the average blood velocity is 20.0 cm/s? 3. Estimate magnetic field strength 1 mm away from the axon if magnitude of axon current is I = 4.1×10 A. Suppose, that axon is long straight current caring wire. Magnetism.pdf MAGNETIC FIELD. MAGNETISM IN NATURE 1. The magnitude of the magnetic force F on a charge q moving at a speed v in a magnetic field of strength B is given by F = qvBsin0, where 0 is the angle between the directions of v and B. This force is often called the Lorentz force. In fact, this is how we define the magnetic field strength B in terms of the force on a charged particle moving in a magnetic field. The SI unit for magnetic field strength B is called the tesla (T). 2. Force on a moving charge in a magnetic field. Magnetic force can cause a charged particle to move in a circular or spiral path. In this case, the magnetic force supplies the centripetal force. mv? = qvB, here, r is the radius of curvature of the path of a charged particle with mass m and charge q , moving at a speed v perpendicular to a magnetic field of strength B (0 = 1) . 3. The Hall Effect The magnetic field also affects charges moving in a conductor. Figure 1 shows what happens to charges moving OB (out of paper) RHR-1 F. through a conductor in a magnetic field. The field is perpendicular to the electron drift velocity and to the width (a) i O of the conductor. Note that conventional current is to the right in both parts of the figure. In part (a), electrons carry the current and move to the left. In part (b), positive charges carry the current and move to the right. Moving electrons feel a magnetic force toward one side of the conductor, leaving a net positive charge on the other side. This separation of charge creates a voltage ɛ , known as the Hall emf, across the conductor. The creation of a voltage across a current-carrying conductor by a magnetic field в (b) -E O F. is known as the Hall effect: qE = qvB, or E = vB. Note that the electric field E is uniform across the conductor because the magnetic field B is uniform, as is the conductor. For a uniform electric field, the relationship between electric field and voltage is E = ɛ/l , where l is the width of the conductor and ɛ is the Hall EMF. Entering this into the last expression gives ɛ = Blv where ɛ is the Hall effect voltage across a conductor of width I through which charges move at a speed v. The Hall effect can be used to measure fluid flow in any fluid having free charges, such as blood. 4. Magnetic force on a current-carrying conductor F = IlBsin0, here F is magnetic force on a length 1 of wire carrying a current I in a uniform magnetic field B. 5. The torque t on a current-carrying loop of any shape in a uniform magnetic field is t = NIABsin0, where N is the number of turns, I is the current, A is the area of the loop, B is the magnetic field strength, and O is the angle between the perpendicular to the loop and the magnetic field. 6. The magnetic field strength (magnitude) produced by a long straight current-carrying wire is found by experiment to be B = 20, where I is the current, r is the shortest distance to the wire, and the constant uo = 4x×10¯7 T · m/A is the permeability of free space. 7. Magnetic field produced by a current-carrying circular loop. There is a simple formula for the magnetic field strength at the center of a circular loop. It is B =, where R is the radius of the loop. One 2πr' Нol 2R way to get a larger field is to have N loops; then, the field is B = N Ho!. 2R Holil2 , here two parallel currents I1 and I2, 8. Magnetic force between two parallel conductors %3D 2nr separated by a distance r, l is length of the conductors.