Lab 5: Resistance, current, and circuits Phy208 Spring 2008 Name_____________________________ Section________ Your TA will use this sheet to score your lab. It is to be turned in at the end of lab. You must use complete sentences and clearly explain your reasoning to receive full credit. What are we doing this time? You will complete four related investigations. PART A: Build resistor circuits on circuit board, investigating current flow and voltages. PART B: Measure resistance and determine resistivity of last week’s conductive paper. PART C: Measure voltages and current flow in an extended medium (conductive paper). PART D: Use the ideas of current flow in an extended medium to understand how an electrocardiogram obtains information about the heart through measuring the effects of currents in a living (conducting) body. Why are we doing this? So you can understand basic ideas about current, resistance, voltage, and electric fields in conductors and simple circuits, and so you see how to extend these ideas to more complicated systems that you might actually come across someday. What should I be thinking about before I start this lab? You should be thinking about what you have learned about work, electric potential energy, electric potential, and electric fields. Because in this lab you set up situations in which charges are continuously moving in conductors between points of different potential, and so something is doing work. You will see that this something is the “power supply”. That’s why it is called a power supply (or voltage supply), and why it needs to be plugged into the wall! You might also be thinking that this contradicts the statement that a conductor in equilibrium has the same potential everywhere, and zero electric field. If the electric field is zero, why are the charges moving?Lab 5 2 A. Resistor Circuits Resistor circuits are made of two different types of conductors connected together, “wires” and “resistors”. Charges flow continuously through these conductors, so there are electric fields in the conductors, and the electric potential is not constant in them. These conductors are not in equilibrium. The charge flow (current) is sustained by a voltage (power) supply. This supplies charge to the circuit, and drives it around the circuit. Ohm’s law says that the electric potential difference (voltage drop) between two ends of a conductor is proportional to the charge/unit time (current) flowing through it, V=IR, with R the resistance. A “wire” has very low resistance, so that the voltage drop across it is extremely small. A “resistor” has a larger, measurable, voltage drop. You build circuits by plugging in elements to the circuit board shown below. Holes connected by black lines are electrically connected by conducting wires, so all points connected by black lines are at the same electric potential. You build a circuit by plugging in resistors across the gap between crosses. The resistors are built into plastic blocks with banana-plug connectors that exactly bridge the gaps. After you plug in a resistor, there will still be unused holes in each cross. You will use the remaining holes to connect the variable voltage source to supply your circuit with charge, and to connect the digital multimeter to measure potential differences and currents at various points in the circuit. These 5 points connected together Resistor block goes hereLab 5 3 30V DC voltage source 10.03 10 KΩ Keithley DMM 1) Measuring current and voltage. Build the circuit below. The digital mulitimeter (DMM) can be configured to measure current, voltage, or resistance. Here you want to measure current. Press the ‘A’ button for amps, then ‘2m’ for a 2 mA scale. The DMM now acts as an ammeter, and it displays how much current flows through it. In this mode it is very low resistance. So it acts like a connecting wire while measuring current, and doesn’t alter the properties of the circuit. Current flowing into the red terminal of the DMM and out of the black terminal gives a positive reading on the display. Set the voltage supply to 10V: what is your measured current? Which way do the electrons move? How much work is done by the power supply to move one electron around the circuit? Which way does the current flow out of the voltage source?Lab 5 4 2) Now you will add a 10 KΩ resistor as shown below, and measure the current. Before you do this, predict your measured current in the space below. Remember Ohm’s law, and that wires have zero voltage drop across them. Now build the circuit below. Here the ‘wire’ is a plug-in block that is a metal conductor. What is your measured current? Explain whether this agrees or disagrees with your prediction. 30V DC voltage source 10.03 10 KΩ Keithley DMM 10 KΩ Wire WireLab 5 5 3) Voltage drops around the circuit, and measuring current with voltage drops. In part 2) you saw that the DMM can measure current directly, but it must be inserted in the path of the current so that current can flow through it. This makes it difficult to quickly probe current in different parts of the circuit. If you press the ‘V’ button, the DMM will display the electric potential difference between its red and black terminals. In this mode, almost no current at all flows through the DMM – it acts like an extremely large resistor while measuring voltage. Explain how you use the DMM to measure the current through a resistor by measuring electric potential difference (voltage drop) between the two ends of the resistor. Now make the circuit below Use the voltmeter to determine the current through each of the 10 KΩ resistors by measuring voltage drops. Write the current below. Current through R1: Current through R2: 30V DC voltage source 10.03 10 KΩ 10 KΩ R1 R2Lab 5 6 In the next step you will put a 10 KΩ resistor (call it R3) in parallel with R2. But before making the circuit, predict whether currents through R1 and R2 will increase or decrease. Current through R1: Current through R2: Explain: Now build the circuit below, and determine the voltages and currents. Voltage across R1: Current through R1: Voltage across R2: Current through R2: Voltage across R3: Current through R3: Voltage across wire block: Current through wire block Explain how this agrees or disagrees with your prediction.
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