03 ohms law Aug 8 2008

03 ohms law Aug 8 2008 - 23 1EXPERIMENT #3: OHM'S LAW...

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1EXPERIMENT #3: OHM'S LAW EXPERIMENT PRE-LAB INSTRUCTIONS PART I - OHM'S LAW When George Simon Ohm investigated the relationship between voltage and current, he found that generally , the ratio of supply voltage (E) to the resulting current (I) was constant. He meant that if the applied EMF were doubled, the current would also be doubled. Stated mathematically: E / I produces a constant ratio The constant of proportionality is called the resistance (R) of the circuit. So Ohm's Law states that generally R is constant. E (in Volts) / I (in Amperes or Amps) = R (in Ohms) Use the memory aid below, called the Ohm’s Law Triangle, for calculating any of the three values when two other values are known: E/I=R E/R=I IxR=E The resistance of most electrical conductors is constant and when R is constant the circuit (resistor) is said to be linear. When R changes with changes in applied current, the resistance is called non-linear. Example: A voltage of 5 kV is applied to a resistor and the resultant current is found to be 2mA, what is the resistor? R = E / I = 5,000V / 0.002 Α = 2,500,000 = 2.5M EXPERIMENT #3: OHM'S LAW EXPERIMENT Ohm's Law Triangle E vs. I Characteristic of a Linear Resistance E vs. I Characteristic of a Non-Linear Resistance 23 E R I
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PART II - MULTIRANGE METERS - CHOICE OF RANGE Philips, Hioki, Simpson, and Micronta multimeters are examples of analogue meters. In such a meter the magnitude of the deflection of the pointer represents a definite magnitude of, say voltage or current. A selector switch enables the user to choose whether the meter will read voltage or current, and also the range of measurement possible, e.g. 0 to 3 V or 0 to 300 V. Experience has shown that when you are uncertain about how large the voltage or current to be measured may be, you should choose the highest range then systematically switch down ranges until you get a reading in the upper half of the scale. Analogue meters tend to be less accurate in the lower part of the scale for reasons you will learn in your Measurements Course. While it may not always be possible, do use the range that gives you a reading in the upper part of the
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03 ohms law Aug 8 2008 - 23 1EXPERIMENT #3: OHM'S LAW...

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