Series Circuits
Section: Physics | Syllabus: Cambridge Lower Secondary Checkpoint Science (0893)
What is a Series Circuit?
- A series circuit is a circuit where components are connected in a single loop or path – there is only one route for the current to flow
- Key features of series circuits:
- components are connected one after another in a single loop
- there is only one path for current to flow
- if one component breaks, the whole circuit stops working
- the same current flows through all components
- voltage is shared between components
- Real-life examples:
- Old Christmas lights: if one bulb breaks, all the lights go out
- Simple torch: battery, switch, and bulb in series
- Doorbells: battery, button (switch), and buzzer in series
A series circuit: components are connected one after another in a single loop
Current in Series Circuits
- In a series circuit, the current is the same at all points in the circuit
- Why current is the same everywhere – think of current like water flowing through a single pipe:
- the same amount of water enters and leaves each section
- current (charge) is not used up as it flows through components
- all the charge that leaves the battery must return to it
- since there's only one path, the same current flows through every component
- If you place ammeters at different points in a series circuit, they will all show the same reading
Worked Example: Current Measurement in Series Circuit
Museli reads an ammeter showing 0.5A at one point in a series circuit with a battery and two lamps, and wants to find the current through each component.
- Step 1: apply the series current rule. Current through the battery = 0.5A
- Step 2: apply it to each lamp. Current through lamp 1 = 0.5A; Current through lamp 2 = 0.5A
- Step 3: conclude. The current everywhere in the circuit is 0.5A – current is not used up, so the same current that leaves the positive terminal returns to the negative terminal
Voltage in Series Circuits
- In a series circuit, the total voltage supplied by the battery is shared between all the components. The voltages across all components add up to the battery voltage
- Vtotal = V1 + V2 + V3 + ...
- where Vtotal is the total voltage from the battery/power supply, and V1, V2, V3 are the voltages across each component
- Why voltage is shared – think of voltage like energy given to charges:
- the battery gives energy to the charges
- as charges flow through each component, they transfer some energy to it
- by the time charges return to the battery, they've transferred all their energy
- the energy given by the battery equals the total energy transferred to all components
- Voltage sharing follows the principle of energy conservation. The total energy supplied by the battery equals the total energy transferred by all components
Worked Example: Identical Components
Muumbe has a 12V battery with three identical lamps in series, and wants to find the voltage across each lamp.
- Step 1: write down the total voltage. Total voltage = 12V
- Step 2: divide equally among identical lamps. Each lamp gets 12V ÷ 3 = 4V
- Step 3: check. 4V + 4V + 4V = 12V ✓ – each lamp has 4V across it
Worked Example: Finding Unknown Voltage
Muchindu has a circuit with a 9V battery and two lamps in series, and a voltmeter across lamp 1 reads 3V.
- Step 1: write down the formula. Vtotal = V1 + V2
- Step 2: substitute the values. 9V = 3V + V2
- Step 3: calculate the answer. V2 = 9V − 3V = 6V
Effects of Adding Cells in Series
- When you add more cells in series (making a battery), the total voltage increases
- Total Voltage = sum of all cell voltages
| Number of Cells | Each Cell Voltage | Total Voltage |
|---|---|---|
| 1 cell | 1.5V | 1.5V |
| 2 cells in series | 1.5V each | 3.0V |
| 3 cells in series | 1.5V each | 4.5V |
| 4 cells in series | 1.5V each | 6.0V |
- What happens when you add more cells:
- Voltage increases – more energy per charge
- Current increases – more charge flows per second
- Bulbs get brighter – more energy transferred
- Motors spin faster – more power available
- All cells must be connected in the same direction (positive to negative). If one cell is backwards, it reduces the total voltage
Worked Example: Calculating Total Voltage
Museli connects four cells, each providing 1.5V, in series in the same direction, and wants to find the total voltage.
- Step 1: add the cell voltages. 1.5V + 1.5V + 1.5V + 1.5V
- Step 2: calculate the answer. Total voltage = 6.0V
Effects of Adding Lamps in Series
Adding more components to a series circuit has a significant effect on the brightness of lamps.
- When you add more lamps in series, the lamps get dimmer. Why:
- Step 1: total resistance increases – each lamp adds resistance to the circuit, and more resistance means more opposition to current flow
- Step 2: current decreases – higher resistance means less current flows, since the same voltage with more resistance gives less current
- Step 3: voltage is shared among more components – each lamp gets less voltage, which means less energy per lamp
- Step 4: lamps are dimmer – less current and less voltage means less power, and less power means dimmer light
| Circuit | Voltage per Lamp | Brightness |
|---|---|---|
| 6V battery + 1 lamp | 6V | Bright |
| 6V battery + 2 lamps | 3V each | Dimmer |
| 6V battery + 3 lamps | 2V each | Very dim |
- More lamps in series equals dimmer bulbs, because voltage is shared and current decreases
Advantages and Disadvantages of Series Circuits
- Advantages:
- Simple to construct – easy to build with one loop
- Uses less wire – only one path needed
- One switch controls everything – convenient for some applications
- Cells add voltage – can increase power easily
- Disadvantages:
- If one component fails, everything stops – no redundancy
- Cannot control components individually – all on or all off
- Components share voltage – each gets less than full voltage
- Adding components makes them dimmer/slower
Key Rules Summary
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