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Grade 9 Science – Physics – Electricity
Series and Parallel Circuits
Series Circuit
In the diagram, two resistors are connected in a series circuit with a battery.
The total resistance (RT) for the whole circuit is very simple. It's
just R1 plus R2. Using these two facts, we can work out the
voltage across each resistor.
Suppose R1 = 1Ω and R2= 4Ω. If the battery supplies 2.5 V,
what is the voltage across each resistor?
Step 1 – Work out RT
RT
= R1 + R2
=1+4
=5Ω
Step 2 – Use Ohm’s Law I = V/R
I
=V/RT
= 2.5 / 5
= 0.5 A
Step 3 – Knowing the current flowing through each resistor, calculate the voltage across each
resistor
Resistance difference across R1
V1
= I X R1
= 0.5 X 1
= 0.5 V
Resistance difference across R2
V2
= I X R2
= 0.5 X 4
= 2.0 V
Practice Questions
1. Let R1 = 2Ω, R2= 3Ω, and V=5V what is the voltage across R1 andR2?
2. Let R1 = 3Ω, R2= 3Ω, and V=6V what is the voltage across R1 andR2?
3. Let Let R1 = 2Ω, V=5V and I= 1A what is the value of resistor R2?
Parallel Circuits
Current can either go through resistor R 1 or resistor R2
but not both. To find out the resistance of the whole
circuit, we can't just add together the resistors as we
did in the series circuit, we have to apply Ohm's law to
each branch of the circuit.
Replacing the resistors with light bulbs, answer the
following questions.
1. You disconnect one of the bulbs, does the
other bulb stay lit? Provide one reason with your answer.
2. You reconnect the disconnected bulb, does the other dim or brighten? Provide one
reason with your answer.
3. Would it be reasonable to say "each branch of the circuit behaves as if the other
branch weren't there"?
Resistance in a Parallel Circuit
Using a simple step-by-step method to understand the process.
Step1 - Consider each branch on it's own, as if the other branch didn't exist, AND use Ohm's law
to work out the current flowing through each branch.
Step 2 - Add the currents together to find out the total amount of current flowing through the
whole circuit.
Step 3 - Apply Ohm's law again to work out the total resistance of the circuit.
Summary - In Parallel Circuits, each branch acts as if the other branches were not there. The
Voltage across each branch is the same. Ohm's law can be applied to each branch separately.
The total current is just the sum of the currents through each branch.
Suppose V=2V R1 = 1Ω and R2= 1Ω.
 Applying Ohm's law to the branch containing R1 gives
I1
=V/R1
=2/1
2A
 Applying Ohm's law to the branch containing R2 gives
I2
=V/R2
=2/1
=2 A
 Total current = I1+I2 = 4A
 Applying Ohm's law again, to the whole circuit.
RT
=V/IT
= 2/4
= 0.5Ω
Practice Questions
1. Calculate the resistance of the above pair of resistors if they had been in series rather than
parallel.
2. A parallel circuit has two resistors, one is 2 Ω the other is 3Ω what is the total resistance of
the circuit?
3. Three resistors are in parallel (three seperate branches) their resitances are 2Ω,2Ω and 1 Ω
what is the resistance of the circuit?