Parallel Circuits
Section: Physics | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is a Parallel Circuit?
- A parallel circuit is a circuit where components are connected in separate branches – there are multiple paths for electric current to flow
- Key features of parallel circuits:
- components are connected in separate branches
- there are multiple paths for current to flow
- has junctions where wires split and rejoin
- if one component fails, others keep working
- current splits at junctions
- voltage is the same across all branches
- Real-life examples:
- House lighting: each light can be turned on/off independently
- Christmas lights (modern): if one bulb breaks, the rest stay on
- Appliances at home: fridge, TV, lights all work independently
- Office lighting: multiple bulbs maintain the same brightness
A parallel circuit: components are connected in separate branches, with junctions where current splits and rejoins
Voltage in Parallel Circuits
- In a parallel circuit, the voltage is the same across all branches – each branch gets the full battery voltage
- Why voltage is the same in all branches: each branch is connected directly across the battery terminals, with one end connected to positive and the other to negative. This means each branch "sees" the full voltage of the battery, no matter how many branches there are
- Vbranch 1 = Vbranch 2 = Vbranch 3 = Vbattery
Worked Example: Voltage Across Parallel Lamps
Muumbe has a 6V battery with 3 lamps connected in parallel, and wants to find the voltage across each lamp.
- Step 1: apply the parallel voltage rule. Voltage is the same across all branches
- Step 2: write down each lamp's voltage. Voltage across lamp 1 = 6V; lamp 2 = 6V; lamp 3 = 6V
- Step 3: conclude. Each lamp gets the full 6V from the battery
- Compare to series: in series, voltage is shared (12V split between 3 lamps = 4V each). In parallel, voltage is the same (12V battery = 12V across each branch)
Current in Parallel Circuits
- In a parallel circuit, current splits at junctions. The total current from the battery equals the sum of currents in all branches
- Itotal = I1 + I2 + I3 + ...
- where Itotal is the total current from the battery, and I1, I2, I3 are the currents through each branch
- What happens at junctions (where wires split or join):
- current splits when it reaches a junction, dividing into the branches
- part of the current flows down one branch, the rest down other branches
- current flowing into a junction equals current flowing out (current is conserved)
- after passing through branches, currents combine back to the original total
- Electrons are physical matter – they cannot be created or destroyed. The total number of electrons going into a junction must equal the number coming out, which is why current is conserved
- Does current always split equally? No – current only splits equally if the branches have identical resistance. If branches have different resistance, more current flows through the branch with less resistance (the "easier path"); less current flows through the branch with higher resistance (the "harder path")
Worked Example: Equal Resistance – Current Splits Equally
Museli has a circuit with a battery supplying 6A total current and two identical lamps in parallel, and wants to find the current through each lamp.
- Step 1: note the lamps are identical, so current splits equally
- Step 2: divide the total current. Current through lamp 1 = 3A; lamp 2 = 3A
- Step 3: check. 3A + 3A = 6A ✓
Worked Example: Finding Missing Branch Current
Muchindu's parallel circuit has three branches with a total current of 9A from the battery. Branch 1 has 4A and branch 2 has 2A.
- Step 1: write down the current rule. Itotal = I1 + I2 + I3
- Step 2: substitute the values. 9A = 4A + 2A + I3, so 9A = 6A + I3
- Step 3: calculate the answer. I3 = 9A − 6A = 3A
Effects of Adding Lamps in Parallel
- When you add more lamps in parallel, the lamps maintain their brightness
- Why lamps stay bright: each lamp continues to receive the full battery voltage, since every branch connects directly across the battery and voltage doesn't decrease when you add branches
- each lamp gets full voltage (every branch connects directly across the battery)
- each lamp gets the same current as before (adding a branch doesn't affect existing branches)
- each lamp works independently (lamps don't "compete" for voltage or current)
- What does change: the total current from the battery increases, since more branches create more paths for current. The battery has to supply current to all branches, but each individual lamp still gets the same current and voltage
Compare: Series vs Parallel
| Action | Series Circuit | Parallel Circuit |
|---|---|---|
| Add more lamps | Lamps get dimmer | Lamps stay the same brightness |
| Voltage per lamp | Decreases (shared) | Stays the same (full battery voltage) |
| Total current | Decreases | Increases |
Advantages and Disadvantages of Parallel Circuits
- Advantages:
- components work independently – if one fails, others keep working
- each component gets full voltage – works at designed efficiency
- can control components separately – individual switches for each branch
- components maintain performance – brightness doesn't decrease when you add more
- reliable – one broken component doesn't affect the rest
- Disadvantages:
- uses more wire – need separate paths for each branch
- more complex to build – more connections and junctions
- higher total current – battery drains faster with more branches
- need thicker wires – to handle higher total current safely
- Despite the disadvantages, parallel circuits are used in homes because you can turn lights/appliances on and off independently, each device gets full voltage, and if one device breaks, others keep working
Series vs Parallel – Complete Comparison
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