phy124lab3 - phy124:lab_3[Stony Brook Physics for Life...

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PHY 124 Lab 3 - DC circuits Important! You need to print out the 2 page worksheet you find by clicking on this link and bring it with you to your lab session. [ ] If you need the .pdf version of these instructions you can get them here [ /labs/dokuwiki/pdfs/phy124lab3.pdf] . Goals The purpose of this laboratory is to observe the relationship between voltage drop across, and current through, electrical circuit components. You will also gain familiarity with connecting circuits and with voltmeters (measuring voltage) and ammeters (measuring current). Equipment 1 DC Power Supply 1 Voltmeter 1 Ammeter 1 board with resistive components 7 wires 4 clamps phy124:lab_3 [Stony Brook Physics for Life Sciences] ... 1 of 6 2/17/2010 9:17 AM
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Introduction Ohmic components obey Ohm's law, (3.1) where V is voltage and I is current through a resistor with resistance R. Ohmic components will keep a constant resistance with varying voltage and current. This can be seen as a linear relationship on a V vs. I graph. Standard circuit resistors are ohmic. Other kinds of electrical components may not be and in these R may depend on factors such as the temperature (as it does in light bulbs), the direction of current flow (for example in diodes), or the light intensity falling on the component (light sensitive diodes). You will use a voltmeter to measure V, the voltage drop across the component, and an ammeter to measure I, the current flow through the component. You must keep in mind that ammeters must be connected in series in circuits, while voltmeters must be connected in parallel across the circuit component whose voltage drop is to be measured. The sketch below shows the proper setup. Fig 2 Technical note: non-zero meter readings are obtained only when some current flows through the meters. Hence, the meters have two connections. The current flowing through the meters influences the measurement. You want this effect to be very small. Thus “good” voltmeters have a large resistance compared to R, which allows only a small current to pass through. Since the voltmeter is in parallel to the resistor, the current through R is essentially determined by R and only a very small current flows through the voltmeter. The equivalent resistance of R and the voltmeter in parallel is close to the resistance of R because if then . The voltmeter you use has the desired property i.e. it’s resistance is much larger than the resistance to be measured, R. In an ideal case a “Good” ammeter has a very small resistance compared to R, which allows current to easily pass through the ammeter and be measured. When this is the case and the resistor and the ammeter are in series, so the current measured in the ammeter is essentially determined by R. The equivalent resistance of R and the ammeter is close to resistance of R because if then . This is only approximately true
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