Question E6D11:
Which type of core material decreases inductance when inserted into a coil?  

Choices:  
A: Ceramic  
B: Brass  
C: Ferrite  
D: Aluminum    

Correct Answer is  B: Brass  

Correct answer: B — Brass

Why it’s correct:
- Inductance depends on core permeability:  
  $$L \propto \mu = \mu_0 \mu_r$$
- Brass is diamagnetic with $\mu_r < 1$, so inserting brass lowers the effective permeability seen by the coil and thus decreases $L$.
- Being a good conductor, brass also supports eddy currents that oppose the coil’s magnetic field (Lenz’s law), further reducing effective inductance and Q.

Why the others are incorrect:
- Ceramic (A): An insulator with $\mu_r \approx 1$, so it has little to no effect on inductance; mainly serves as a mechanical support.
- Ferrite (C): A magnetic material with $\mu_r \gg 1$, which significantly increases inductance when inserted.
- Aluminum (D): Paramagnetic with $\mu_r$ slightly greater than 1, so, in principle, it tends to increase inductance. Though it is conductive (causing losses and possible detuning via eddy currents), it is not the standard choice for purposely decreasing $L$—that role is filled by brass slugs.

Useful formulas and rules of thumb:
- Solenoid inductance with a core:  
  $$L \approx \frac{\mu_0 \mu_r N^2 A}{\ell}$$  
  where $N$ = turns, $A$ = core cross-sectional area, $\ell$ = magnetic path length.
- Rules of thumb:
  - Ferrite/powdered iron core → increases $L$.
  - Brass slug → decreases $L$.
  - Ceramic former → little change in $L$.
  - Any metal near a coil → tends to reduce Q due to eddy-current losses.