*Question T5A08:
Which of the following is a good electrical insulator?*  

*Correct Answer is  B: Glass*  

### AI Response: (may contain errors)  

To determine which material is a good electrical insulator, we need to understand the properties of each option:

1. **Correct Answer: Glass**
   - Glass is a good electrical insulator because it has a high resistivity, meaning it does not allow electric current to flow easily. This property makes it suitable for applications where electrical isolation is required.

2. **Why the other choices are incorrect:**
   - **A: Copper**
     - Copper is a highly conductive metal, commonly used in electrical wiring. It has low resistivity, which allows electric current to flow freely, making it a poor insulator.
   - **C: Aluminum**
     - Similar to copper, aluminum is also a good conductor of electricity. It is often used in power lines and electrical components due to its low resistivity.
   - **D: Mercury**
     - Mercury is a liquid metal that conducts electricity. While it has some unique properties, it is not an insulator and has low resistivity, allowing current to flow.

3. **Useful Concepts:**
   - **Resistivity ($\rho$)**: This is a measure of how strongly a material opposes the flow of electric current. Materials with high resistivity are good insulators, while those with low resistivity are good conductors.
   - **General Rule of Thumb**: 
     - Insulators typically include materials like rubber, glass, and ceramics, while conductors include metals like copper, aluminum, and mercury.

In summary, glass is the correct answer because it effectively prevents the flow of electric current, while the other options are all conductive materials.