The materials resistance of which decreases with increases in temperature (i.e. the temperature coefficient of resistance is negative) are called

Options

A. conductors
B. insulators
C. semiconductors
D. all of the above

Answer

To solve the question regarding the materials whose resistance decreases with an increase in temperature, we can follow these steps:

### Step 1: Understand the Concept of Temperature Coefficient of Resistance
The temperature coefficient of resistance (α) indicates how the resistance of a material changes with temperature. If α is negative, it means that the resistance decreases as the temperature increases.

### Step 2: Identify the Types of Materials
The materials can be broadly classified into three categories:
1. Conductors (e.g., metals)
2. Insulators (e.g., rubber, glass)
3. Semiconductors (e.g., silicon, germanium)

### Step 3: Analyze Conductors
For conductors, as the temperature increases, the lattice vibrations increase, which leads to more collisions between charge carriers (electrons) and the lattice. This results in an increase in resistance. Therefore, conductors have a positive temperature coefficient of resistance.

### Step 4: Analyze Insulators
In insulators, the number of free charge carriers is very low at room temperature. However, as the temperature increases, some electrons gain enough energy to break free from their bonds, increasing the number of charge carriers. This leads to a decrease in resistance. Thus, insulators can have a negative temperature coefficient of resistance.

### Step 5: Analyze Semiconductors
Semiconductors behave similarly to insulators at low temperatures. However, as the temperature increases, more electrons are excited into the conduction band, significantly increasing the number of charge carriers. This also results in a decrease in resistance, indicating that semiconductors have a negative temperature coefficient of resistance.

### Step 6: Conclusion
From the analysis:
- Conductors: Resistance increases with temperature (positive coefficient).
- Insulators: Resistance decreases with temperature (negative coefficient).
- Semiconductors: Resistance decreases with temperature (negative coefficient).

Thus, the materials whose resistance decreases with an increase in temperature are **insulators and semiconductors**.

### Final Answer
The materials whose resistance decreases with an increase in temperature are called **insulators and semiconductors**.

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