Series Circuit Calculations - BackgroundIn this activity you'll have a chance to see how the voltage, current, and
resistance are calculated in multiload series circuits (circuits with more than
one electricity-using device). A series circuit is a circuit with only one possible path through which electrons
can flow. Let's review Ohm's law because you'll use it often. Ohm's law is the formula
for how voltage, current, and resistance in conductors are related. You need
to remember only one of the three equations representing this law. By now you
should remember VIR for V = I R. The two other forms of Ohm's law can
be found by dividing voltage by the remaining measure. See Activity 1 for more
information about Ohm's law. Abbreviations and Units
Voltage is abbreviated V (unit volts, V) and resistance is abbreviated
R (unit ohms, Ω). Current is abbreviated I (unit amperes,
or amps, A). Thus, V = I R. Series Circuit Formulas
Current
Current is the rate at which electrons flow through a circuit. The more resistors
you add, the slower the rate of electron flow. In a series circuit the current
through any of the loads or wires is the same and equals the total current,
abbreviated ITotal or IT . We can write
this as IT = I1 = I2 = I3 ,
etc. Voltage
You can think of voltage of a dry cell or battery as the force that pushes electrons
through a circuit. In a series circuit the total voltage equals the sum of the
voltages across the batteries wired in series in the circuit. The total voltage
also equals the sum of the voltages across each load (lights, etc.). The total
voltage is abbreviated VTotal or VT .
We can write this as VT = V1 + V2 + V3
, etc. Resistance
Electricity-using devices provide resistance to current. In a series circuit
the total resistance (in ohms) equals the sum of the resistances of each device
using electricity. The total resistance is abbreviated RTotal
or RT . We can write this as RT = R1
+ R2 + R3 , etc. Remember, V = I x R. If you hold the voltage constant (add
no additional dry cells or batteries) and increase the resistance, current
must decrease. In other words, as you add electricity-using devices in
series, the rate at which electrons flow slows down. As R
goes up, I must go down so that I x R still equals
V. V = I ↓ x R ↑ |
Summary Table
Ohm's law can be used to solve for any of the individual values or for the total
value of each measure within the series circuit. Ohm's Law states V = I R, and | V1 = I1 x R1 | V2 = I2 x R2 | V3 = I3 x R3 | VT = IT x RT |
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