(a) Sketch the form of the magnetic flux pattern due to a current flowing (i) in a long solenoid (ii) through two long straight parallel wires when the directions of the current are opposite. (Neglect the earth's magnetic field). (b) Draw a labelled diagram of an electric bell and explain how it works (c) An electric bell takes a current of 0.2A groin a battery of two dry cells connected in series. Each cell has an e.m.f. of 1.5V and an internal resistance of 1.0. (i) Calculate the effective resistance of the bell (ii) What current would the bells take in the cells were arranged in parallel?
Explanation
[b][/b]: When the key is closed, current flows in the circuit thereby magnetising the electromagnet. The soft iron armature is thus attached to the electromagnet, causing the hammer to strike the gong. When the armature is attracted, a gap is created at the contact and the circuit breaks. The electromagnet loses its magnetism and the armature falls back to its original position by the pull of this spring. This process of make and break is repeated thereby making the bell to ring continuously
(i) E = IR - 0.2(2 + R) R = Ressistence of bell 3 = 0.2(2 + R) 15 = 2 + R R = 15 - 2 = 13e (ii) E = 1(0.5 _ R) 1.5 = I(0.5 _ 13) = I(13.5) I = = 0.11A
