Note: Your homemade circuits, especially the parallel circuits, will look quite a bit different from the neat rectangular diagrams shown above.

Transcription

1 Name Period BUILDING SERIES AND PARALLEL CIRCUITS WIRING DIRECTIONS Materials per group: two D-cells and holders, three lightbulbs and holders, six pieces of insulated wire with stripped ends 1) Below are diagrams showing three bulbs in series and in parallel. Build these same circuits using two 1.5-volt D-cells (flashlight-type), three bulbs, some wire with stripped ends. Connecting wires through springs in a circuit board will help you keep your wiring in place. 2) Record how the brightnesses of the bulbs in the series and parallel circuits change as you increase the bulbs in the circuits from one to two to three. SERIES CIRCUIT PARALLEL CIRCUIT Note: Your homemade circuits, especially the parallel circuits, will look quite a bit different from the neat rectangular diagrams shown above. QUESTIONS 1) Which bulbs are brighter? a. The three bulbs wired in series. b. The three bulbs wired in parallel. c. They're the same. 2) What happens to the brightness as you add bulbs in series? a. The bulbs get brighter. b. The bulbs get dimmer. c. The bulbs stay the same. 3) What happens to the brightness as you add bulbs in parallel? a. The bulbs get brighter. b. The bulbs get dimmer. c. The bulbs stay the same. 4) What do you think these lighting differences suggest about the voltage across the bulbs in series circuits? a. The voltage across each bulb is less each time a similar bulb is added. b. The voltage across each bulb is more each time a similar bulb is added. c. The voltage across each bulb stays the same each time a similar bulb is added. 5) What do you think these lighting differences suggest about the voltage across the bulbs in parallel circuits? a. The voltage across each bulb is less each time a similar bulb is added. b. The voltage across each bulb is more each time a similar bulb is added. c. The voltage across each bulb stays the same each time a similar bulb is added.

2 USING METERS BACKGROUND Voltmeters are connected across the device whose voltage you wish to determine. One wire is connected to the positive end of the voltmeter and the positive side of the power source (battery) or other device (lightbulb, resistor, motor, buzzer, etc.). The other wire is connected to the negative end of the voltmeter and the negative side of the power source or other device. Basically, you are wiring the voltmeter in parallel with, or across, the device. Voltage is measured with a closed circuit so that current is flowing through the device. Ammeters are connected in series with the circuit whose current you wish to determine. (Remember that the same current runs through all devices in a series circuit.) One wire from the ammeter is connected to the positive terminal of the power source (battery, solar cell, etc.). The other wire is connected to the negative terminal of the power source. Current is measured with a closed circuit so that current from the battery is flowing through the circuit. Series Circuit Using Meters 1) Connect the three light in series. 2) Use the voltmeter to find the voltage drop across each light and the two batteries Light 1 (V) Light 2 (V) Light 3 (V) Two Batteries (V) 3) Find the total voltage drop across all three light by adding the three voltage drops together. Total voltage drop: V (This should be equal to the voltage drop across the two batteries) 4) Now connect the ammeter in series in the circuit and find the current through the lights. Current through the lights in A: A 5) Calculate the resistance of each light using the voltage drop for each and the current. Resistance of Light 1 (Ω) Resistance of Light 2 (Ω) Resistance of Light 3 (Ω)

3 Parallel Circuit Using Meters 1) Connect the three light in parallel. 2) Use your measured values of the resistance of each light from the activity above to calculate the total resistance of the parallel circuit. Total resistance of the parallel circuit: Ω 3) Now calculate the total current through the circuit using the total resistance of the parallel circuit from part 2 and the voltage drop across the two batteries from the activity above. Calculated total Current through the lights: A 4) Use the ammeter to find the current through each light. The ammeter must be put in series which each light to determine the current as shown in the figure below. Current through Light 1 (A) Current through Light 2 (A) Current through Light 3 (A) 5) Find the total current though all three light by adding the three currents together. Measured total Current: A 6) Compare the calculated total current from part 3 to the measured total current in part 5 by finding a percent difference. Percent Difference: %

4 Circuits Assignment Use the following situation to answer problems 1-8. Two students are studying circuits and how they behave by using batteries, a lightbulb, a resistor, a voltmeter and an ammeter. They study the relationship between current, voltage and resistance in a circuit in Ohm s Law: V = IR. One student reads that a component is nonohmic if its resistance changes as a function of current. If a component is ohmic then as the current changes the resistance would stay the same and by plotting a Voltage-Current graph of data, the slope would represent the resistance and it would be constant. If the Voltage-Current graph did not have a constant slope then this would imply the resistance changes with current and the component is nonohmic. The students decide to test to see if the lightbulb and the resistor are ohmic. They connect a voltage source and the resistor in series and measure the voltage drop across the resistor and the current through the circuit. (See image to the right.) They then increase the voltage and take the measurements again. They repeat this four more times. They then repeat the experiment but this time they replace the resistor with the lightbulb. Their data is shown in the tables below. Lightbulb Resistor Voltage (V) Current (A) Voltage (V) Current (A) ) Make a graph of the Voltage vs Current for both the lightbulb and the resistor LightBulb: Voltage-Current Graph

5 Resistor: Voltage-Current Graph ) Review the introduction information and the lightbulb: Voltage-Current Graph and determine if the lightbulb is ohmic or nonohmic. a) Ohmic b) Nonohmic 3) Describe how you are able to determine if the lightbulb is ohmic or nonohmic using the Voltage-Current graph. 4) What happens to the resistance of the lightbulb as greater currents pass through it as shown on your graph? a) The resistance increases b) The resistance decreases c) The resistance stays the same 5) Review the introduction information and the resistor: Voltage-Current Graph and determine if the lightbulb is ohmic or nonohmic. a) Ohmic b) Nonohmic 6) Describe how you are able to determine if the resistor is ohmic or nonohmic using the Voltage-Current graph. 7) What happens to the resistance of the resistor as greater currents pass through it as shown on your graph? a) The resistance increases b) The resistance decreases c) The resistance stays the same 8) If either of the graphs indicate an ohmic relationship, use the graph(s) to find the resistance of the component(s). Show your work below and include the proper units.

6 Extra Credit: Using the schematic of a circuit above, answer the following. Assume that the Battery has a voltage of 12 V. 1) Find the total resistance of the circuit. 2) Find the current passing through R1. 3) Find the current passing through R6. 4) Find the voltage drop across R3.