Ch. 13 - Electricity II. Electric Current

Electric 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 Difference Voltage Difference The force that causes electric charges to flow large separation of charge creates high voltage measured in volts (V) Ball bouncing down stairs, electrons move around but net is toward positive. Ball net is down. Electron flow Current is from high voltage to low Electrons flow from low voltage to high - - - - - - - - - - - - - - - - Low voltage High voltage

Circuit closed path through which electric current flows A current will only flow as long as there is a voltage difference. Sources of the voltage difference? Series and parallel are next section.

Batteries To keep electric charges continually flowing in an electric circuit, a voltage difference must be maintained. Power supplies such as batteries!

Dry 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 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 a closed circuit

Wet-cell Batteries Contains two connected plates made of different metals in a conducting liquid solution (electrolyte). Example: car battery Chemical reactions transfer electrons from the lead (negative) plate to the lead dioxide (positive plates)

Wet-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 Chemical reactions transfer electrons from the lead (negative) plate to the lead dioxide (positive plates)

Electric 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 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?

Resistance Copper - low resistance Tungsten - high resistance Not only is electrical energy converted to thermal energy but also to light!

Resistance 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

I = V / R Ohm’s Law Ohm’s Law V: voltage difference (volts) I: current (amperes) R: resistance (ohms)

Ohm’s Law R V I R = 160  I = V ÷ R V = 120 V I = (120 V) ÷ (160 ) A 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 Current 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!