Core of electromagnets are made of ferromagnetic materials which have

Options

A. low permeability and high retentivity
B. high permeability and low retentivity
C. low permeability and low retentivity
D. high permeability and high retentivity

Explanation

To solve the question regarding the properties of ferromagnetic materials used in the cores of electromagnets, we need to understand two key concepts: permeability and retentivity.

### Step-by-Step Solution:

1. **Understanding Permeability:**
- Permeability is a measure of how easily a material can become magnetized when exposed to an external magnetic field.
- For electromagnets, we want materials that allow magnetic fields to pass through them easily, which means they should have **high permeability**.

2. **Understanding Retentivity:**
- Retentivity (or remanence) refers to the ability of a material to retain its magnetism after the external magnetic field is removed.
- Since electromagnets are designed to be temporary magnets, we want materials that do not retain significant magnetism once the external field is removed. Therefore, they should have **low retentivity**.

3. **Analyzing the Options:**
- Now, let's analyze the given options:
- **Option A:** Low permeability and high retentivity - Incorrect, as we need high permeability.
- **Option B:** High permeability and low retentivity - Correct, as this matches our requirements for electromagnets.
- **Option C:** Low permeability and low retentivity - Incorrect, as we need high permeability.
- **Option D:** High permeability and high retentivity - Incorrect, as we need low retentivity.

4. **Conclusion:**
- Based on the analysis, the correct answer is **Option B: High permeability and low retentivity**.