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