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Kreutter: Circuits 2 Voltage/Current/Resistance Lab Voltage, Current, and Resistance Lab Objectives: Measure current. Describe the relationship between resistance of a circuit and the total current flowing through a circuit. Describe the relationship between voltage and the total current flowing through a circuit. Make and use graphs that show the relationships between current and resistance and current and voltage. Safety Precautions: Wires and circuits may become hot. Wires are sharp and can cut or prick you. Materials: Power Supply Christmas tree light bulbs—they are resistors. Insulated copper wire Ammeter (measures current) Inquiry: Given the above materials, how can you design an experiment to find the relationship between current, voltage, and resistance? Write down your ideas. Kreutter: Circuits 2 Voltage/Current/Resistance Lab To build a circuit: 1. Always make all connections before turning on the power supply. To adjust any connections, TURN OFF THE POWER SUPPLY! 2. The red lead should come from the positive terminal of the power supply, then connect to a component (the positive terminal of the ammeter). Then use another alligator clip (black) to clip the opposite (negative) end of the component to the next component (resistor, bulb, etc). Always connect the positive lead to the component first so you can “follow” the current through the circuit as you are building it. The load in this case would be the resistor. Be sure to connect the positive lead from the power supply to the positive terminal of the ammeter. To connect multiple resistors (loads), use alligator clips to fasten them together in series. Your final connection should be to the negative terminal of the power supply. IF YOU SEE OR SMELL SMOKE, (this shouldn’t happen as we are using GFCI outlets and the power supplies also “trip” if too much current is drawn), LET ME KNOW IMMEDIATELY. IF YOU ARE ABLE TO TURN OFF THE POWER SUPPLY YOU SHOULD DO SO, BUT DO NOT TOUCH ANY COMPONENTS! Procedure if you need help: Part A 1. Create a circuit with the power supply (V= 1.5 V), the ammeter, and one light bulb (one resistor). Since you only give current one path, this is a series circuit. 2. Record values for current and resistance in table 1. For resistance, record the number of light bulbs, i.e., “1”, since we do not know the exact value of resistance the light bulb provides. For current, read the value given by the ammeter. Be very careful to not overload the ammeter. Keep the voltage on 1.5 V! 3. Double the resistance by putting 2 bulbs into the circuit instead of one. Again, record values for current and resistance. 4. Repeat step 3 with 3x and 4x the resistance. Kreutter: Circuits 2 Voltage/Current/Resistance Lab Table 1 Voltage (V) 1.5 Resistance Current 1.5 1.5 1.5 Part B: 5. Recreate the circuit with 1x resistance. Verify current and record current and voltage in Table 2. 6. Now set the voltage at 3 V. Record values for voltage and current in Table 2. 7. Repeat step 6 with a voltage of 4.5 and 6 V. Record your values for voltage and current. Table 2 Voltage (V) Resistance Current Kreutter: Circuits 2 Voltage/Current/Resistance Lab Analysis: 1. Graph current versus resistance. Resistance should be your independent variable and current should be your dependent variable. 2. Graph current vs. voltage. Again, current is the dependent variable. 3. Other than the values of the resistors, what factors could have affected the current in Part A? 4. Other than the added batteries, what factors could have affected the current in part B? How might the effect of these factors be reduced? 5. Using your graph, describe the relationship between resistance and current. 6. Why do you think this relationship between resistance and current exists? Kreutter: Circuits 2 Voltage/Current/Resistance Lab 7. Using your second graph, describe the relationship between voltage and current. 8. Why do you think this relationship between voltage and current exists? 9. What would the current be in a circuit with a voltage of 3.0 V and a resistance of 20 kΩ? How do you know? 10. Use your data to devise a mathematical relationship between voltage, current and resistance. Hint: Assume your graph of current vs. voltage is a straight line that goes through the origin. 11. How well does your data match this formula?