric ______ – continuous flow of
electric charge
 measured in _______ (A) – 1 coulomb
per second
 two types


_________ current – charge flows one way
 Found

in devices like flashlights and calculators
_________ 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: ___________________________
_______________________________
conductors transmit charge because they have
free electrons in their structure
___________ – material through which charge
cannot easily flow

Examples: ___________________________
Challenge Question
Q: If a lightening storm occurs, you are
relatively _______ 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 _________ reaches you.
Resistance
opposition to the ______ __ ________
 measured in _______ (Ω)
 occurs because electrons moving through
a conductor collide with other electrons,
ions and atoms
 depends on material’s ______, thickness
and _____________

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 _________ __ _______


batteries, solar cells and generators are all
sources of electrical energy
charges flow from areas of _____
potential to areas of ____ 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 _______ in an electric
field just like the gravitational potential
energy of an object depends on its height
above the earth
 __________ ________ = the difference in
electrical potential energy of two places in
an electric field

measured in ________ (V)
 also called voltage

Ohm’s Law

relationship between current, resistance
and voltage in a circuit


first discovered by Georg Ohm (1826)
______ = 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.