ELE1001: Basic Electrical Technology L-10 ELECTROMAGNETISM Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Objective To conceptualize the basic laws in electromagnetism. Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Contents Introduction Flux produced by a current carrying conductor Magnetic field produced by a solenoid Force acting on a conductor Direction of Force: Fleming’s left hand rule Faraday’s laws of Electromagnetic Induction Lenz’s law Direction of induced emf : Fleming's right hand rule Dynamically induced emf Statically induced emf Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Introduction Electromagnetism defines the relationship between magnetism and electricity. Electric generator, motors, transformers, etc. work based on the principle of electromagnetic induction Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Flux produced by a current carrying conductor An electric current flowing in a conductor creates a magnetic field around it. Determination of direction of magnetic field: Maxwell’s right hand grip rule : Assume that the current carrying conductor is held in right hand so that the fingers wrap around the conductor and the thumb is stretched along the direction of current. Wrapped fingers will show the direction of circular magnetic field lines. + . Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Magnetic field produced by a solenoid Determination of direction of magnetic field: Right hand grip rule for solenoid: If the coil is gripped with the right hand, with the fingers pointing in the direction of the current, then the thumb, outstretched parallel to the axis of the solenoid, points in the direction of the magnetic field inside the solenoid Dept. of E&E, MIT, Manipal

Force acting on a conductor ELE1001: Basic Electrical Technology Force acting on a conductor A current carrying conductor, placed in a magnetic field, experiences a force. This is the principle of electric motor. Force on a single conductor = flux density  length of conductor  current f = B I l sinθ Newton This force can be increased by increase in any one of these components. θ Example 5.1 A conductor carries a current of 20 A and is at right- angles to a magnetic field having a flux density of 0.9 T. If the length of the conductor in the field is 30 cm, calculate the force acting on the conductor. Determine also the value of the force if the conductor is inclined at an angle of 300 to the direction of the field.  Solution: B=0.9T; I=20A; L=0.3m. Force 𝐹=𝐵𝐼𝑙=(0.9)(20)(0.3) When the conductor is at right angles to the field F=5.4N When the conductor is inclined at 300 to the field, then the force 𝐹=𝐵𝐼𝑙𝑠𝑖𝑛𝜃=(0.9)(20)(0.3)𝑠𝑖𝑛〖30〗^0 = 2.7N Example 5.2 Determine the current required in a 400 mm length of conductor of an electric motor, when the conductor is situated at right-angles to a magnetic field of flux density 1.2 T, if a force of 1.92 N is to be exerted on the conductor. If the conductor is vertical, the current flowing downwards and the direction of the magnetic field is from left to right, what is the direction of the force? Solution: F=1.92N; L=400mm=0.40m; B=1.2T Since 𝐹=𝐵𝐼𝑙; 𝑡ℎ𝑒𝑛 𝐼=𝐹/𝐵𝑙=1.92/(1.2)(0.4) =4𝐴 Dept. of E&E, MIT, Manipal

Direction of Force: Fleming’s left hand rule ELE1001: Basic Electrical Technology Direction of Force: Fleming’s left hand rule Let the thumb, first finger and second finger of the left hand be extended such that they are all at right-angles to each other. If the first finger points in the direction of the magnetic field, the second finger points in the direction of the current, then the thumb will point in the direction of the motion of the conduct Field (Mechanical force) Current (Second finger) thumb (First finger) Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Faraday’s laws of Electromagnetic Induction An induced emf is set up whenever the magnetic field linking that circuit changes. The magnitude of the induced emf in any circuit is proportional to the rate of change of the magnetic flux linking the circuit. Suppose a coil has N turns and flux through it changes from an initial value of Φ1webers to the final value of Φ2 webers in time t seconds. Then, remembering that by flux-linkages mean the product of number of turns and the flux linked with the coil, we have Initial flux linkages=NΦ1 ,and final flux linkages=N Φ2 ∴𝐼𝑛𝑑𝑢𝑐𝑒𝑑 𝑒.𝑚.𝑓 𝑒=(𝑁Φ2 −𝑁Φ1 )/𝑡 Wb/s or volt Putting the above equation in differential form, we get 𝑒=−𝑁 𝑑Φ/𝑑𝑡 volt Negative sign is given to signify the fact that the induced e.m.f sets up current in such a direction that magnetic effect produced by it opposes the very cause producing it Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Lenz’s law An electro magnetically induced emf always acts in such a direction to set up a current opposing the motion or change of flux responsible for inducing the emf. Dept. of E&E, MIT, Manipal

Direction of induced emf : Fleming's right hand rule ELE1001: Basic Electrical Technology Direction of induced emf : Fleming's right hand rule If the first finger of the right hand is pointed in the direction of the magnetic flux, and if the thumb is pointed in the direction of motion of the conductor relative to the magnetic field, then the second finger, held at right angles to both the thumb and the first finger represents the direction of emf. Dept. of E&E, MIT, Manipal

Dynamically induced emf ELE1001: Basic Electrical Technology Dynamically induced emf The voltage induced in the conductor due to relative motion of conductor and magnetic field Either conductor or magnetic field is moving Principle of Electric generator Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Statically induced emf The voltage induced in the conductor due to change in the magnetic field Conductor is stationary Magnetic Field is changing in a stationary Magnetic System; Eg: Transformer Alternatig current (ac) produces varying magnetic field. Hence induced emf is developed in transformer without any relative motion between field and conductors. Dept. of E&E, MIT, Manipal

ELE1001: Basic Electrical Technology Summary An electric current flowing in a conductor creates a magnetic field around it. A current carrying conductor, placed in a magnetic field, experiences a force. (eg. motor). Whenever the magnetic field linking with a circuit changes an emf is induced in it. Change in magnetic field can be achieved statically (eg. transformer) or by dynamically (eg. generator). Dept. of E&E, MIT, Manipal