CALCULATORS / MAGNETICS AND INDUCTIVE DEVICES

Magnetic-circuit reluctance calculator

Uniform linear core reluctance; supplied relative permeability, no saturation/fringing.

magnetics-magnetic-circuit.napkin / editable

SCENARIO COMPARISON

Compare the numbers.

This source describes an analytical model. Its quantities and checks are the view.

Scenario	reluctanceA/Wb	fluxWb	densityT
	795775	0.0000125664	0.125664
	636620	0.000015708	0.15708

All calculated quantities are listed with the inputs. Switch to Source to inspect the equations.

SCENARIOS / CALCULATED VALUESSAME SOURCE

Change an input to see the consequences.

flux density screen0.13 T✓ passes

BASELINE INPUTS

length100 mmarea100 mm^2relativePermeability1000 turns100 current0.1 AmuZero0.00000125664 H/m

CALCULATED

reluctance795770 A/Wb

flux0.000012566 Wb

density0.12566 T

Edits change the baseline source.

THE SOURCE

Edits below update the same model.

# Magnetic-circuit reluctance
# Scope: Uniform linear core reluctance; supplied relative permeability, no saturation/fringing.
# Status: illustrative worked example; engineering review pending.
# Inputs are example values. Checks are editable example requirements, not standards.

length = 100 mm
area = 100 mm^2
relativePermeability = 1000
turns = 100
current = 0.1 A
muZero = 0.0000012566370614 H/m

reluctance := length/(muZero*relativePermeability*area) [A/Wb]
flux := turns*current/reluctance [Wb]
density := flux/area [T]

check flux density screen: density <= 0.3 T

scenario alternative: length = 80 mm

Calculated from your source · not saved

Baseline · Baseline

flux density screen0.13 T ✓ passes

alternative · alternative

flux density screen0.16 T ✓ passes

Illustrative first-order example. Checks are editable requirements, not design standards. The broader topic may require additional equations, data, and validation.

Formula and units

This example uses the following equations. Input names and units match the editable source above.

reluctance: reluctance = length/(muZero*relativePermeability*area) [A/Wb]
flux: flux = turns*current/reluctance [Wb]
density: density = flux/area [T]

Original worked example

These are the baseline values in the downloadable source. Edits above let you test your own values.

Given

length: 100 mm
area: 100 mm^2
relative permeability: 1000
turns: 100
current: 0.1 A
mu zero: 0.00000125664 H/m

Calculated

reluctance: 795775 A/Wb
flux: 0.0000125664 Wb
density: 0.125664 T

Assumptions and limits

Uniform linear core reluctance; supplied relative permeability, no saturation/fringing.

The requirement checks use the limits written in this example. Passing a check does not establish that the model covers every load, material behavior, or operating condition in a real design.

Download this .napkin example ↓

Take it further with your agent

Napkin keeps the inputs, equations, checks and assumptions together in one study. Your agent sets it up; you change the numbers.

Try with Claude ↗

codex 'Use Napkin (https://napkin.engineering/mcp, or the local Napkin plugin) to build a study: Magnetic-circuit reluctance. Inputs: path lengths; areas; permeability curves; turns/current. Outputs: flux; MMF; gap drop. Model scope: Magnetic-circuit approximations with calibrated material data. Read napkin_capabilities and the current snapshot. Ask me for the values that matter, label assumptions with units, author .napkin source, validate it and apply it with napkin_source using the current revision, then open the study. Add checks for the limits I supply. Keep unsupported analyses explicit.'

Claude needs the Napkin connector; Codex needs the Napkin plugin. Set up your agent ↗

The broader study

The topic brief below describes the broader scope. The worked example above states the particular calculation it implements.

Inputs: path lengths, areas, permeability curves, turns/current
Outputs: flux, MMF, gap drop
Modes: calculate, compare, sweep
Drawing: Magnetic circuit and flux plot

EXECUTABLE WORKED EXAMPLE · FIRST-ORDER MODEL · CHECK THE SCOPE & ASSUMPTIONS