Umair Gaines
Electric dipoles and magnetic dipoles are fundamental concepts in electromagnetism. An electric dipole is defined as a system with two equal and opposite charges separated by a distance, resulting in an overall polarity. The electric dipole moment, measured in coulomb-meters, quantifies the separation of positive and negative charges within the system. On the other hand, a magnetic dipole is characterised by a closed loop of electric current or a pair of magnetic poles, with each magnetic object exhibiting dipolar behaviour due to its two poles. The magnetic dipole moment, also known as the magnetic moment, quantifies the torque caused by a magnetic force on a dipole per unit of magnetic field strength. While magnetic monopoles have not been observed in nature, magnetic dipoles are prevalent in atomic and macroscopic systems, playing a crucial role in various technologies such as compasses, MRI machines, and computer hard drives.
| Characteristics | Electric Dipole | Magnetic Dipole |
|---|---|---|
| Definition | An electromagnetic phenomenon involving the separation of positive and negative electric charges in any electromagnetic system. | The limit of either a closed loop of electric current or a pair of poles as the size of the source is reduced to zero while keeping the magnetic moment constant. |
| Simple Example | A pair of charges of equal magnitude but opposite sign separated by a small distance. | A single loop of wire with a constant current passing through it. |
| Permanent State | An electret. | A bar magnet. |
| Poles | Two equal and opposite charges that exist separately. | Two equal and unlike poles that do not exist separately. |
| Pole Movement | Charges can move freely. | Pole does not move. |
| Field Type | Electric field. | Magnetic field. |
| Field Calculation | Linear force. | Lorenz force. |
| Field Strength | The dipole moment has a magnitude and direction, expressed as a vector quantity. | The field strength is symmetric under rotations about the axis of the magnetic moment. |
| Field Direction | The dipole direction aligns with an external electric field. | The magnetic field between poles is in the opposite direction to the magnetic moment. |
| Unit | Coulomb-metre (C⋅m). | A*m^2 or J/T. |
| Transition | Occurs when an atom behaves like an oscillating electric dipole. | Occurs when an atom behaves like an oscillating magnetic dipole. |
| Effect on Transition Intensity | Much higher effect compared to magnetic dipole transition. | Lower effect compared to electric dipole transition. |
| Application in Imaging | N/A | Magnetic dipole helps in locating the NMR of protons and nuclei of hydrogen in Magnetic Resonance Imaging (MRI). |
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What You'll Learn
Electric dipole moment
In physics, an electric dipole is an electromagnetic phenomenon involving the separation of positive and negative electric charges within an electromagnetic system. This separation results in the generation of electromagnetic waves. A simple example of this system is a pair of charges of equal magnitude but opposite signs, separated by a small distance. The electric dipole moment is a measure of this separation of charges and the overall polarity of the system.
The SI unit for the electric dipole moment is the coulomb-metre (C·m), while another unit of measurement used in atomic physics and chemistry is the debye (D). The electric dipole moment can be expressed using vector algebra, with the dipole moment vector pointing from the negative charge to the positive charge. This vector tends to align itself with an external electric field.
The concept of the electric dipole moment is important in understanding the behaviour of substances in the presence of external electric fields. Dipoles tend to align themselves with the direction of the external field, which can be constant or time-dependent.
Experiments have been conducted to measure the electric dipole moment of particles like electrons and neutrons, with predictions of a nonzero electric dipole moment for the neutron and proton, which have not been experimentally observed.
In certain circumstances, atoms, molecules, or nuclei can behave like oscillating electric dipoles, generating electromagnetic waves. This behaviour is described by quantum mechanics, which determines whether an atom can act as an oscillating electric dipole or must undergo a "forbidden transition" and behave as an oscillating magnetic dipole during a change in quantum state.
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Magnetic dipole moment
A magnetic dipole is the closed circulation of an electric current system or a pair of poles. A simple example is a single loop of wire with a constant current running through it. A bar magnet is an example of a permanent magnetic dipole moment.
The magnetic dipole moment is a combination of the strength and orientation of a magnet or other object or system that exerts a magnetic field. The dipole moment of a bar magnet points from its magnetic south to its magnetic north pole. The magnetic moment can be defined as a vector (or pseudovector) relating the aligning torque on the object from an externally applied magnetic field to the field vector itself. The strength and direction of the torque depend on the magnitude of the magnetic moment and its orientation relative to the direction of the magnetic field.
The magnetic dipole moment of an object determines the magnitude of the torque the object experiences in a given magnetic field. When the same magnetic field is applied, objects with larger magnetic moments experience larger torques. The magnetic field of a magnetic dipole is proportional to its magnetic dipole moment. The dipole component of an object's magnetic field is symmetric about the direction of its magnetic dipole moment.
