The Magnetic Fields Interface

The interface (

), found under the branch when adding a physics interface, is used to compute magnetic field and induced current distributions in and around coils, conductors, and magnets. Depending on the licensed products, stationary, frequency-domain, small-signal analysis, and time-domain modeling are supported in 2D and 3D. Note that the frequency and time-domain formulations become ill-posed when approaching the static limit. You can extend the useful frequency range downward by adding a low conductivity.

The physics interface solves Maxwell’s equations, which are formulated using the magnetic vector potential and, optionally for coils, the scalar electric potential as the dependent variables.

The main node is Ampère’s law, which adds the equation for the magnetic vector potential and provides an interface for defining the constitutive relations and its associated properties, such as the relative permeability.

When this physics interface is added, these default nodes are also added to the — , , (the default boundary condition), and . Then, from the toolbar, add other nodes that implement boundary conditions and external currents. You can also right-click to select physics features from the context menu.

The physics-controlled mesh is controlled from the node’s window (if the is ). There, in the table in the section, find the physics interface in the column and select or clear the check box in the column on the same table row for enabling (the default) or disabling contributions from the physics interface to the physics-controlled mesh.

Information from the physics, such as the presence of an infinite elements domain or periodic condition, will be used to automatically set up an appropriate meshing sequence.

	In the COMSOL Multiphysics Reference Manual see the Physics-Controlled Mesh section for more information about how to define the physics-controlled mesh.

The is the default physics interface name.

The is used primarily as a scope prefix for variables defined by the physics interface. Refer to such physics interface variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the field. The first character must be a letter.

The default (for the first physics interface in the model) is mf.

This section allows the specification of a background magnetic vector potential (that generates a background magnetic flux density).

Select an option to — (the default) or . When is selected, choose the Background field specification — (the default) or . Mathematically, the background field is always specified as a background magnetic vector potential. The option will apply an appropriate magnetic vector potential generating the specified magnetic flux density. In axisymmetric geometries, the uniform background field is applied in the axial direction.

For , enter values or expressions for the Ab (SI unit: Wb/m). The defaults are 0 Wb/m. For , in 2D or 3D components, enter the values or expressions of the components of the Bb (SI unit: T); in 2D axisymmetric components, enter the value or expression of the Bb,z (SI unit: T). The defaults are 0 T. The specified background magnetic flux densities must be uniform (space-independent), but they can be a function of time.

When a background field is active, this solves for the relative (perturbated) field only. This setting, together with the External Magnetic Vector Potential feature, is useful to introduce an external field generated by systems not included in the model.

This section is only available in 2D and 2D axially symmetric components.

The current vector has the same direction as the magnetic vector potential. This setting also controls the direction in which applied and induced currents can flow in the model. The default option is to solve for the out-of-plane component only for 2D and 2D axisymmetric components.

Select — (the default), , or for the magnetic vector potential.

When or is selected, the Gauge Fixing for A-Field node becomes available.

For 2D components, enter a value or expression for the global d (SI unit: m). The default of 1 m is typically not representative for a thin domain. Instead it describes a unit thickness that makes the 2D equation identical to the equation used for 3D components.

Use the Change Thickness (Out-of-Plane) node (described for the Electrostatics interface) to define specific geometric entities (for example, domains) instead of a global setting for the thickness.

Select the check box to switch on the sweep and invoke a parametric sweep over the Lumped Port features. Enter a to indicate the name of the model parameter that controls the port activated in each step of the sweep. The default is PortName. The name given must match the model parameter, defined under , that is object of a node in the current .

The lumped parameters computed can be subject to a . To activate this functionality, enter a file path or click to navigate to a file. Select a for the Touchstone export — (the default), , or . Select the desired operation to perform from the list — (the default) or . The latter is useful when the model is solved multiple times with different settings. Enter a Zref (SI unit: Ω). The default is 50 Ω.

To display this section, click the button (

) and select in the dialog box.

When the check box is selected, any features that are incompatible with the study will generate an error message when trying to solve or show the default solver. No solver will be generated. Clear the check box and you will be able to run the model, possibly with runtime errors instead. It is available to allow the advanced user to tweak any feature and use it outside of its intended study scope.

The dependent variable is the A. You can change both its field name and the individual component variable names. If the new field name coincides with the name of another magnetic vector potential field in the model, the physics interfaces share degrees of freedom and component names. The new field name must not coincide with the name of a field of another type, or with a component name belonging to some other field. Component names must be unique within a model, except for fields of the same type sharing a common field name.

Select the shape order for the dependent variable — , (the default), or . For more information about the section, see Settings for the Discretization Sections in the COMSOL Multiphysics Reference Manual.