Question E6D05:
How do ferrite and powdered iron compare for use in an inductor core?  

Choices:  
A: Ferrite cores generally have lower initial permeability  
B: Ferrite cores generally have better temperature stability  
C: Ferrite cores generally require fewer turns to produce a given inductance value  
D: Ferrite cores are easier to use with surface-mount technology    

Correct Answer is  C: Ferrite cores generally require fewer turns to produce a given inductance value  

To compare ferrite vs powdered iron cores, remember that inductance scales with core permeability.

Why the correct answer (C) is correct:
- Ferrite typically has much higher relative permeability ($\mu_r$) than powdered iron. Since inductance is proportional to $\mu_r N^2$, a higher $\mu_r$ means you need fewer turns $N$ for the same inductance.

Key formula:
- For a given core: $$L = A_L N^2$$
  - $L$ = inductance
  - $N$ = turns
  - $A_L$ = core’s inductance per turn squared (set by core material and shape)
- Rearranged: $$N = \sqrt{\frac{L}{A_L}}$$
  - Ferrite cores have higher $A_L$ than powdered iron, so $N$ is smaller.

Quick example:
- Suppose $L = 10\,\mu H$.
- Ferrite: $A_L \approx 1000\,\text{nH/turn}^2 \Rightarrow N \approx \sqrt{10000/1000} \approx 3.2$ turns.
- Powdered iron: $A_L \approx 50\,\text{nH/turn}^2 \Rightarrow N \approx \sqrt{10000/50} \approx 14$ turns.

Why the other choices are incorrect:
- A: “Ferrite cores generally have lower initial permeability” — Incorrect. Ferrite generally has higher initial permeability than powdered iron.
- B: “Ferrite cores generally have better temperature stability” — Incorrect. Powdered iron is typically more temperature-stable; ferrite permeability varies more with temperature depending on mix.
- D: “Ferrite cores are easier to use with surface-mount technology” — Not generally true. Both materials are available in SMT forms; core material doesn’t inherently make SMT assembly easier.

Rules of thumb:
- Ferrite: higher permeability, fewer turns; good for high-inductance chokes, broadband transformers, EMI suppression.
- Powdered iron: lower permeability, more turns; better Q at HF and more tolerant of DC bias (less saturation), and better temperature stability.