Ohm's Law

INTRODUCTION: Variations in current intensity and potential difference occur when conductors of different resistance are connected in parallel, in series, or in series-parallel. What you learned last week in the DC Circuits lab will be applied here.

THEORY: Ohm's Law is not a fundamental law of Physics like F = ma; it is merely a convenient relationship between resistance, current, and voltage that occurs when certain constraints are applied. For instance, one condition is that the current source is infinite, meaning no matter how small a resistance is offered. In practice all power supplies have limited current supply capabilities. Also, we assume the voltage of the source doesn't drop when huge amounts of current are required; again a fallacy. Finally, we assume the resistance doesn't cause any losses to heat, a very great fiction. Still, for reasonable values:

V = IR

That is, the slope of a V(I) graph is R. This relationship can be checked by measuring the current through and voltage across each resistor.

When measuring current be sure to place the ammeter in series with the components in question. Current flows through a circuit. When measuring voltage be sure to place the voltmeter in parallel with the resistance. Voltage drops across a resistor.

No doubt you noticed in the DC Circuits lab that voltages recorded across the series components very nearly summed to the power supply voltage. And it's almost certain that you discovered that voltage measured across each component in the parallel configuration was quite close to the power supply setting. Therefore, your astute observations should help you with this experiment as well.

TASK:

To verify Ohm's Law

EQUIPMENT:

• Voltmeter
• Ammeter
• Resistors
• Variable D.C. power supply,
• wires
  

PROCEDURE:

Set the circuits below.  Please be careful when you setup the input voltage.

Circuit 1 (Series)

1. Construct this circuit below.

2. Let R₁ = 180 Ohms, R₂ = 220 Ohms, R₃ = 330 Ohms

3. Set the power supply to 12 volts.

4. Measure the current (in milliamps) through and voltage drop (in volts) across each resistor

Circuit 2 (Parallel)

1. Construct the circuit below.

2. Let R₁ = 180 Ohms, R₂ = 220 Ohms, R₃ = 330 Ohms.

3. Set the power supply to 3 volts.

4. Measure the current (in milliamps) through and voltage drop (in volts) across each resistor.

5. Also measure the current (in milliamps) between the + side of the battery or power supply and R₁.

Circuit 3

1. Using your components construct a circuit with R₁ in series with a parallel branch containing R₂ and R₃.

2. Let R₁ = 180 Ohms, R₂ = 220 Ohms, R₃ = 330 Ohms.

3. Set the power supply to 6 volts.

4. Measure all the necessary voltages and currents.

CALCULATIONS:

1. For circuit # 1 (series):
   
   1. Calculate the total resistance.
   2. Calculate the theoretical current (in Amps) value using the total resistance value and the value of the input voltage.
   3. Calculate the theoretical value of the voltage drop across each resistor by using the theoretical current value.
   4. Calculate the percent error for the current. (use the average of your measured values (in Amps) as your experimental value)
   5. Calculate the percent error for the voltage drops across each resistor.
      
      
2. For Circuit # 2 (parallel):
   
   1. Calculate the total resistance.
   2. Calculate the theoretical value of the total current (in Amps) by using the total resistance and the value of the input voltage.
   3. Calculate the theoretical value of the current (in Amps)  through each resistor. You may assume that the voltage across each resistor is the same as the input voltage.
   4. Calculate the percent error for the total current.
   5. Calculate the percent error for the values of the currents through each resistor.
      
      
3. For circuit # 3 (combination):
   
   1. Calculate the total resistance.
   2. Calculate the total current  (in Amps) by using the value of the total resistance and the input voltage.
   3. Calculate the theoretical value of the voltage drop across R₁
   4. Calculate the theoretical value of the voltages drops across the parallel combination.
   5. Calculate the theoretical values of the currents (in Amps) through R₂ and R₃.
   6. Calculate the percent error for the values of the currents through each resistor.
   7. Calculate the percent error for the values of the voltage drops across R₁ and the parallel combination.