electric current. the rate of flow of electric charges unit of measure is coulombs per second, or...
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Electric Current
Electric Current• The rate of flow of electric charges• Unit of measure is coulombs per second, or
amperes
• I – rate of flow (current) (amps)• q – charge (coulomb)• t –time (sec)
t
qI
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)
R – Resistance (OHM (Ω))V – Potential Difference (Volts, V)I – Current (Amps, A)
I
VR
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
• R - Resistance• ρ – resistivity (ohm·meter)• L – length of wire (meter)• A – Cross-sectional area (m2)
A
LR
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
SH
OW
WO
RK
Electrical Power• Power is the rate at which work is being
done• Derivations
P – Power (Watts)V – Potential Difference (Volts, V)I - Current (Amperes, Amps)
R
VRIVIP
22
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
R
tVRtIVItPtW
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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