A7LAM CSCA ACADEMYCSCA Academy / Physics / Electromagnetism
CSCA Physics · Lesson 8 of 16DC circuits
Circuits en courant continu · دارات التيار المستمر
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Circuits are about three quantities: voltage pushes, current flows, resistance opposes. Ohm's law ties them together.
Key ideas
- Ohm's law: U = R·I (volts, ohms, amperes).
- Series resistors add: R = R₁ + R₂. Same current through each.
- Parallel resistors: 1/R = 1/R₁ + 1/R₂. Same voltage across each.
- Electrical power P = U·I = R·I² = U²/R.
Formulas to know
- U = RI
- P = UI
- Energy E = P·t
Worked example
Two 6 Ω resistors are connected in parallel to a 12 V battery. Find the total current.
- 1/R = 1/6 + 1/6 = 1/3, so R = 3 Ω.
- I = U / R = 12 / 3.
Answer: I = 4 A
Practice questions
1. A 4 Ω and an 8 Ω resistor in series. Total resistance?
2.67 Ω · 4 Ω · 12 Ω ✓ · 32 Ω
Series: 4 + 8 = 12 Ω.
2. A lamp draws 0.5 A from 220 V. Its power?
110 W ✓ · 440 W · 220 W · 0.5 W
P = UI = 220 × 0.5 = 110 W.
3. Current through a 10 Ω resistor with 5 V across it?
50 A · 2 A · 0.5 A ✓ · 15 A
I = U/R = 5/10 = 0.5 A.