ELECTRICAL POTENTIAL ENERGY AND RESISTANCE REVIEW Last class

ELECTRICAL POTENTIAL ENERGY AND RESISTANCE

REVIEW • Last class we discussed charge and current • Charge (q) is a measure of the excess or deficit of electrons and is measured in Coulombs. Where 1 Coulomb equals 6. 24 x 1018 electrons • Current (I)is a measurement of how quickly electrons travel through a circuit. It is measured in Amperes. Where 1 ampere is equal to 1 C/s passing a point on the circuit.

WATER WHEEL ANALOGY • A simple electric circuit can be viewed as a system of pipes that channel water around using a pump in order to make a wheel spin

GETTING ENERGY FOR A CIRCUIT • Water is pumped up to give it the ability to fall on the wheel with a lot of energy and make it spin. The higher the water is pumped, the more energy it will have and the faster the wheel will spin

ELECTRICITY • In an electrical circuit, the battery is the pump and the electrons are the water. The battery releases the electrons into the circuit and gives them energy

ELECTRICAL POTENTIAL ENERGY • As the water falls and moves the water wheel it loses energy • Same thing with electrons. The load provides resistance and the resistance takes potential energy away from the electrons. When electrons return to the battery, they have lost all of their energy. The battery then re-energizes the electrons and sends them back into the wires.

ELECTRIC POTENTIAL • This potential energy that the battery gives the electrons is called the voltage (V).

RESISTANCE • The greater the voltage, the greater the current. In other words, the more energy the electrons receive from the battery, the faster they will move through the circuit. • But there is one more factor; resistance • Resistance impedes, or slows down the flow of electrons. Wires are made of copper wire which conducts electricity well and provides virtually no resistance. • But loads, like light bulbs, provide more resistance and take some of the energy from the electrons. This causes the current to decrease.

RESISTANCE • There are three things that affect resistance in an object. They are: 1. The material – Insulators provide more resistance than conductors 2. The cross sectional area 3. The length Long thin loads provide more resistance than short wide loads

VOLTAGE, CURRENT & RESISTANCE • There appears to be a fixed relationship between the three variables • That relationship is V = IR or • Voltage = Current x Resistance

V = IR • That means that current and resistance are inversely related. That means that as resistance increases, current decreases by the same proportion. • This formula holds true for the entire circuit and for each individual load

LEARNING CHECK • The following electric circuit is powered by a 20 V power source (battery). The resistance provided by the light bulb is 5 ohms ( ). What is the current of the circuit?

LEARNING CHECK • Simple!! • Since V = IR • I = V/R • So, substituting • I = 20/5 = 4 Amperes or 4 Coulombs per second

SERIES VS PARALLEL • In circuits that have several loads, the value of one of the variables (i. e. voltage, current or resistance) at one point in the circuit could be different than at another point. • The next slide shows how the variables for the entire circuit compare to the values at a single load based on the type of circuit

SERIES VS PARALLEL SERIES • VT = V 1 + V 2 + V 3 • IT = I 1 = I 2 = I 3 • RT = R 1 + R 2 + R 3 PARALLEL • VT = V 1 = V 2 = V 3 • IT = I 1 + I 2 + I 3 • 1/RT = 1/R 1+1/R 2 +1/R 3