PHYS 1407 – Conceptual Physics II

Ohm’s Law

 

Leader: ___________________                      Recorder: __________________________

Skeptic: ___________________                     Encourager: ________________________

 

Materials

2 x DMM

Battery eliminator

2x alligator clips

Pasco Circuit Board

Analog Ammeter

Laptop (for graph)

 

Introduction

      In this activity we will examine the factors that determine the amount of current that flows in a resistive circuit.  Part 1 will examine the correct use of meters.  Part 2 will look at the relationship between potential difference and current for conductor’s called Ohm’s law.

 

Part 1  Use of Voltmeters and Ammeters

In the following procedure, disconnect your circuits when not making observations to preserve battery life.

      Use the Pasco circuit board and connect the two D cells in series and connect the battery across two light bulbs in series.  Your circuit should appear something like the following schematic.

 

 

 

 

 

 

 

 

 


Observe the bright ness of the light bulbs, and open the circuit.

      When we depict a meter on a schematic we show it by a circle with a letter inside designating the type of meter.  Thus a voltmeter will be a circle with a V inside and an ammeter will be a circle with an A inside.  Potential differences are measured across objects so when using a voltmeter – or a DMM in voltmeter setting – we place the probes on either side of the object.

 

Q1)  Close the circuit and use the DMM to measure the potential difference across each resistor and record the values.  Both values should be positive so reverse the orientation of the leads if you measure a negative value.  V1 = ________    V2 = ­­­__________

 

Q2)  Find the sum of the potential differences

 

V = V1 + V2 = _______

 

Q3)  Measure the potential difference across both resistors and record

 

V = ________

 

Q4)  How does your answer to Q3)  compare to Q2)?  Explain why this should be the case.

 

Q5)  Measure the potential difference across the battery and record

 

V = _______

Open the circuit again.

 

Q6)  How does your answer to Q5)  compare to Q2)?  Explain why this should be the case.

 

 

Now we will find out what happens when we connect a voltmeter incorrectly.  Instead of using the voltmeter across a resistor, let us connect it in series with the circuit.  Connect the following circuit. Note to put a new element in series, you must make a break in the current circuit.

 

Q7)  What happens to the light bulbs when you place the voltmeter in series with the circuit?

 

Q8)  Does this suggest that a voltmeter has a high resistance or a low resistance?  Explain.

 

      Now we will examine using an ammeter.  Current flows through a circuit so to measure it, the current must flow through the ammeter.  Thus when we use an ammeter correctly it must be placed in series in the circuit. 

     

      Remove the voltmeter from the circuit and reconnect the battery across the two light bulbs in series.  Take a piece of alligator clip wire and short out one of the light bulbs by connecting the wire form one side of the socket to the other.

 

Q9)  What happens to the other light bulb?

 

Q10)  Did the current increase or decrease when the first light bulb was shorted out?

 

Disconnect the short circuit.

 

Now we will use an ammeter incorrectly and see what happens.  It is very common for people to connect an ammeter like it was a voltmeter, namely to connect it across a circuit element instead of putting it in series with the circuit.  Clip one alligator clip lead onto the black common terminal on the ammeter and the other onto the 5 A terminal.  This means that the ammeter will read 0 – 5 A.  If the lead was connected to the 500 mA terminal it would read 0 – 500 mA and so on.  Connect the ammeter across one of the light bulbs and observe what happens.  Disconnect once you have made your observation.

 

Q11)  Describe what happens.  Did you short out the light bulb?

 

When you connect an ammeter across a circuit element, you create a short circuit and increase the current in the circuit.  This can be dangerous and at the very least can damage the ammeter.  Ammeters must be connected in series with the circuit.

 

Connect the ammeter in series with the light bulbs between the positive terminal of the battery and the first light bulb.

 

Q12)  Are both light bulbs now lit?

 

Q13)  Record the current.  Note if the needle seems to be deflecting to the left, reverse the leads.

 

P14)  Does it matter where in the circuit you place the ammeter?  Explain.

 

 

Connect the ammeter so that it is in series between the two light bulbs.

 

Q15)  Record the current.

 

Q16)  How does the current recorded in Q15)  compare the current recorded in Q14)? 

 

 

Connect the ammeter in series so that it is between the light bulb and the – terminal of the battery.

 

Q17)  Record the current.

 

Q18)  How does the current recorded in Q)  compare the currents recorded in Q15) and Q14)? 

 

Q19)  What does your answer to Q18)  suggest about the behavior of current in a series circuit?

 

Part 2 Ohm’s Law

Procedure

1.  Set-Up

      Place the 100 Ω resistor between empty coils on the Pasco circuit board as shown.  Construct the circuit shown below using the 100 Ω resistor.  In the circuit diagram the symbol for the potential difference with an arrow drawn through it indicates a variable potential difference.

 


.Make sure that the power supply is turned off.  Configure a second DMM to be used as an ammeter by plugging one lead into COM and the other into the socket marked 2 A.  Connect the ammeter in series with the resistor and the other DMM configured as a voltmeter across the resistor.  This way we will measure the potential across the resistor and the current through it.  Before turning on the power supply, check your circuit with the instructor.  Incorrect circuits will result in blown fuses and possible damage to the DMM.

 

2.  Using the lowest 5 settings on the power supply, collect and record Potential Difference versus current data for the 100 Ω resistor. 

 

D20)  Record your data in the table below.

 

3.  Reverse the leads to the power supply only, so that you apply a negative potential difference across the circuit.  Measure and record Potential Difference (in V) versus Current (in A) in your data table for the lowest 5 settings on the power supply.

 

D21)  Add this data to your table.

Current (A)

Potential Difference (V)

 

 

 

 

 

 

 

 

 

 

0

0

 

 

 

 

 

 

 

 

 

 

 

Data Analysis

      Using your data for both positive and negative potential differences, construct a graph of Voltage versus Current for with LoggerPro.  (Remember it is y vs. x)

 

Q22)  Do your data seem to lie along a line?

 

 

Q23)  Does the line seem to go through the origin? (Or at least very close?)

 

 

Q24)  What type of relationship is represented by a line that goes through the origin?

 

 

D25)  Click on the linear regression button .  LoggerPro will use a statistical procedure and find the line which most closely matches the data – referred to as a best fit line.

 

Print and attach your graph.

 

Q26)  How does the slope of the line compare to the resistance of the resistor.

 

 

 

Q27)  Fill in the blank with the appropriate type of mathematical relationship.  For a resistor, the potential difference is ______ to the current.

 

This idea is Ohm’s law.