College of Engineering & Science living with the lab Ohm’s Law © 2012 David Hall 1.5V

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electric current and voltage living with the lab 3 electric resistance electron supply - electron shortage +

living with the lab 4 units for electric current The electric current I is the amount of charge passing a point per unit time. Class Problem A battery powers a flashlight. If the battery supplies a steady current of 1.3A over 8 minutes, how many electrons leave the negative terminal of the battery during this time period? teams of 2

living with the lab 5 units of voltage the unit for voltage V is the volt 1.5V 1C One Coulomb flows through each light bulb. The voltage across a bulb is doubled when two batteries are used. The work done when two batteries are used is twice as much as the work done when one battery is used, even though the same number of electrons flow through each bulb. Doubling the voltage doubles the work... and gives off much more light.

living with the lab 6 voltage / water pressure analogy depth of water between surface of lake and surface of river depth of water between surface of lake and surface of river Glen Canyon Dam in Arizona, USA The work done per gallon of water passing through a turbine will double (theoretically) when the water depth (pressure) is doubled. Wikipedia Likewise, the work done per Coulomb of charge passing through a resistor will double (theoretically) when the voltage is doubled. voltage & pressure are both measures of a potential difference

living with the lab 7 units of resistance The resistance R is a measure of the frictional resistance encountered by electrons as they attempt to pass through a material. a pile of electrical resistors

living with the lab 8 circuit symbols We frequently draw diagrams that represent various types of electric circuits. The most simple diagram is that of a direct current (DC) power source and a resistive element such as a light bulb or resistor. straight lines depict conductors with an assumed resistance of zero 1.5V + - circuit symbol for a DC power source volts in this case 220 Ω circuit symbol for a resistor ohms in this case For resistors, you don’t need to worry about which side is + and which side is – (they will work either way).

living with the lab 9 We can put together a series of circuit symbols to depict an electric circuit. circuit diagrams 6V Ω The voltages of power sources arranged end-to-end in this way can be added. Thus, four 1.5V batteries in series has a total voltage of 6V.

living with the lab 10 Ohm’s law 6V Ω Ohm’s law relates voltage, current and resistance as follows... I conventional current Class Problem Use Ohm’s law to determine... (a)the current delivered to the resistor (light bulb) for the circuit above. (b)the number of electrons that leave the battery pack over 2 minutes. teams of 2