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LAB_2 - Resistance

# LAB_2 - Resistance - Lab 2 Basic Measurement Concepts And...

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Lab 2 31 Basic Measurement Concepts And Practices Resistance Summary In 1827 G.S. Ohm, a German scientist, proved experimentally that the electrical current flowing in certain materials (metals) resulted in a voltage drop across the element that was directly proportional to the current. This relationship is known as Ohm’s Law. Ohm’s Law is written as V = IR , where V is voltage measured in Volts, I is current measured in amps and R is measured in ohms. This law is very important. It enables us to solve for the current flow in an element if the voltage across it and its resistance are known. Or the voltage may be determined for if both the current flow and the resistance are known. When the resistance of an element remains constant, the voltage across the element plotted against the current through the element is a straight line with a slope of R (Fig. 1). Such elements are called linear resistive circuit elements. Every material exhibits resistance. Conductors such as metal wires and ionized gases have very low resistance. Insulators such as air, glass wood and plastic, have resistances which are extremely high compared to conductors. Semi conductors such as silicon and germanium have resistances between insu- lators and conductors. Super conductors have extremely low resistances. Resistance is measured in a variety of ways. In this experiment you will use a digital multimeter and a power supply to measure the resistance of different components. "All that glistens may not be gold, but at least it contains free electrons.” - John Desmond Bernal English crystallographer and discoverer of X-ray diffraction from protein crystals. Figure 1: Current - voltage relationship for a resistor. When the voltage across a resistor is plotted against current through it, the slope of the line gives the resistance.

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Lab 2 32 Educational Objectives After performing this experiment, students should be able to: 1. Measure resistance using the DMM. 2. Use the function keys of the DMM. 3. Program the DC power supply. 4. Plot and interpret characteristic current-voltage curves. 5. Measure the characteristic current-voltage curves of diodes. 6 . Measure the resistance of a thermistor as a function of temperature and voltage. 7 . Examine the resistivity of conductive foam strips. Background Information In this experiment the energy source is a programmable DC power supply. You should particu- larly read the description of the model you are using. When considering a resistor in a circuit, we use Ohm’s law to determine the resistance of a linear element. where V is voltage in Volts (V), I is current in Amperes (A) and R is in Ohms (Ω ). Figure 2 depicts a quick way of remembering how this law can be used to solve for the third quantity among V , I and R , once the other two quantities are given.
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