Transcript Electric Current - Hudson Falls Middle School

Electric Current
Electric Current
• The rate of flow of electric charges
• Unit of measure is coulombs per second, or
amperes
q
I
t
• I – rate of flow (current) (amps)
• q – charge (coulomb)
• t –time (sec)
Sample Problem
• 10 Coulombs goes through a conductor in 5
seconds. What is the current?
Conductivity
•
•
•
•
•
Ability to conduct a current
Metals – Good
Liquids – Vary
Solutions (Electrolytes) – Good
Gases – Good
Conditions Necessary for a Current
• A difference in potential is required to
maintain a flow of charge between 2 points in
a conductor
Flow of positive charges*
+
Flow of electrons
V
Motion Inside a Conductor
•
•
•
•
Electrons are accelerated by an electrical field
Collisions with fixed particles of the conductor
Create heat
“Plinko”
Resistance
• The opposition to the electron moving
through a conductor is called Resistance (R)
• The unit of resistance is OHM (Ω)
V
R
I
R – Resistance (OHM (Ω))
V – Potential Difference (Volts, V)
I – Current (Amps, A)
Sample Problem
• When a conductor has a potential difference
of 110 volts placed across it, the current
through it is 0.50 ampere. What is the
resistance of the conductor?
• How many electrons pass through this coil per
second?
Factors that Affect Resistance
• Length of Wire
– Directly proportional to its length (length up,
resistance up)
• Cross-Sectional Area
– Inverse relationship (thicker the wire, less
resistance)
• Temperature
– Resistance INCREASES when Temperature
INCREASES
• Type of Material (Resistivity)
– Insulator vs Conductors
Resistivity of Materials
• How well a substance resists carrying a
current is known as resistivity (ρ).
• The unit is ohm·meter (Ω·m)
• Reference Table
Resistivity
L
R
A
•
•
•
•
R - Resistance
ρ – resistivity (ohm·meter)
L – length of wire (meter)
A – Cross-sectional area (m2)
Sample Problem
• Calculate the resistance at 200 C of an
aluminum wire that is 0.200 meter long and
has a cross-sectional area 1.00x10-3 m2 .
Electric Circuits and Ohm’s Law
• Circuit means “closed path”
• Typically circuits have a source of
potential difference, a resistance and
wires
Interpreting a Completed
Circuit
• Current travels from (+) to (-)
• Electrons travel opposite
Graphs of Ohm’s Law
SHOW WORK
Electrical Power
• Power is the rate at which work is being
done
2
• Derivations
V
2
P  VI  I R 
P – Power (Watts)
V – Potential Difference (Volts, V)
I - Current (Amperes, Amps)
R
Sample Problem
• A lamp designed for a 120 Volt circuit is
labeled 60 watts. What is the operating
resistance of the lamp?
Electrical Work and Energy
• The work required to maintain a current
is equal to power x time
2
V t
W  Pt  VIt  I Rt 
R
2
Sample
• How much electrical energy is supplied
to a 60 watt bulb in 10 seconds?
• How much energy is produced by a 50
volt source that generates a current of
5.0 amperes for 2.0 minutes