be protected by a circuit breaker to guarantee that the current through the wire can’t exceed its rating. This is to prevent the wires in your house from getting too hot and starting a fire. Of course in special cases, you do want the resistor to get hot. If the resistor is in an incandescent light bulb, it will get hot enough to glow and emit light. This is the reason that we represented a light bulb as a resistor in the first chapter. You also use resistors to generate heat if you have an electric oven, toaster, hair dryer, etc. The heating elements in these devices are all resistors that convert electrical energy to heat. While real wires have resistance, the lines in our circuit schematics do not. These lines are perfect conductors, so there is no voltage difference between any points on this line. If you want to model the resistance of a real wire, you need to represent that resistance by explicitly adding a resistor to you circuit diagram. As mentioned earlier, for most circuits, the resistance of the wire is so small it can be ignored. The range of resistance we use in our circuits is quite large. Recalling that the inverse of the resistance (1 /R ) is the slope of the iV characteristic curve of a resistor, these large ranges in resistance will result in different-looking iV curves. Some circuits will use resistors larger than 10 MΩ, which is nearly a horizontal line near zero, to model the small amount of current that flows 2 All material, except for superconductors which are a macro-scale manifestation of a quantum phenomenon, have resistance. These no-resistance materials, which only exist at cold temperatures, seem magical if you are used to normal conductors with loss. They have interesting properties, like current can flow in a loop forever without any voltage driving them. Since currents cause magnetic fields, superconducting circulating currents are used in MRI machines to make large magnetic fields, and they can even levitate objects.
30 CHAPTER 2. ELECTRICAL DEVICES V Figure 2.6: A battery. On the bottom left is the circuit symbol for a battery, and on the right is a more accurate iV plot of a battery. While the line is pretty vertical, there is some slope to it. We can model this real battery by combining a perfect voltage source with a resistor. The output voltage of this combination is going to be equal to the sum of the voltage drop across the voltage source and the resistor. When current is flowing out of the voltage source, it will flow into the resistor, so the charges will leave with less energy (voltage) then they started with, so the voltage across the combination decreases as the current becomes more negative (the current reference direction is into the positive terminal). through a device which doesn’t take much current (like a good voltage meter). Other circuits might need resistors less than 10 mΩ which is a nearly vertical line, to measure the resistance of a thick wire carrying a large current (like battery jumper cables). The difference of resistance between
- Spring '19
- Hassan Kasfy