Experiment on Resistance, Ohm's Law, and Power Dissipation, Lecture notes of Law

Download Experiment on Resistance, Ohm's Law, and Power Dissipation and more Lecture notes Law in PDF only on Docsity! 1     Resistance and Ohm's Law Goal: To test Ohm's law with a carbon resistor, measure resistances in series and parallel, and to measure the current-voltage characteristics of a light bulb. Lab Preparation For many conductors, especially metals, the current flowing through a device is proportional to the voltage difference applied to the device: the ratio of the voltage to the current is a constant. This is Ohm's law. This ratio for a particular device defines its resistance: 𝑅 = ! ! where R is measured in ohms (Ω). For many materials under ordinary lab conditions, the ratio is practically constant, and the device is said to be ohmic. In some situations the ratio ! ! of a device may vary with changes in the conditions of the measurement such as large changes in the applied voltage or temperature. In this experiment you will test whether a carbon resistor (a common electronic component) obeys Ohm's law by measuring the current and voltage and calculating the ratio of ! !  . If Ohm's law holds, the ratio should be a constant. According to Ohm's law, a graph of current as a function of voltage (an I vs. V curve) will form a straight line for a simple resistor. The slope of the line is the reciprocal of the resistance, ! ! , which is called the conductance. When two or more resistors are connected together, a single equivalent resistance can replace them. Most combinations of resistors can be broken down into two kinds: series and parallel (Figure 1). Figure 1a: Series Figure 1b: Parallel For two or more resistors connected in series, the same current must pass through each resistor. The equivalent resistance is the sum of the individual resistances: Req = R1 + R2 + R3 + .... For two or more resistors in parallel, the same voltage drop is present across each resistor. The equivalent resistance is given by: ! !!" = ! !! + ! !! + ! !! +  . .. R R2 R3 R1 R2 R3 R1 R2 R3 R1 R2 R3 2     The power dissipated by a resistor (or light bulb) is P = VI. The dissipated energy appears as heat and if the resistance is hot enough as visible light. Procedure I. Ohmic behavior. A. The resistors provided are made from a carbon-based composite material. Identify the values of the resistors provided by interpreting the color coded bands using the reference provided. Record these values along with the tolerances in a table. B. Have your lab instructor check your circuit before closing the switch. When building circuits always try to arrange components as in the schematic to make wiring errors easier to spot. Add voltmeters last, since they usually can be added or subtracted from a circuit without significantly altering the currents and voltages present in the other components. Build the circuit of Figure 2 with a 33 kΩ resistor. Connect to the battery pack's 6 V terminal. Remember to have your lab instructor check your circuit. Measure the current in µA (be very careful reading the ammeter) and the actual voltage drop across the resistor using the voltmeter. Record your values of V and I in a table that has columns for V, I and R. Calculate R. Figure 2 Repeat this process for the battery pack hooked up to 4.5 V, 3.0 V, and 1.5 V. Does R have a constant value? C. Make a graph of I as a function of V (put I along the y-axis and V along the x-axis). Is the graph linear? If so, find the slope of the line and from the slope find the resistance. Check to see if this value is consistent with the value found in part A. Print out a copy of the graph to include in your report.