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Electric Circuits
Voltage, Current, Resistance, and Power
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An electric circuit is a closed loop through which electric charge can flow. The three fundamental quantities - voltage, current, and resistance - are linked by Ohm's Law: V = IR. Voltage (measured in volts) is the electrical pressure driving charge through the circuit. Current (amperes) is the rate at which charge flows. Resistance (ohms) opposes that flow.
Components connected in series share the same current but split the voltage. Components in parallel share the same voltage but split the current. Most real circuits combine both arrangements. Being able to simplify complex circuits into equivalent resistances - and then work backward to find individual currents and voltages - is the core skill of circuit analysis.
Key Facts
- Ohm's Law: V = IR (voltage = current × resistance)
- Power: P = IV = I²R = V²/R
- Series resistors: R_total = R₁ + R₂ + ... (same current through all)
- Parallel resistors: 1/R_total = 1/R₁ + 1/R₂ + ... (same voltage across all)
- In series: voltage splits, current is the same throughout.
- In parallel: current splits, voltage is the same across each branch.
Vocabulary
- Voltage (V)
- The electrical potential difference that drives current through a circuit, measured in volts.
- Current (I)
- The flow of electric charge past a point per unit time, measured in amperes (A).
- Resistance (R)
- Opposition to current flow, measured in ohms (Ω).
- Power (P)
- The rate of energy transfer in a circuit, measured in watts (W = J/s).
- EMF
- Electromotive force: the energy supplied per unit charge by a battery or source.
Common Mistakes to Avoid
- Thinking current is 'used up' as it flows through resistors. Current is the same throughout a series circuit - only energy (voltage) is consumed.
- Forgetting that adding resistors in parallel decreases total resistance, allowing more total current to flow.
- Mixing up which quantity is the same vs which splits: in series current is equal; in parallel voltage is equal.
- Using without checking that and are for the same component when finding power in a specific resistor.
Practice Questions
- 1 Two resistors (4 Ω and 6 Ω) are connected in series to a 20 V battery. Find the current and voltage across each resistor.
- 2 The same two resistors are now in parallel with 20 V. Find the total current drawn from the battery.
- 3 A light bulb with resistance 240 Ω is connected to 120 V. What is the power dissipated?