Physics
Current, Voltage, and Ohm’s Law
Updated Oct 28, 2026
Relate charge flow, potential difference, and resistance.
What you’ll learn
Relate charge flow, potential difference, and resistance.
ភ្ជាប់ចរន្ត បន្ទុក ផលសងប៉ូតង់ស្យែល និងរេស៊ីស្តង់។
Meaning and use
Current is charge transferred per unit time; voltage is energy transferred per unit charge. For an ohmic component under unchanged physical conditions, voltage is proportional to current. V = IR can define resistance at a point, but a constant R model is not valid for every device over all currents.
ចរន្តជាបន្ទុកឆ្លងកាត់ក្នុងមួយពេល។ តង់ស្យុងជាថាមពលក្នុងមួយបន្ទុក។ ច្បាប់អូមមាន R ថេរពេលលក្ខខណ្ឌរាងកាយដូចសីតុណ្ហភាពមិនប្រែ។
Formulas
Charge flow and Ohm’s law
\[I=\frac qt,\qquad V=IR\]
Resistance
\[R=\frac VI\]
Symbols and units
| Symbol | Meaning | Unit or restriction |
|---|---|---|
| I | Electric current | ampere (A) |
| q | Charge transferred | coulomb (C) |
| t | Time interval | s |
| V | Potential difference | volt (V) |
| R | Resistance | ohm (Ω) |
Conditions and limits
- Use a constant-resistance model only when the component and conditions justify it.
- R = V/I requires nonzero current; I = q/t requires positive time.
- គណនាតាមគំរូសៀគ្វីវ៉ុលទាបដែលបានផ្ដល់ មិនមែនការណែនាំតខ្សែភ្លើងផ្ទះ។
Worked examples
Current through a resistor
A 6 V source acts across a 3 Ω constant resistor.
- Rearrange \(I=V/R\).
- Calculate \(6/3=2\) A.
2 A
ត្រូវប្រើតង់ស្យុងនៅលើរេស៊ីស្តង់ដែលកំពុងគណនា។
Charge per time
12 C passes a point in 4 s.
- Use \(I=q/t\).
- Calculate \(12/4=3\) A.
3 A
1 A ស្មើបន្ទុក 1 C ក្នុង 1 s។
Common mistakes
| Instead of | Use | Why |
|---|---|---|
| Current is used up in a resistor. | Charge flow is continuous in a steady series path. | The resistor transfers energy; it does not consume charge. |
| Every component has constant resistance in every condition. | Check the ohmic-model conditions. | Heating and device type can change the relationship. |
Practice
Solve these before revealing the worked solutions. Include units where relevant.
សាកល្បងដោះស្រាយមុនបើកមើលចម្លើយ។ សូមបង្ហាញជំហាន និងឯកតាដែលពាក់ព័ន្ធ។
- Find current for V = 12 V and R = 4 Ω.
- Find voltage for I = 0.5 A and R = 10 Ω.
- Find current when 20 C flows in 5 s.
Answers and steps
Show solutions / បង្ហាញដំណោះស្រាយ
-
3 A
I = V/R = 12/4.
-
5 V
V = IR = 0.5 × 10.
-
4 A
I = q/t = 20/5.
Quick recap
- Distinguish charge flow from energy transfer.
- Use the voltage across the component.
- State the constant-resistance assumption.