*Question T5D02:
What formula is used to calculate voltage in a circuit?*  

*Correct Answer is  A: E = I x R*  

### AI Response: (may contain errors)  

To calculate voltage in a circuit, we use Ohm's Law, which is represented by the formula:

$$E = I \times R$$

where:
- $E$ = voltage (in volts)
- $I$ = current (in amperes)
- $R$ = resistance (in ohms)

### Explanation of the Correct Answer:
- **Correct Answer: $E = I \times R$**
  - This formula states that the voltage across a resistor is equal to the current flowing through it multiplied by the resistance. It is fundamental in electrical engineering and is used to analyze circuits.

### Explanation of Incorrect Choices:
- **Choice B: $E = I / R$**
  - This is incorrect because it suggests that voltage is equal to current divided by resistance, which does not align with Ohm's Law.

- **Choice C: $E = I + R$**
  - This is incorrect as it implies that voltage is the sum of current and resistance, which is not a valid relationship in electrical circuits.

- **Choice D: $E = I - R$**
  - This is also incorrect because it suggests that voltage is the difference between current and resistance, which does not reflect the relationship defined by Ohm's Law.

### Useful Formulas and Rules of Thumb:
- Ohm's Law is a foundational principle in electronics. Remember:
  - If you know any two of the three variables ($E$, $I$, $R$), you can calculate the third.
- For example:
  - To find current: $$I = \frac{E}{R}$$
  - To find resistance: $$R = \frac{E}{I}$$

Understanding these relationships will help you solve various circuit problems effectively.