electric current. the rate of flow of electric charges unit of measure is coulombs per second, or...

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

22

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