The magnetic moment is a quantity that describes the magnetic strength of an entire object. The magnetic moment of a current loop is the product of the current flowing in the loop and the (vector) area of the loop. The magnetic moment is typically measured with devices called magnetometers.
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Electric dipole transition
An electric dipole transition is the dominant effect of an interaction of an electron in an atom with an electromagnetic field. The electric dipole matrix elements control the rates of electric dipole transitions.
The electric dipole transition rate may be zero between certain electron states due to selection rules, particularly the angular momentum selection rule. The angular momentum selection rule forbids an electric dipole transition between two states. The selection rules can be written in a more general form, allowing for a transition from a state to a state of a hydrogen-like atom but disallowing a transition from a p to an s state.
The absorption cross-section associated with atomic transitions between an initial state of energy and a final state of energy can be calculated using equations. The net absorption cross-section can be calculated by summing over all final states and integrating over all possible angular frequencies of radiation.
The electric dipole moment is a measure of the separation of positive and negative electrical charges within a system, or its overall polarity. A simple electric dipole consists of two equal and opposite charges that are infinitesimally close together, although real dipoles have separated charges. The dipole moment vector points from the negative charge to the positive charge.
In contrast, a magnetic dipole is the closed circulation of an electric current system, such as a single loop of wire with a constant current. The only known mechanisms for creating magnetic dipoles are by current loops or quantum-mechanical spin. A magnetic dipole moment can also be generated by an electron, which has an intrinsic magnetic property.
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Magnetic dipole transition
In electromagnetism, a magnetic dipole is the limit of either a closed loop of electric current or a pair of poles as the size of the source is reduced to zero while keeping the magnetic moment constant. It is a magnetic analogue of the electric dipole, but the analogy is not perfect. A magnetic dipole transition describes the dominant effect of the coupling of the magnetic dipole moment of an electron to the magnetic part of an electromagnetic wave.
The electronic states of atoms and molecules normally don't have a static electric dipole moment, but many states have a static magnetic dipole moment. The classical magnetized top model can be used to describe magnetic resonances for atoms with static magnetic dipole moments between different Zeeman-split sublevels without needing a full quantum mechanical description.
The selection rules for allowed magnetic dipole transitions are:
> ΔJ=0,±1(except J=0→J=0)
Where J is the total angular momentum quantum number. Magnetic dipole transitions and electric quadrupole transitions couple states with the same parity.
The most celebrated example of a magnetic dipole transition in physics is the spontaneous decay of the triplet state of a hydrogen atom to the corresponding singlet state. This transition, which takes place at a very slow rate, produces the well-known hydrogen 21 cm line.
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Differences between electric and magnetic dipoles
Electric dipoles and magnetic dipoles are both characterised by their dipole moment, a vector quantity. However, they exhibit several key differences.
A magnetic dipole is formed by the arrangement of two unlike magnetic poles at very small distances. It represents the north and south poles of different magnets divided by a small distance. Magnetic dipoles are essentially tiny magnets where charge flows in a loop. They are of microscopic to subatomic dimensions. Electrons circulating around atomic nuclei, electrons spinning on their axes, and rotating positively charged atomic nuclei are all examples of magnetic dipoles.
On the other hand, an electric dipole is formed by physically separate charges. The electric dipole moment points from the negative charge towards the positive charge, and its magnitude is determined by the strength of each charge multiplied by the separation between them. Unlike magnetic dipoles, the charges in electric dipoles can move freely.
The magnetic field of a magnetic dipole is continuous everywhere. In contrast, the electric field of an electric dipole diverges or converges at the point charges. The direction of an electric field is defined as the direction of the force on a positive charge, with electric field lines pointing away from positive charges and towards negative charges.
While magnetic dipoles are characterised by the flow of electric charge around a loop, the magnetic moment of a magnetic dipole is not due to a current loop but is an intrinsic property of the electron. A bar magnet, for example, owes its magnetism to the intrinsic magnetic dipole moment of the electron.
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Frequently asked questions
An electric dipole is a system with equal positive and negative charges separated by a distance. The dipole moment is a measure of the separation of these charges and the system's overall polarity.
A magnetic dipole is a tiny magnet with a north and south pole, with a magnetic current flowing in a loop. It is created by having two equal but opposite magnetic charges near each other.
A water molecule can be modelled as an electric dipole. Electrons circulating around atomic nuclei and rotating positively charged atomic nuclei are examples of magnetic dipoles.
