Magnetism and Electricity

Magnetism and Electricity

1. Iron is a temporary magnet that is used as an electromagnet because it is easily magnetised and demagnetises

   • Electromagnets can switch on and off and change the strength of the magnetic field by altering the current
     • Magnetic field is a region in which a magnetic force is exerted
       • THe magnetic flux lines represent the magnitude and direction of the field (from North to South)

2. Steal is a permanent magnet used as a bar magnet

3. Current electricity

   • Flow of electrons from negative to positive terminal while the conventional current is the flow of electric charge from positive to negative terminal
   • Electrical current electricity is the rate of flow of charges (coulomb)
     • D.C flow in the same direction BATTERIES
     • A.C continuously changing the direction of flow GENERATORS
     • Ampere is the current intensity when one coulomb of charge passing through a conductor in one second
   • Voltage of potential difference is the amount of energy that a unit of charge transfers between one point and another
     • voltage of electromotive force is the maximum P.D produced by an electric cell
       • Chemical energy to electrical energy
     • Electric energy to heat energy
   • REsistance oppose the flow of electrons as the electrons collide with the ions of the metal
     • As the length increase the resistance increase. as the diameter of the cross section are increase the resistance decrease. for conductors As the temperature increase the metal ions vibrate more providing greater resistance to the flow of electrons
       • In filament lamp, as the voltage increase the current increase so it gets hotter increasing the resistance decreasing the current
     • the variable resistor can change the length and shape to alter the resistance
   • Series connection circuits
     • Current is the same. Voltage is divided in a volt share calculation (provided by POTENTIOMETER). Resistance is added
   • Parallel connection circuits
     • Current is distributed at the junctions according to the resistance. Voltage is the same. Total resistance have a certain rule to wor out

4. Electromagnetic effects

   • The magnetic effect of a current
     • A conductor wire carrying current creates a magnetic field aroud it increased by increasing the current
     • In a coil, the right hand grip is used where thumb is the direction of current and the other 4 is the direction of field. while In a solenoid, the thumb is the position of the North pole and the other 4 is the direction of current
       • if the current is into the page it is represented by an X sign and have a clockwise field. if the current is out of the page it is represented by a dot and have an anticlockwise field
         • Used in a RELAY in which large current circuit can be operated by a low current circuit
           • when the switch is closed the iron core inside the solenoid is magnetised so it attract the armature causing the contacts in the secondary circuit
   • Force on a current carrying conductor
     • when a current carrying conductor wire is placed in a magnetic field of a magnet a force is produced on a conductor causing it to move
       • the direction is determined by the Fleming Left hand rule
         • if only one finger is reversed the direction is reversed but if two are reversed the direction is the same
       • the force value produced can be increased by increasing the current intensity in the conductor, the length of the wire and the strength of the magnetic field
     • Turning effect of D.C motors
       • when the current is switched on in the coil it exists in opposite direction along the 2 opposite sides of the coil generating equal but opposite motor force which form a torque that rotate the coil.
         • forces remain in the same direction ensuring the continuous rotation even after the reverse of the first half turn
           • the motion stop after 180 turn but continue because of inertia
       • Commutator act as a current carrying switch after every half revolution of the coil. The carbon brushes serve to make contact between the battery and the rotating commutator
       • the turning effect can be increased by increasing the number of turns of the coil, increasing the strength of the magnetic field and increasing the current
   • Electromagnetic induction
     • A changing magnetic field can induce an E.M.F in a circuit
       • A. moving a wire in a fixed magnetic field. When a wire is moving perpendicular to a strong magnetic field and cutting the magnetic field lines a small emf and current is induced in the wire.
       • B. moving a magnet near a coil [The direction of the induced emf opposes the change causing it Lenz]
       • If the direction of movement is reversed the direction of the induced current is reversed. No induction if the magnet is held still. The direction is detected by Fleming Right hand rule
       • the magnitude of the induced emf increases by moving the magnet / the coil or the wire at a higher speed, using stronger magnet and increasing the number of turns of the coil inside the magnetic field
     • A.C generators
       • by means of any external energy source mechanical energy is converted into electrical energy
       • The two slip rings rotate with the coil and provide the alternate positive and negative induce current to the output carbon brushes which maintain the contact with rotating coil. slip rings and brushes slide past each other during rotating the coil.
       • In vertical positions, the size of the alternating current and voltage is minimum while it is maximum in horizontal positions
   • Mutual electromagnetic induction of transformers
     • When an alternating current exists through the primary coil wrapped around one side of soft magnetic iron core, the alternating voltage causes an alternating magnetic field inside the iron core passing to the secondary coil, changing flux likage of varying the field in the secondary coil cuts the wiresof the secondary coil which induces an alternating voltage emf and current in the secondary coil
     • the output of voltage of step up transformer is higher than the input while the output voltage of a step down transformer is lower than the input
     • used to modify the voltages of alternating power supply by changing the number of turns of the primary and secondary coils
     • by stepping up the voltage the current produced will be less which reduce the I^2R which appear as heat loss which is wasted because of the heating effect of current on the wires which also decrease the cost of transmission cables, this voltages is then stepped down before supplied
     • A.C can be transmitted over long distance with low power loss and can be changed to D.C by rectification can be stepped and switched unlike D.C