Lenz's Law

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A very important law that connects electricity and magnetism is Lenz's law. When written as an equation, this law involves two mathematical concepts, a negative sign and a first derivative. Students can perform mathematical operations involving both of these concepts. The difficulty arises, and this happens in many equations, when we try to put a physical interpretation on what the negative sign and the derivative are trying to "tell us". In Physics we need insight into what an equation is saying. To a physicist an equation is a connection between quantities that have physical meaning; an equation describes how nature behaves. As Lenz's law is a common topic in HSC, IB, Cambridge International and first year university examination papers it is worthwhile to compile a list of the statement of Lenz's law as given by the current popular Physics textbooks to see how experts interpret it.

1. Halliday, Resnick and Walker Fundamentals of Physics (14th edition) An induced current has a direction such that the magnetic field due to the current opposes the change in the magnetic flux that induces the current

2. Serway and Jewett Physics for Scientists and Engineers (8th edition) The induced current is in the direction that creates a magnetic field that opposes the change in magnetic flux through the area enclosed by the loop.

3. Sears, Zemansky, Young and Freedman University Physics (14th edition) The direction of any magnetic induction effect is such as to oppose the cause of the effect

4. Knight Physics for Scientists and Engineers (4th edition) There is an induced current in a closed, conducting loop if and only if the magnetic flux through the loop is changing. The direction of the induced current is such that the induced magnetic field opposes the change in flux.

5. Cutnell and Johnson Physics (9th edition) The induced emf resulting from a changing magnetic flux has a polarity that leads to an induced current whose direction is such that the induced magnetic field opposes the original flux change.

6. Tipler and Mosca Physics for Scientists and Engineers (7th edition) The induced emf is in such a direction as to oppose, or tend to oppose, the change that produces it.

Which do you think is clearest?

Matter into Energy?

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One common statement in student's examination responses is that "energy is released in nuclear fission because matter is converted into energy". Particles of matter are not converted into energy in nuclear fission. During nuclear fission the proton-neutron combination in U-235 is rearranged into a more stable combination, such as Ba-141 and Kr-92 and three neutrons,  releasing some binding energy that was "stored" in the U-235 nucleus. We have the same number of protons and neutrons after reaction. There is no annihilation of any particle in nuclear fission. When an electron and a positron (the electron antiparticle) do meet they annihilate each other producing two gamma rays which carry away energy, but this does not occur in this reaction.

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