Section 6: Current, Voltage, and
Resistance in Parallel and Series Circuits
Lesson 2



Go over the HW.
o Discuss 1 & 2 carefully, first in small groups and then with the entire
class.
o (10 minutes)
Physics Talk – Current in parallel and series circuits
o Tell the class that you are going to explain how series and parallel circuits
are different and how you will be able to measure current, voltage, and
resistance.
o Remind the class that in series circuits (one path) when one bulb goes out,
or the wire is cut or the switch is thrown, all the other bulbs will go out.
And for a parallel circuit, when one bulb goes out, or the wire is cut, or the
switch is thrown, only that bulb goes out, the others stay on.
o When a switch is thrown to the on position, it means that the circuit is
closed and that the current can flow. When a switch is thrown to the off
position it means that the circuit is open, and the current cannot flow.
Draw a schematic for a simple switch showing the closed and open
position.
o Draw a simple series circuit on the board with three resistors and a 6 V
battery. Point out that there is a single path and that the current that flows
at any point in the circuit must be equal. Revisit the Electron Shuffle
analogy. IT = I1 = I2 = I3 = ….
o Draw a simple parallel circuit on the board with three identical resistors
and a 6 V battery as shown on page 647. Point out that at any junction the
current entering the junction is equal to the current leaving the junction.
This is Kirchoff’s First Law and is an example of conservation of charge.
Make note of the current at junction A, 9A enters and 6A + 3A leaves. It
would be good to put arrows on the current showing the direction of flow.
Compare that to junction D where 3A and 6A enter the junction and 9A
leaves. The formula for parallel circuits shows that the total current
leaving the battery is equal to the sum of the currents in each loop.
IT = I1 + I2 + I3 + ….
o (10 minutes)
Physics Talk – Voltage in parallel and series circuits
o Look at the series circuit that you drew for the class earlier. Mention that
the battery is providing a certain amount of energy per charge to each
resistor in the circuit. As a result the potential drop (voltage) can be
calculated for each resistor based on the current flowing in the circuit
times the resistance. (This is Ohm’s Law, V = IR) You will find that the

total voltage is equal to the sum of the voltages for each resistor.
VT = V1 + V2 + V3 + ….
o This is Kirchoff’s Second Law, and since it is based on voltage, joules per
coulomb, is an example of conservation of energy.
o Look at the parallel circuit that you drew for the class earlier. Using the
Electron Shuffle model the energy provided must be equal for each
resistor. This means that the potential energy drop (voltage) must be the
same for each resistor.
VT = V1 = V2 = V3 = ….
o (10 minutes)
Physics Talk – Resistance in parallel and series circuits
o It is not necessary to derive the formulas for total resistance but your
students need to be able to use them.
o Total Resistance in a series circuit is RT = R1 + R2 + R3 + ….
o A good model for this is the supermarket analogy. A shopper in a
supermarket must take his basket of groceries to a checkout counter. But
in this “weird” supermarket, you must go through checker #1 and then
checker #2 and then checker #3 – in series. These checkers represent
resistance to getting out of the supermarket and the more of these checkers
that you have to go through represents greater and greater resistance. For
instance if the first checker required 10 minutes and the second required
20 minutes and the third 30 minutes, your resistance to getting out of the
market would be 60 minutes – just add the resistance from each checker.
o Total Resistance in a parallel circuit is 1/RT = 1/R1 + 1/R2 + 1/R3 + ….
o Using the supermarket analogy again. In a normal supermarket shoppers
have many checkout counters in parallel to choose from and they only
have to go through one, only one resistance, before leaving the store. The
total resistance can be calculated by using the formula. But as resistors are
added to a parallel circuit, the total resistance decreases. In a supermarket,
the greater the number of checkout counters the easier, the lower the
resistance, to getting out of the store.
o One of the classic problems that students are asked is how much is the
total resistance if given several resistors in parallel. An easy answer is that
the total resistance must be less than the least resistance. This must be
true because if you add another resistor, the current now has another path
which means less resistance.
o Q – Three resistors, 1 , 10  and 100 , are connected in parallel.
What is the total resistance? Answer – It must be less than 1 .
o (12 minutes)