---
title: "Deperming and graphite in non-uniform field"
subject: "Electrical Engineering Material"
chapter: 4
key: "semester_3/electrical_engineering_material:deperming-graphite"
source: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/deperming-graphite
site: BCT Engineering
---

# Deperming and graphite in non-uniform field

Electrical Engineering Material · Chapter 4: Magnetic Materials

- Asked in: 2071 Chaitra, 3+3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/electrical_engineering_material/deperming-graphite
- Whole chapter: https://www.bctengineer.com/md/semester_3/electrical_engineering_material/chapter-4

## Question

Explain deperming method of demagnetization. If you place graphite in a non-uniform magnetic field what will happen?

## Short answer

Deperming demagnetizes steel ships by passing DC pulses of alternating polarity and decreasing amplitude, driving the material round shrinking hysteresis loops that spiral to B=0. Graphite, diamagnetic, in a non-uniform field is repelled from the stronger-field region towards the weaker one; it can levitate above magnets.

## Answer

#### Deperming method of demagnetization
**Deperming** is the process of removing the **permanent (remanent) magnetism** of large steel bodies, mainly **ships and submarines**. A steel hull becomes magnetized by the earth's field during construction (welding, riveting, hammering) and in service. This magnetism can trigger magnetic mines and disturb compasses.

**Procedure**
1. Heavy cables are wrapped round the hull (or the ship enters a deperming station with built-in coils).
2. Large **DC current pulses of alternating polarity and decreasing magnitude** are passed: +I₁, −I₂, +I₃, … with I₁ > I₂ > I₃ ….
3. Each pulse drives the steel round a smaller hysteresis loop, so the B–H point spirals towards the origin.
4. The magnetic signature is measured and the process is repeated until the remanent field is nearly zero.

```text
   B
   ^  /\      loops shrink
   | /  \  /\
 --+------\/--\/-> H
   |  ending near B = 0
```

It is the same principle as AC demagnetization with a decaying field, applied with DC pulses to a very large object. (Degaussing, by contrast, uses coils carrying a steady current on board to cancel the remaining field.)

#### Graphite in a non-uniform magnetic field
Graphite is **diamagnetic**, and strongly so (one of the largest diamagnetic susceptibilities, $\chi \approx -10^{-4}$, especially pyrolytic graphite).
- In a field it develops a magnetization **opposite** to $H$.
- Its energy is $U = -\tfrac{\chi V B^2}{2\mu_0}$, which is positive for $\chi < 0$, so it is lowest where $B$ is weakest.
- Hence in a **non-uniform** field, graphite feels a force $F = \dfrac{\chi V}{2\mu_0}\nabla(B^2)$ directed from the **stronger field region towards the weaker** region: it is **repelled** by the magnet's poles.
- A thin piece of pyrolytic graphite can even **levitate** stably above an array of strong NdFeB magnets.
- In a uniform field it feels no net force, only a weak magnetization opposing the field.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are ships and submarines depermed?
   A. To remove permanent magnetism that could trigger magnetic mines or disturb compasses
   B. To increase their buoyancy
   C. To make them magnetic
   D. To remove corrosion
   Answer: A. Hulls acquire permanent magnetism during building and service. (Key point: Deperming purpose)

2. What current pattern is used in deperming?
   A. Steady DC of constant magnitude
   B. DC pulses of alternating polarity with decreasing magnitude, I1 > I2 > I3
   C. Pulses of increasing magnitude
   D. AC at constant amplitude
   Answer: B. Decreasing alternating pulses spiral the B-H point toward the origin. (Key point: Pulse pattern)

3. What kind of magnetic material is graphite?
   A. Ferromagnetic
   B. Paramagnetic
   C. Strongly diamagnetic
   D. Ferrimagnetic
   Answer: C. Graphite has one of the largest diamagnetic susceptibilities. (Key point: Graphite type)

4. In a non-uniform field, graphite experiences a force:
   A. Toward the stronger field
   B. Zero in any field
   C. Perpendicular to the field gradient only
   D. From the stronger field toward the weaker field, so it is repelled
   Answer: D. Energy U = -χVB^2/2μ0 is lowest where B is weakest when χ < 0. (Key point: Force direction)

## Related in Electrical Engineering Material

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