Lab 2: Ohm’s Law Only 10 more labs to go!! When trying to understanding electricity it is helpful to draw analogies to the classical mechanics. PE flow, current Gravitational Field In this example we have flow of water (current) from the storage tank because the gravitational field (force) is pushing the water downwards (in order to decrease the water’s potential. Electric current flow Analogous to the water flow due to the gravitational field, charge will flow from it’s storage device (a battery) when in the presences of an electric field. Electric Field + + + + + PE

When charges flow we can do work, in fact batteries are characterized by the amount of work they can do. Electric potential- the amount of work each Coulomb of charge delivers, measured in VOLTS, V So 110 V means that 110 Joules of work can be delivered per 1 Coulomb of charge The current, I in a circuit is defined as the amount of charge that flows per second. Current, I, is measured in Amperes or Amps, A. 1 A of current means that 1 Coulomb of charge has flowed past a point in a second. We electrical current is not free to flow we call this Electrical Resistance

As charge flows through something that has resistance, some of the charge’s energy is dissipated. We call this type of device a resistor. Resistance is measured in ohms, . a bad, circuit diagram of a resistor Electric potential, current and resistance are related through Ohm’s law: As current flows through a resistor, the charges dissipate energy. Each Coulomb of charge will deposit one joule of energy per volt. If one Coulomb of charge flows per second, then one Watt of power is dissipated. Remember: Also remember from before:

We have several variations of the power equation: What if I gave you two light bulbs, one rated at 60 W and the other at 100 W at 120 Volts. Which bulbl will have the largest resistance? From the equation above:

The first thing you are going to do today: V A Construct a graph of current vs. voltage. Increment the voltage by 5 to 7 Volts , when the voltage exceeds 40 Volts, increment by 10 to 15 Volts. What’s the slope of this line? From Ohm’s law:

The next procedure you’re going to examine the lamp circuit: V A Construct a graph of current vs. voltage. Increment the voltage by 5 to 7 Volts , when the voltage exceeds 40 Volts, increment by 10 to 15 Volts. Current vs. Voltage for a Lamp Notice how the slope of the tangent line is changing. (For smaller voltages the slope is higher therefore the resistance is lower) In this case the temperature of the bulb is increasing with increasing power. As the temperature increases the resistance also increases I Current (Amps) V Voltage (Volts)

The next procedure you’re going to examine a light emitting diode (LED) circuit. An LED is a special semiconductor device. According to device physics: If I take the natural log of both sides: Construct a graph of the natural log of current vs. voltage. Increment the voltage by 5 to 7 Volts for all data points. On a log graph On a regular graph I I Voltage Voltage