Fundamentals of Electricity and DC Circuits
Scope
Fundamentals of Electricity and DC Circuits tests charge, current, potential difference, resistance, Ohm’s law, series and parallel networks, Kirchhoff’s laws, power, energy and resistivity.
Core rules
- Current is rate of charge flow, I = Q/t; one ampere is one coulomb per second.
- Ohm’s law: V = IR at constant temperature.
- Series: same current, R_T = R1 + R2 + …, voltages add.
- Parallel: same voltage, 1/R_T = 1/R1 + 1/R2 + …; for two resistors R_T = R1R2/(R1 + R2).
- Power: P = VI = I²R = V²/R. Energy in kWh = watts × hours ÷ 1000.
- KCL: currents at a node sum to zero. KVL: voltages around a closed loop sum to zero.
Worked example
A 12 V battery feeds 4 Ω and 6 Ω in series. R_T = 10 Ω, so I = 12/10 = 1.2 A. The 4 Ω drop is 4 × 1.2 = 4.8 V; the 6 Ω drop is 7.2 V. Total power = 12 × 1.2 = 14.4 W, or I²R_T = 1.44 × 10 = 14.4 W. If the same resistors were parallel, R_T = 24/10 = 2.4 Ω and the battery current would be 5 A. That is a different circuit, not an interchangeable one.
Traps
- Adding parallel resistances directly instead of using reciprocals.
- Mixing kΩ with mA without converting to ohms and amperes.
- Assuming a lamp’s resistance stays fixed when its voltage changes.
- Confusing energy in kWh with power in kW.