Faraday’s Law

Question 1

Faraday’s law

Answer 1

1. Describes the induction of emf in a conductor
2. The emf produced in the conductor

Question 2

Firstlaw

Answer 2

• Whenever a conductor is placed in a changing magnetic field, an electromotive force (EMF) is induced
• If the conductor is part of a closed circuit (MRI), this EMF will cause an electric current to flow
• When EMF is induced and there is a voktage within the coil – the faster the change in the magnetic field -the greater the change in the EMF coil

Question 3

2nd law

Answer 3

• Flux - is a measure of the number of electric or magnetic field lines passing through a surface in a given amount time
• Flux linkage of thr coil – the product of the number of turns in the coil and flux associated with the coil - The magnetic flux linkage is the amount of field passing through a coil of wire. It is the flux ‘linked’ to a wire.
• The magnitude of emf induced in the coil is equal to the rate of change of flux that linkages with the coil
• It is possible to increase EMF induced in a coil by : increasing number of turns in the coil(increasing flux), increasing the magnetic field strength, increasing the speed of relative motion between the coil and the magnet
• The flux will impact the current or the emf induced
• If we imagine a moving magnet through a coil of wire the motion of the magnetic field across the coil will induce an emf which will cause an electric current to flow through the wire and the faster the magnetic field changes (either by moving the magnet or varying it’s magnetic strength) then the greater will be the induced EMF.

Question 4

Faraday’s law of applicability in MRI

Answer 4

• The production of electricity by a rotating magnet actually describes how an MR signal is produced
• An MR scanner is an electrical generator with a computer that interprets the induced electrical currents to produce an image of in vivo magnetism.
• The creation of an MR image requires; an in vivo magnetic source, a force (torque) -hydrogen atoms that causes the magnetic source to rotate and an appropriately placed coil of wire to receive the signal
• In MRI we have active nuclei that will have a net charge and are spinning automatically, they will align to the magnetic field and they will get that magnetic moment and will align to that external magnetic field. With rf pulses we can effect that alignment and generate the current and signal that we want.

Question 5

Electromagnetic induction in MRI

Answer 5

• The principle of electromagnetic induction is used to generate the mr signal
• Inner workings of MRI - Magnetic coils in different directions and Rf transmitter and receiver coils
• Electromagnetic induction in MRI-
1. A strong static magnetic field is applied – called B0 (causes hydrogen nuclei in the body to align)
2. Then we apply the rf pulse (temporaril disturbs the alignment)
3. When the rf pukse is switched off the protons relax and realign with the magnetic field
4. This relaxation causes a change in the magnetic flux which induces a voltage in the receiver coil of the MRI machine and this signal is what is used to mainky create the image.

Question 6

Faraday’s law relating to safety

Answer 6

• Induced currenfs – rapidly changing magnetic fields, particularly during gradient switching, can induce electrical currents – Eddy currents in conductive materials, including the human body.
• Can lead to heating and possible injury
• Therefore this is why metallic objects are harmful as rhey act as conductors in which induced currents may cause localized heating, burns or displacement forces on ferromagnetic objects.

Question 7

Rf heating

Answer 7

• Rf pulses can induce currents in the body, potentially causing heating and burns particularly if the patient is in contact with conductive surfaces
• It is important to screen patients and that patients are positioned appropriately- such as avoiding a loop in the patient,avoiding wires on the patient – can be a conductor to the magnet