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Electricity Circuits
Series vs Parallel
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Electric circuits provide a controlled path for electric charge to move and transfer energy to devices such as bulbs, motors, and speakers. Two of the most important circuit layouts are series circuits and parallel circuits. They behave differently because components are connected in different ways. Understanding the difference helps explain why a string of old holiday lights can go out all at once, while lights in a house usually work independently.
In a series circuit, charges have only one path, so the same current passes through every component. In a parallel circuit, charges can split among multiple branches, so each branch can have its own current while receiving the same voltage from the source. These rules let engineers predict brightness, battery drain, and what happens when one component fails. The key ideas are Ohm's law, equivalent resistance, current splitting, and voltage sharing.
Key Facts
- Ohm's law: V = IR
- Series current is the same through every component: I_total = I_1 = I_2 = I_3
- Series voltage divides across components: V_total = V_1 + V_2 + V_3
- Series equivalent resistance: R_total = R_1 + R_2 + R_3
- Parallel voltage is the same across every branch: V_total = V_1 = V_2 = V_3
- Parallel equivalent resistance: 1/R_total = 1/R_1 + 1/R_2 + 1/R_3
Vocabulary
- Circuit
- A circuit is a closed conducting path that allows electric charge to flow.
- Current
- Current is the rate at which electric charge flows through a point in a circuit.
- Voltage
- Voltage is the electric potential difference that pushes charges through a circuit.
- Resistance
- Resistance is a measure of how strongly a material or component opposes electric current.
- Equivalent resistance
- Equivalent resistance is the single resistance value that could replace a group of resistors without changing the total current from the source.
Common Mistakes to Avoid
- Adding parallel resistors like series resistors is wrong because parallel branches create extra paths and always reduce the total resistance below the smallest branch resistance.
- Assuming current is used up by bulbs is wrong because current is conserved in a closed loop, while electrical energy is transferred to the bulbs.
- Saying every resistor in a series circuit gets the full battery voltage is wrong because the source voltage is divided among the series components.
- Thinking one broken bulb stops every circuit is wrong because an open component stops a series loop but other branches in a parallel circuit can still operate.
Practice Questions
- 1 A 12 V battery is connected to two resistors in series, 4 ohms and 8 ohms. Find the total resistance, the circuit current, and the voltage across each resistor.
- 2 A 9 V battery is connected to two resistors in parallel, 6 ohms and 3 ohms. Find the equivalent resistance, the total current from the battery, and the current in each branch.
- 3 Two identical bulbs are connected to the same battery, first in series and then in parallel. Explain which arrangement makes the bulbs brighter and what happens if one bulb burns out in each arrangement.