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PHYSICS 220/230
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PHYSICS 220/230
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Introduction: The multimeter, which you have already used, is a device that will measure voltage, current, or resistance. Observe the controls for function selection and scale factor selection on the front of your multimeter. Note what ranges it will cover. Also, where the probe wires connect to your instrument, note the panel markings indicating the maximum allowable current and voltage that can be measured.
When making electrical measurements, always remember the following rules. To measure the voltage across a circuit element, the multimeter must be connected across (in parallel with) that circuit element. To measure current through a circuit element, the multimeter must be connected in line (in series) with that circuit element. To measure resistance, the resistor should be completely disconnected from the circuit and placed between the probes of the multimeter.
If ever the voltage, current, or resistance being measured by the multimeter exceeds the maximum value of the range selected, an over-flow indication "1. " will be displayed. You should then select a higher range. In general, you should start on the highest range possible if you have no idea of the value of the variable you are measuring.
To make sense of this lab, you must keep in mind that light bulb resistance depends strongly on temperature, which is governed by the current passing through the bulb. For the bulbs used in this lab, THE RESISTANCE INCREASES BY ABOUT A FACTOR OF TEN as the current goes from very small values (like those used by the FLUKE meter when it measures resistance) to operational values (i.e. the current that flows when a potential difference of 5V is applied across the bulb).
When predicting and explaining bulb brightness, remember that the BRIGHTNESS of a light bulb is proportional to the POWER dissipated by the bulb (that is, the product of the current passing through the bulb and the voltage drop across the bulb).
Objectives: Consider, build, and test several different light bulb circuits. Make predictions. Devise and document your experimental procedure. Explain your observations!
1) Measurement of Resistance: Use the resistance mode of the FLUKE multimeter to measure the individual resistance of each of the three bulbs. Note that the multimeter has its own internal voltage source (a battery), so no external power supply is needed to make these measurements. Since resistance varies from light bulb to light bulb, you may notice minor differences in their behavior. Compute the average resistance, and use this value for all future predictions of current and voltage. For this lab, let's try to focus our attention on visible differences in brightness as the bulbs are configured in different circuits.
2) Measurements of Current and Voltage
(a) Bulbs in parallel: Consider a two-bulb circuit with the bulbs connected side-by-side and the combination connected to a 5V source. Using the average resistance from Part (1), predict the results of your future measurements by calculating in your lab notebook the voltage across and current through each bulb.
Now use the Science Workshop Output for your DC Voltage source. Set the output voltage to 5V and measure its value with the FLUKE meter. Then build the parallel circuit and use the multimeter to measure the individual bulb voltages and currents. Use the FLUKE multimeter in its voltage mode to measure the voltages. Use the FLUKE meter in its current mode to measure the currents. (NOTE: measuring the individual currents can be tricky: you will need to ensure that the current junction occurs BEFORE the current passes through the ammeter in each branch of the circuit.)
Compare your measurements with your predicted values. Why are they different?
(b) Bulbs in series: Now consider a two-bulb circuit with the bulbs connected one after the other and the combination connected to a 5V source. Using the average resistance from Part (1), predict the results of your future measurements by calculating in your notebook the voltage across and current through each bulb.
Now build the circuit and use the multimeter to measure the voltages and currents.
How bright are the series-connected bulbs in comparison to the parallel-connected bulbs? Explain any differences that you observe.
3) Bulb Brightness in Three-Bulb Circuits
(a) Consider a three-bulb circuit such that 2 of the bulbs are in series with each other, this series combination is in parallel with the third bulb, and the three-bulb circuit is connected to a 5V source. Before testing the circuit, predict the relative brightness of each of the bulbs. In particular, how do you expect the brightness to compare with the bulb brightness in the parallel and series connections of Part (2) above?
Now build the circuit, connect the 5V power supply, and test your predictions.
Explain the brightness of each bulb.
(b) Consider a three-bulb circuit such that 2 of the bulbs are in parallel with each other, this parallel combination is in series with the third bulb, and the three-bulb circuit is connected to a 5V source. Before testing the circuit, predict the relative brightness of each of the bulbs. In particular, how do you expect the brightness to compare with the bulb brightness in the parallel and series connections of Part (2) above?
Now build the circuit, connect the 5V power supply, and test your predictions.
Explain the brightness of each bulb.