Ohm's Law and Parallel Circuits

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Name ____________________________ Hour _____

Lab Objectives:

    1. Measure voltages and current in a Parallel circuit.
    2. Find the resistance of a resistor in a Parallel circuit without measuring its resistance directly.
    3. Find the total resistance of a Parallel circuit.

Parallel Circuits facts:

    1. The current is NOT the same in all parts of a DC circuit when resistors and loads are connected in Parallel with the voltage source. The total current is the sum of the current in each branch of the circuit. Itot = I1 + I2 + I3 + …..
    2. In DC circuits with resistors or loads in parallel, the voltage drop across each resistor is the same.
    3. Total resistance (Rtot) in a parallel DC circuit is found using this equation: 1/Rtot = 1/R1 + 1/R2 + 1/R3 + …..
    4. The total resistance in a parallel circuit is always less than the smallest resistance in any one of the parallel branches.
    5. For any resistor, given the current through it and the voltage across it, you can use Ohm's law to find the resistance.

Ohm's Law: Since there is only one current path in a series circuit, there's only one value of current flow. Current flow value is determined by the voltage source and the total resistance in the circuit. According to Ohm's law, the current is

                                                        I = DV / Re

Where: I = current in amperes
DV = source voltage in volts
Re = circuit resistance in ohms

Procedure:

1. Use your internet browser and open the web site listed above.

Click on the "pop-ups" button in the lower left-hand corner of the screen. Use the pop-up feature to select three resistors of 200, 100, and 10 ohm resistance. Record their individual resistance value in column #2 of Data table #1. Calculate the circuits total resistance and record it below:

Total Resistance = ________ ohms

2. Connect the three selected resistors in parallel with a switch, battery and wires. Turn Pop-ups off.

3. Place the ammeter in the circuit in several different locations and measure the total current in the circuit. Record the value below.

Measured Circuit Current through the 100-ohm resistor =I1= ______ amps

Measured Circuit Current through the 200-ohm resistor =I2= ______ amps

Measured Circuit Current through the 200-ohm resistor =I3= ______ amps

Measured Circuit Current from the battery =Itot = ______ amps

4. Use the voltmeter to measure the voltage source (battery). Record the total voltage below.

Source Voltage = _____________ volts

5. Calculate the circuit current using Ohm's law and the data recorded in steps #1 and #4 above:

Calculated Circuit Current = Circuit Voltage / Circuit Resistance

Itot =  Vs / Rtot   = _______ amperes

6. Use the voltmeter to measure the voltage across each resistor. Record your values in column #4 in the table below.

Resistor #

Current through each resistor

Value of Resistance from Pop-ups

Measured value of Voltage across Resistor

Calculated Value of Resistance

R1

A

W

= V1

W

R2

A

W

= V2

W

R3

A

W

= V3

W

Totals

Itot                 = A

Rtot              = W

= Vtot

Rtot=      W

 

Using Ohm's law calculate the resistance of each resistor and record it in Column #5 of Data Table #1. The formula to use is:

                                                    R = DV / I

In the fifth row of the data table determine the values and record the total resistance, total current, voltage total and the calculated total resistance.

Wrap-up Questions:

1. Does the value of current measured in step #3, agree with that found in step #5?

  1. Does the measured source voltage equal the voltage drops across each resistor recorded in the data table?
  2. Does the total resistance of the circuit equal the sum of the resistors placed in the circuit? (1/Rtot = 1/R1 + 1/R2 + 1/R3)

    3.  

  3. How are the ways the voltmeter and ammeter placed in the circuit different? (Which is placed in series and which is in parallel to the component to be measured? Which will have all the current flowing into it?
    4.

     

Procedure Part 2

    Repeat the above experiment using three lamps to replace the resistors. The lamps are loads with a resistance value

    1. Use your internet browser and open the web site listed above.

    Click on the "pop-ups" button in the lower left-hand corner of the screen. Use the pop-up feature to determine the resistance value of the three lamps. Record their individual resistance value in column #2 of Data table #2.

                            Total Resistance = ________ ohms

    2. Connect the three selected lamps in parallel with a switch, battery and wires. Turn Pop-ups off

    3. Place the ammeter in the circuit in several different locations and measure the total current in the circuit. Record the value below.

    Measured Circuit Current through the lamp 1 =I1= ______ amps

    Measured Circuit Current through the lamp 2 =I2= ______ amps

    Measured Circuit Current through the lamp 3 =I3= ______ amps

    Measured Circuit Current from the battery =Itot = ______ amps

    4. Use the voltmeter to measure the voltage source (battery). Record the total voltage below.

                                    Source Voltage = _____________ volts

    5. Calculate the circuit current using Ohm's law and the data recorded in steps #1 and #4 above:

                        Calculated Circuit Current = Circuit Voltage

    Circuit Resistance

                Itot = Vs / Rtot = _______ amperes

    6. Use the voltmeter to measure the voltage across each resistor. Record your values in column #4 in the table below.

    Data Table #1



    Resistor #

    Current through each resistor

    Value of Resistance from Pop-ups

    Measured value of Voltage across Resistor

    Calculated Value of Resistance

    Lamp 1

    A

    W

    = V1

    W

    Lamp 2

    A

    W

    = V2

    W

    Lamp 3

    A

    W

    = V3

    W

    Totals

    Itot = A

    Rtot = W

    = Vtot

    Rtot= W

     

Using Ohm's law calculate the resistance of each resistor and record it in Column #5 of Data Table #2. The formula to use is:

                                                                R = DV / I

In the fifth row of the data table determine the values and record the total resistance, total current, voltage total and the calculated total resistance.

Wrap-up Questions:

1. Does the value of current measured in step #3, agree with that found in step #5?

2. Does the measured source voltage equal the voltage drops across each resistor recorded in the data table?

3. Does the total resistance of the circuit equal the sum of the resistors placed in the circuit? (1/Rtot = 1/R1 + 1/R2 + 1/R3)

 

4. How are the ways the voltmeter and ammeter placed in the circuit different? (Which is placed in series and which is in parallel to the component to be measured? Which will have all the current flowing into it?)

 

5. What is the affect of adding more loads in parallel in terms of the circuit's current, resistance, and voltage across the loads?

 

6. Compare and contrast the use of resistors in series and parallel DC circuits.