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Lab 1 - Resistance

3 resistance electrical resistance is a physical

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3 Resistance Electrical resistance is a physical property that describes how strongly a material opposes the flow of electric current; electric current being the motion of charges within that material. The current is driven by an electric force exerted on those charges. by way of an electric field inside the material pointing along it’s length. “How does this electric field arise?”, you may ask. This field comes about by the application of a potential difference (Δ V ) between the two ends of the material. For example, consider straightening a paper clip and sticking an end in each of the two slots of one of your household electrical outlets; the potential difference between the two slots ( 110 V ) will cause a current to flow through the length of the paper clip (this is why you were told not to do this as a kid). The amount of current depends on how much resistance is in the material; generally speaking, the greater the resistance the less the current and visa versa. 3.1 On what does resistance depend? The value of an object’s resistance is determined by its physical structure (shape, size and composition). To better understand this we will look at the resistance of graphite as follows: Get out a sheet of paper, a ruler, a number 2 pencil and your DMM. On your sheet of paper measure out and draw a rectange of approximate dimensions 1cm by 4cm with your pencil. Now, with your pencil, solidly fill in the rectangle you just drew. The darker you make the pencil the better. Now draw two more rectangles - one that’s twice as wide ( 2cm) and one that’s twice as long ( 8cm). Fill these in as well, making them as dark as the first. These rectangles are your resistors of different dimensions. Note that they are made of the same material and are equally dark. In other words they have the same material composition. Set your DMM to measure resistance on the kΩ scale. This should be the right order of magnitude for the resistors you’ve made; if not it may read “OL”. In this case adjust the measuring scale up or down an incriment until a numerical value is displayed. With each resistor place the two probes at opposite ends of the rectangle. Have the probes within 2-3mm of the end so that it is completely contacting the graphite. Hold the probes very still such that the value displayed on the DMM is roughly constant. Record this value. Repeat this step with the other two rectangles. Do you note a difference in the values? How does changing the length and width of the resistor change the value of its resistance? 2
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4 Circuit Components and Electronics Board Throughout the quarter you will be working with several types of circuit components to study the various phenomena encountered in Electricity and Magnetism. We will now look at some of the basic aspects of these in this section.
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