Ch. 13 - ElectricityCh. 13 -

ElectricityII. Electric CurrentII. Electric Current

Electric CurrentElectric Current Current

Rate of flow of charges through a conductor Usually the flow of electrons. depends on # of e- passing a point in a given

time measured in amperes (A) (1 A = 6250 million

billion electrons past a point every second)

Voltage DifferenceVoltage Difference

Voltage Difference The force that causes electric

charges to flow large separation of charge creates

high voltage measured in volts (V)

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Low voltage High voltage

Electron flow

Current is from high voltage to lowElectrons flow from low voltage to high

CircuitCircuit

closed path through which electric current flowsA current will only flow as long as there is a

voltage difference. Sources of the voltage difference?

BatteriesBatteries To keep electric charges continually flowing in

an electric circuit, a voltage difference must be maintained. Power supplies such as batteries!

Dry Cell BatteriesDry Cell Batteries A cell = two electrodes surrounded by an

electrolyte The electrolyte enables the charge to flow

from one electrode to the other.

Dry Cell BatteriesDry Cell Batteries Electrode 1 – carbon rod; Electrode 2 – zinc container Electrolyte – moist paste of chemicals Completing the circuit causes a chemical reaction and

electrons are transferred. Carbon rod becomes positive, zinc accumulates

electrons (neg) creating a voltage difference and a current will flow in aclosed circuit

Wet-cell BatteriesWet-cell Batteries Contains two connected plates made of different metals

in a conducting liquid solution (electrolyte). Example: car battery

Wet-Cell BatteriesWet-Cell Batteries Lead-Acid batteries: contain a series of six wet cells

of lead and lead dioxide plates in a sulfuric acid solution.

Voltage difference approx 12V

Electric OutletsElectric Outlets

create a voltage difference Example: wall socket Usually has a higher voltage difference then a

battery. Approx 120 V in US for most sockets

Why do cell phones get hot on Why do cell phones get hot on a long call?a long call?

Materials resist the flow of electrons and convert electrical to thermal energy

Resistance Measured in Ohms (Ώ)

Resistors reduce the current through the circuit to prevent overload (some energy transferred to resistor)

Why do light bulbs glow?Why do light bulbs glow?

ResistanceResistance

Copper - low resistance

Tungsten - high resistanceNot only is electrical energy converted to thermal energy but also to light!

ResistanceResistance Resistance depends on…

the conductor• less resistance in a

better conductor

wire thickness• less resistance in

thicker wires wire length

• less resistance in shorter wires temp

less resistance at low temps

Ohm’s LawOhm’s Law

Ohm’s Law

I = V / RV: voltage difference

(volts)

I: current (amperes)

R: resistance (ohms)

Ohm’s LawOhm’s LawA lightbulb with a resistance of 160 is

plugged into a 120-V outlet. What is the current flowing through the bulb?

GIVEN:

R = 160 V = 120 V

I = ?

WORK:

I = V ÷ R

I = (120 V) ÷ (160 )

I = 0.75 A

I

V

R

Alternating Current and Direct Alternating Current and Direct CurrentCurrent

AC – current from household electrical outlets. In US power grids alternate current changes

direction 120 times per second. DC – battery powered devices use DC. Current

never changes direction. An AC/DC adapter is a device that converts the

alternating current of an outlet into direct current. Lets you charge a cell phone battery!