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# Tutor profile: Elena L.

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Elena L.
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## Questions

### Subject:Physics (Electricity and Magnetism)

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Question:

What is the quasi-static approximation?

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Elena L.

While at high frequencies the electric and magnetic field are always coupled, as described by Maxwell’s equations, at the low frequency range a quasi-static approximation can be used. Such approximation consists on decoupling the E and H-field, because the dimensions of the exposed body are electrically small compared to the field wavelength. Specifically the approximations are strictly related to the dimensions of the exposed object with respect to the incident wave. In the static or quasi-static state (ω → 0), the E and H fields are completely decoupled, and therefore can be solved independently. For static field Maxwell’s equation are heavily simplified into decoupled electrostatic and magnetostatic equations. Conversely, in quasi-static state only one of the two time derivative becomes important for the calculation depending on the relative importance of the two dynamic coupling terms. The Quasi-static approximation implies that the field at a given time are determined indipendently on what the sources of the field were at an earlier time, because the process under consideration is much slower than the propagation time of an electromagnetic wave . Hence, quasistatics approxi- mation assumes that the field strengths change so slowly in time (quasistatic) that the E and H fields induced by those changes (the contributions to E and H from the ∂/∂t terms in Maxwell’s equations) are sufficiently small, and by consequence the induced fields (∝ (∂/∂t)^2) can be neglected (i.e., fields are decoupled); only the original and first-order induced fields are therefore of interest.

### Subject:Biomedical Science

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Question:

Is MRI dangerous for patients?

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Elena L.

### Subject:Algebra

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Question:

How can I use the scientific notation to find an easy solution to the apparently complicated operation: 0.00015 * 5372 / 15?

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Elena L.

The scientific notation allows to express numbers using a decimal form. For the specific case reported the scientific notation can be very useful to quickly solve the operation. In fact, the expression can be written as: (1.5 * 10^-4 * 5.372 * 10^3) /( 1.5 * 10) we can now group all the coefficients and decimals together, thus the expression becomes: (1.5*5.372/1.5)*(10^-4*10^3 / 10) The first part of the expression can be easily solved because the 1.5 can be simplified as present both at the numerator and denominator. Whereas the second part of the expression can be solved using the properties of the exponents. Hence, the expression becomes (already simplifying 1.5 in the first part): 5.372 * 10^(-4+3-1) Please notice that the 10 at the denominator was brought to the nominator as 10^-1. The final solution of the operation is 5.372*10^-2 that can now be converted back to its decimal notation: 0.05372

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