Lenz's law

Lenz's law /ˈlɛnts/ is a common way of understanding how electromagnetic circuits obey Newton's third law and the conservation of energy. Lenz's law is named after Heinrich Lenz, and it says:

If an induced current flows, its direction is always such that it will oppose the change which produced it.

Lenz's law is shown with the negative sign in Faraday's law of induction:

${\mathcal {E}}=-{\frac {\partial \Phi _{\mathrm {B} }}{\partial t}},$

which indicates that the induced emf (ℰ) and the change in magnetic flux (∂Φ_B) have opposite signs. As such it is a qualitative law that refers to the direction of induced current and its magnetic field without quantitatively defining their magnitudes.

For a rigorous mathematical treatment, see electromagnetic induction and Maxwell's equations.

Opposing currents

If a change in the magnetic field of current i₁ induces another electric current, i₂, the direction of i₂ is opposite that of the change in i₁. If these currents are in two coaxial circular conductors ℓ₁ and ℓ₂ respectively, and both are initially 0, then the currents i₁ and i₂ must counter-rotate. The opposing currents will repel each other as a result.

Lenz's law states that the current induced in a circuit due to a change or a motion in a magnetic field is so directed as to oppose the change in flux or to exert a mechanical force opposing the motion.

Example

Currents bound inside the atoms of strong magnets can create counter-rotating currents in a copper or aluminum pipe. This is shown by dropping the magnet through the pipe. The descent of the magnet inside the pipe is observably slower than when dropped outside the pipe.

When an emf is generated by a change in magnetic flux according to Faraday's Law, the polarity of the induced emf is such that it produces a current whose magnetic field opposes the change which produces it. The induced magnetic field inside any loop of wire always acts to keep the magnetic flux in the loop constant. In the examples below, if the B field is increasing, the induced field acts in opposition to it. If it is decreasing, the induced field acts in the direction of the applied field to try to keep it constant.

Detailed interaction of charges in these currents

In electromagnetism, when charges change positions along electric field lines, work is done on them, whether it involves storing potential energy (negative work) or increasing kinetic energy (positive work).

When net positive work is applied to a charge q₁, it gains momentum. The net work on q₁ thereby generates a magnetic field whose strength (in units of magnetic flux density (1 tesla = 1 volt-second per square meter)) is proportional to the speed increase of q₁. This magnetic field can interact with a neighboring charge q₂, passing on this momentum to it, and in return, q₁ loses momentum.

The charge q₂ can also act on q₁ in a similar manner, by which it returns some of the emf that it received from q₁. This back-and-forth component of emf contributes to magnetic inductance. The closer that q₁ and q₂ are, the greater the effect. When q₂ is inside a conductive medium such as a thick slab made of copper or aluminum, it more readily reacts to the emf sent to it by q₁. The energy of q₁ is not "instantly" consumed only as heat generated by the current of q₂ but is also stored in two opposing magnetic fields. The energy density of magnetic fields tends to vary by the square of the magnetic field's intensity; however, in the case of magnetically non-linear materials such as ferromagnets and superconductors, this relationship breaks down.

Field energy

The electric field stores energy. The energy density of the electric field is given by:

$u={\frac {1}{2}}\varepsilon |\mathbf {E} |^{2}\,,$

In general the incremental amount of work per unit volume δW needed to cause a small change of magnetic field δB is:

$\delta W=\mathbf {H} \cdot \delta \mathbf {B} .$

Conservation of momentum

Momentum must be conserved in the process, so if q₁ is pushed in one direction, then q₂ ought to be pushed in the other direction by the same force at the same time. However, the situation becomes more complicated when the finite speed of electromagnetic wave propagation is introduced (see retarded potential). This means that for a brief period of time, the total momentum of the two charges is not conserved, implying that the difference should be accounted for by momentum in the fields, as asserted by Richard P. Feynman. Famous 19th century electrodynamicist James Clerk Maxwell called this the "electromagnetic momentum". Yet, such a treatment of fields may be necessary in the case of applying Lenz's law to opposite charges. It is normally assumed that the charges in question have the same sign. If they are not, such as a proton and an electron, the interaction is different. An electron generating a magnetic field would generate an emf that causes a proton to change its motion in the same direction as the electron. At first, this might seem to violate the law of conservation of momentum, but of course, such an interaction indeed conserves momentum once taking into account the momentum of electromagnetic fields.

External links

• Eddy Currents and Lenz's Law (audio slideshow from the National High Magnetic Field Laboratory)
• MIT A brief video demonstrating Lenz's law
• A dramatic demonstration of the effect on YouTube with an aluminum block in an MRI
• Eddy currents produced by magnet and copper pipe.