17 EMI 2022 - Tutorial ans with assignment

St. Andrew’s Junior College H2 Physics 17-33 TUTORIAL 17: EMI SOLUTIONS Level 1 Solutions 1 B. Refer to Example 2. [1] 2(a) Magnetic flux is defined as the product of the magnetic flux density and the area normal to the field through which the field is passing. Magnetic flux linkage in a coil is defined as the product of the magnetic flux passing through a coil and the number of turns of the coil. [1] [1] 2(b) The magnetic flux density is defined as the force acting per unit current in a wire of unit length at right-angles to the field whereas magnetic flux is defined as the product of the magnetic flux density and the area normal to the field through which the field is passing. [1] [1] 2(c) Faraday’s Law states that magnitude of an induced emf is directly proportional to the rate of change of magnetic flux-linkage. [1] 2(d) Lenz’s Law states that the direction of the induced emf is such that its effects oppose the change which causes it. The induced emf produces effects which opposes the change in the magnetic flux linkage. Consequently, work has to be done by an external agent to overcome this opposition and energy is conserved. Hence the electrical energy associated with the induced current is derived from the work that is done by the external agent to overcome the change in magnetic flux. If the effects of the induced emf do not oppose the change in magnetic flux, there will be a gain in energy without any work done. This violates the law of conservation of energy. [1] [1] [1] [1] 3 “its axis parallel to a uniform magnetic field ” means the B field vector is perpendicular to the plane containing the cross-sectional area of the coil. EMF =  N B A/t = 120  (80m – 20m)  0.07 / 4.0 = 130 mV [1] [1]

St. Andrew’s Junior College H2 Physics 17-34 Level 2 Solutions 4 A  The current in the solenoid sets up a B-field.  “Original current” = Applied emf ÷ Tot Resistance of circuit.  The iron rod strengthens the B-field, ie increases the flux density B, ( Topic 16: EM Learning Outcome (d); B = ro nI -NOT in Syll). r:relative permeability of the medium (soft iron)  1. When the rod is “far eough” fr solenoid: equivalent to being “not there at all”: current = “original current”  2. When the iron rod is entering the solenoid: (ie while partially in solenoid), the flux linkage increases with time.This induces an emf in the solenoid. By Lenz’s law the direction of the induced emf is opposite that of the applied emf (by the battery); resulting emf decreases, causing the current in the solenoid to decrease (momentarily). (See the dip, the one that occurs earlier in time.)  3. “Deep within the solenoid”: Here B has increased to its max; it