*Question T5D01:
What formula is used to calculate current in a circuit?*  

*Correct Answer is  B: I = E / R*  

### AI Response: (may contain errors)  

To calculate the current in a circuit, we use Ohm's Law, which states:

$$I = \frac{E}{R}$$

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

### Explanation of the Correct Answer:
- **Correct Answer: $I = \frac{E}{R}$**  
  This formula correctly describes how current ($I$) is calculated by dividing the voltage ($E$) across a resistor by the resistance ($R$) of that resistor. This relationship is fundamental in electrical engineering and is essential for understanding circuit behavior.

### Explanation of Incorrect Choices:
- **Choice A: $I = E \times R$**  
  This is incorrect because it suggests that current is the product of voltage and resistance, which contradicts Ohm's Law. Current actually decreases with increasing resistance for a given voltage.

- **Choice C: $I = E + R$**  
  This is incorrect as it implies that current is the sum of voltage and resistance, which has no basis in electrical theory. Current is not simply additive in this context.

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

### Useful Formulas and Rules of Thumb:
- **Ohm's Law**:  
  $$I = \frac{E}{R}$$  
  This is the primary formula for calculating current.

- **Power Formula**:  
  $$P = I \times E$$  
  where $P$ is power in watts (W). This can also be rearranged to find current:  
  $$I = \frac{P}{E}$$

Understanding these relationships will help you solve various circuit problems and prepare effectively for the Extra Class exam.