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CIRCUITS ANALYSIS In any working circuit the current flows from the high potential (+) to the low potential (-). The potential is designated as the voltage, V. A successful circuit will allow you to follow the path of current through the devices. In reality electrons travel from low potential to high (which is from high potential energy to low potential energy if you have a negative charge), but we only recently observed that behavior on the particle scale. Benjamin Franklin established the convention of high potential to low, and we still use his definition. Light Bulb Inside a light bulb, the complete path takes you from the electrical foot, through the filament, and then out through the threads. The entry and exit points are separated by an insulating material (usually glass or ceramic) so that the electrons must pass through the filament and cannot take a short cut (called a short circuit) that bypasses the filament. Figure 1: Anatomy of a Light Bulb1 1. Glass bulb (or "envelope") 2. Low pressure inert gas 3. Tungsten filament 4. Contact wire (goes to foot) 5. Contact wire (goes to base) 6. Support wires 7. Glass mount/support 8. Base contact wire 9. Screw threads 10. Insulation 11. Electrical foot contact 1 Wikipedia 2006 Property of LS&A Physics Department Demonstration Lab Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109 1 Drawing a Schematic When a circuit is drawn, it is not common to draw a literal representation of every component, but to instead use short-hand. The symbols for various components of a circuit are: Light Bulb Resistor Wire Switch Battery A literal schematic for the flashlight you built would be: Property of LS&A Physics Department Demonstration Lab Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109 2 Figure 2: Battery board wired like a flashlight The short-hand schematic of your circuit is: Figure 3: Flashlight schematic The schematic is much simpler and includes all the essentials. The light bulb, and switch are on the right, and the battery is on the left. Current and Voltage Measurements When you purchase a light bulb, the wattage is usually listed. Watts are the units of energy used per second known as the power. Power is equal to the voltage multiplied by the current. P I V The current and voltage of a circuit are measured in the fashion shown below. Voltage: Voltage is the electrical “push” from the battery to produce current. Voltage is measured across the device, because we want to know the potential difference above and below the device. This is a direct current (DC) circuit, which means current only flows in one direction. Use V on the multimeter for these measurements. To make a connection, simply touch the probe to an exposed place in the circuit, like a metal prong or the teeth of an alligator lead. Figure 4: Measuring the VOLTAGE ACROSS the device Property of LS&A Physics Department Demonstration Lab Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109 3 Current: Current is the electrical flow of electrons through the circuit. To measure the current, one must make the current flow through the multimeter. An alligator clip can be removed to do this in the lab. The units for current are Amps (A). Note that the meter must become part of the circuit (be in series with the circuit or device) to measure current, since the current must flow through the meter to make the measurement. Figure 5: Measuring the CURRENT of the circuit. ** WARNING: Do not measure current across a device, you will make a short circuit and blow a fuse. ** Property of LS&A Physics Department Demonstration Lab Copyright 2006, The Regents of the University of Michigan, Ann Arbor, Michigan 48109 4