# Chapter 21

Name: ____________________

Section: ___________________

Resistors in Series and Parallel

Data Table 1: Resistors in Series
Battery voltage: _____V. Expected current (from V=IR): ______ mA

Resistor value (Ω) color code; % variance

R value (Ω) DMM

R value Ω)
calculated from R=V/I

DMMCurrent, mA

DMMvoltage, V

Calculated voltage, V
from V=IR

R1

R2

R3

Req

N.A.

Expected Values Using V = IR, and Req = R1 + R2 + R3

R (Ω)

Current (mA)

Voltage (V)

R1

R2

R3

Req

Data Table 2: Resistors in Parallel
Battery voltage: _____V

R value (Ω) DMM

DMM Voltage, V

DMM Current, mA

Calculated Current, A from V=IR

R1

I1 =

I1 =

R2

I2 =

I2 =

R3

I3 =

I3 =

Req

I =

I =

Expected Values Using V = IR, and !/Req = 1/R1 + 1/R2 + 1/R3

R (Ω)

Current (mA)

Voltage (V)

R1

R2

R3

Req

Data Table 3: (Resistors in Combination)
Battery voltage: _____V.

R Value (Ω) DMM

DMM Voltage, V

DMM Current, A

Calculated Current, A
from V=IR

R1

R2

R3

R2 + R3
Parallel

R1 + R2 +R3

Req

Expected Values: Resistors in Combination

R value (Ω)

Voltage, V

Current, A

R1

R2

R3

R2 + R3(parallel)

Req

*Note: Expected value for current is calculated and known to be the current through R1 and R23. This value was is used to determine voltages across R1 and R23. Then the current through R2 and R3 is calculated by the formula V = IR.

4. Calculations: Now using the equation V = IR, calculate the expected values of the currents and voltages of all resistors tested above. Create new tables for these results. Use only the voltage supplied by the 1.5V battery and the resistance values for the three resistors. When using the current in calculations (V=IR), you must convert mA to A by dividing the mA value by 1000. For example, in a circuit with a 1.5V battery and a 500 Ω resistor, what is the expected current? V = IR or I = V/R = 1.5/500 = 0.003A or 3 mA.

Series Resistors: Req = R1 + R2 + R3. First, find the equivalent resistance Req for all resistors involved. Then find the current through Req. The voltage for each resistor can be calculated using the resistor value, current and the equation V=IR.

Parallel Resistors: 1/Req = 1/R1 + 1/R2 + 1/R3 or Req = (R1 x R2 x R3) / (R1 + R2 + R3). Using the measured voltage, calculate the currents for each resistor. The total circuit current, I, is simply the sum of the currents through each resistor I1, I2, and I3.

Combination: First, find the equivalent resistance R23 of the parallel resistors, then the equivalent resistance Req for the entire circuit. Next, find the current through Req, and this should be the current through R1 and R23. Using that current, find voltages across R1 and R23.Then you can find the current through R2 and R3.

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