*Question T5D03:
What formula is used to calculate resistance in a circuit?*  

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

### AI Response: (may contain errors)  

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

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

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

### Explanation of the Correct Answer:
- **Correct Answer: $R = \frac{E}{I}$**  
  This formula correctly describes how resistance is calculated by dividing the voltage ($E$) across a component by the current ($I$) flowing through it. This relationship is fundamental in electrical engineering and is derived from Ohm's Law.

### Explanation of Incorrect Choices:
- **Choice A: $R = E \times I$**  
  This is incorrect because it suggests that resistance is the product of voltage and current, which does not align with Ohm's Law.

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

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

### Useful Formulas and Rules of Thumb:
- Ohm's Law:  
  $$V = I \times R$$  
  This can be rearranged to find any of the three variables (voltage, current, resistance).

- Power Formula:  
  $$P = V \times I$$  
  This relates power (in watts) to voltage and current, which can also be useful in conjunction with resistance calculations.

Understanding these relationships is crucial for analyzing and designing electrical circuits, especially for the Extra Class exam.