1Chapter 23Electric Current22Flow of Charge• Like all matter, charge seeks to be at its lowest possible energy• When there is a potential difference, charges (actually that particles that posses charge) will move towards to lower potential energy• Think of plumbing: if there is a difference in water pressure, the water will flow from the high pressure to the low pressure33Plumbing Analogy44Electric Current• Electric current – the flow of electrical charge• Electric current is composed of electrons (negative charge)• The amount of charge the flows in a second is called Amperes (sometimes calls Amps)• A current of 5 amps means 5 C of charge flows a second– That allot of electrons!55Voltage Source• We need a potential (voltage) difference to induce a current– This is the pump in our plumbing example• Batteries are a source of potential difference– That’s why the strength of a battery is measured in V• Charge flows through a circuit when there is is potential difference across it– Water will flow though a pipe when there is a pressure difference across it66Electrical Resistance• Electrical resistance is the measure of how much a material resists the flow of charge; units: Ohms (Ω)– Not all conductors conduct equally well• We can think of this resistance to be the size of our pipes in the plumbing example– The larger the pipes, the less resistance• So, the less resistance in a circuit, the more current can flow given the same potential difference77Ohm’s Law• The relationship among voltage, current and resistance is called Ohm’s Law:IVRIRVRVI === or or88Problem Solving: Ohm’s Law• A 12 V source is connected to a circuit with a total resistance of 5 Ω. What is the current in the circuit?• You measure a 2 A current in a circuit that has a total resistance of 10 Ω. What is the voltage?AVRVI 4.2512=Ω==VAIRV 20)10)(2( =Ω==99Direct and Alternating Current• Direct current has a constant value with time• Alternating current has changing values and has many of the same properties of a wave (frequency and amplitude)• The current that comes out of the wall is alternating current with a frequency of 60 Hz1010Speed and Source of Electrons in a Circuit• Electrons are densely packed in conductors and can flow easily. Therefore there can be a large current even though electrons are moving slowly through the material• Electrons move slowly in a metal because they run into each other often and are deflected in all directions1111Electric Power• We define power before to be the amount of work done in a unit of time• Electrical power is defined the same way:–Units: W• You can use Ohm’s Law to replace current or voltageIVPower =1212Electric Circuit• There are 2 types of circuits: series and parallel• Series circuits have everything connected in a line• Parallel circuits have component connected in branches1313Series Circuits• Electric current has but a single pathway through the circuit; the current passing through each electrical device is the same• The total resistance is the sum of all of the resistances in the circuit• The current in the circuit is equal to the voltage of the source divided by the total resistance• Ohm’s Law also applies to the individual components to determine the voltage drop• The sum of the voltage drops over the circuit equals the voltage of the source1414Parallel Circuits• The voltage over each component in a parallel circuit is the same• The total current in the circuit divides over each of the parallel components• The total current in the circuit equals the sum of the currents in its parallel branches• As the number of parallel branches is increased, the overall resistance of the circuit is