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Figure 2.4: Resistors come in many different shapes and sizes. The resistors on the lower right are surface-mount resistors, which are used on printed circuit boards. M. Horowitz 19 Measuring/Reading Resistors Measure a resistor Does it match the value printed on it? E40M Lecture 3 Color First Digit Second Digit Third Digit 1% Resistors Multiplier Tolerance Black 0 0 0 1 Brown 1 1 1 10 ±1% Red 2 2 2 100 ±2% Orange 3 3 3 1000 (=1k) Yellow 4 4 4 10k Green 5 5 5 100k Blue 6 6 6 1000k (=1M) Violet 7 7 7 10M Gray 8 8 8 100M White 9 9 9 1000M (=1G) Gold 0.1 ±5% Silver 0.01 ±10% Gap Figure 2.5: Resistor color code. On many resistors, the three numbers are represented by colors and not by numerals, which makes reading resistors even more fun.
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2.2. RESISTOR 29 The iV curve of a resistor is shown in Figure 2.3 . The slope of the line is 1/R. Resistors are devices that conduct current, but it takes energy to get the charge to move. The more current you want through the device, the harder you need to drive the charge, and the more voltage will “drop” across the resistor. We will find that these type of devices are useful for building different types of circuits, so people have created materials which can create a wide range of resistance. Since charge flowing into a resistor has more potential energy than it leaves with (aka positive current flows into the terminal with a higher voltage), the resistor absorbs energy from the circuit. We know that the power a device absorbs is just i · V and that the iV relationship for a resistor is V = i · R , so the power a resistor dissipates is: Power = i · V = V 2 /R = i 2 R The power a resistor dissipates is proportional to the square of the voltage across it. While this might at first seem surprising, it make sense. Both the current through and the voltage across a resistor increase when the voltage (or current) increases, so a quadratic dependence should be expected. Where does this energy go? It gets converted into heat, and makes the resistor warm, so the heat can flow out. As a result be careful if you touch a resistor you are putting a lot of power into. It might be very hot. In fact, it can get so hot it burns up. It turns out that almost all material, even good electrical conductors have some resistance. 2 Thus, all real wires usually have a small voltage drop across them when current flows through them. In most designs the wires are chosen so this drop is small enough so it can be ignored. Wire resistance is a problem for large structures as well as small ones. Every wonder why the wires connecting to your car battery are so large? It is because they need to supply a large current to start the motor, and thus need to have a low resistance. Similarly the wires that deliver power (110V) through your house also need to be wide to support the large currents they support. In fact, there are rules for the wire size required for different current ratings, and these wires must
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  • Spring '19
  • Hassan Kasfy

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