2 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 timemeasured in amperes (A) (1 A = 6250 million billion electrons past a point every second)
3 Voltage Difference Voltage Difference The force that causes electric charges to flowlarge separation of charge creates high voltagemeasured in volts (V)Ball bouncing down stairs, electrons move around but net is toward positive. Ball net is down.Electron flowCurrent is from high voltage to lowElectrons flow from low voltage to highLow voltage High voltage
4 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.
5 BatteriesTo keep electric charges continually flowing in an electric circuit, a voltage difference must be maintained.Power supplies such as batteries!
6 Dry Cell BatteriesA cell = two electrodes surrounded by an electrolyteThe electrolyte enables the charge to flow from one electrode to the other.
7 Dry Cell BatteriesElectrode 1 – carbon rod; Electrode 2 – zinc containerElectrolyte – moist paste of chemicalsCompleting 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
8 Wet-cell BatteriesContains two connected plates made of different metals in a conducting liquid solution (electrolyte).Example: car batteryChemical reactions transfer electrons from the lead (negative) plate to the lead dioxide (positive plates)
9 Wet-Cell BatteriesLead-Acid batteries: contain a series of six wet cells of lead and lead dioxide plates in a sulfuric acid solution.Voltage difference approx 12VChemical reactions transfer electrons from the lead (negative) plate to the lead dioxide (positive plates)
10 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
11 Why do cell phones get hot on a long call? Materials resist the flow of electrons and convert electrical to thermal energyResistanceMeasured in Ohms (Ώ)Resistors reduce the current through the circuit to prevent overload (some energy transferred to resistor)
13 Resistance Copper - low resistance Tungsten - high resistance Not only is electrical energy converted to thermal energy but also to light!
14 Resistance Resistance depends on… the conductor less resistance in a better conductorwire thicknessless resistance in thicker wireswire lengthless resistance in shorter wirestempless resistance at low temps
15 I = V / R Ohm’s Law Ohm’s Law V: voltage difference (volts) I: current (amperes)R: resistance (ohms)
16 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 VI = ?WORK:I = V ÷ RI = (120 V) ÷ (160 )I = 0.75 AIVR
17 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!