Electromagnetism Flashcards
on increasing current what happens to deflection of compass needle
deflection increases
from magnetic field lines around a wire, what do we observe (with straight wire)
- magnetic field lines form concentric circles around the wire, with their plane perpendicular to the straight wire and with their centres lying on the wire
- when direction of current in wire is reversed, pattern of iron fillings does not change, but direction of deflection of compass needle gets reversed
- on increasing current, field lines become denser and iron filings get arranged in circles up to a larger distance from the wire, showing that the magnetic field strength as increased and it is effective up to a larger distance
right hand thumb rule
if we hold the current carrying conductor in our right hand such that the thumb points in the direction of flow of current, then the fingers encircle the wire in direction of magnetic field lines
from magnetic field lines around a wire, what do we observe (with a coiled wire)
- in the vicinity of wire at P and Q, magnetic field lines are nearly circular
- within space enclosed by wire, magnetic field lines are in same direction
- near the centre of loop, magnetic field lines are nearly parallel to each other, so magnetic field is nearly uniform in a small space at the centre
- at centre, magnetic field lines are along the axis of loop and normal to its plane
- magnetic field lines pass through loop in the same direction (loop acts as dipole)
when does magnetic field line get denser in coiled wire
- strength of current in loop is increased
- number of turns in loop is increased
clock rule to determine polarity at faces of loop
looking at the face of the loop, if the current in wire around that face is in anticlockwise direction, the face has north polarity, while if current at that face is clockwise, face has south polarity
from pattern of magnetic field lines of solenoid, it is found that
- magnetic field lines inside the solenoid are nearly straight and parallel to the axis of solenoid (uniform)
- lines become denser on increasing current in solenoid
- magnetic field is increased if number of turns in solenoid is increased
- magnetic field is also increased if soft iron core is placed along the axis of solenoid, because soft iron has HIGH MAGNETIC PERMEABILITY
similarities between current carrying solenoid and bar magnet
- magnetic field lines of a current carrying solenoid are similar to bar magnet
- when suspended freely, both rest in north-south direction
- both acquire magnetic properties and attract iron filings
dissimilarities between solenoid and bar magnet
- strength of magnetic field due to solenoid can be changed by changing current in it, but bar magnet’s strength cannot be changed
- direction of magnetic field can be changed in solenoid by reversing direction of current in it, but not for bar magnet
define electromagnet
an electromagnet is a temporary strong magnet made by passing current in a coil wound around a piece of soft iron. it is an artificial magnet
when is bar shaped electromagnet used
relay (switching device)
use of horse shoe magnet
dc motor
ac generator
electric bell
why can’t ac current be used in electromagnet
polarity at ends change 50 times in each second
why is steel preferred over soft iron in making a strong permanent magnet
steel has higher magnetic retentivity
uses of permanent magnets
galvanometer, ammeter, voltmeter
magnetic compass
advantages of electromagnet over a permanent magnet
- strong magnetic field
- strength of magnetic field can be easily changed
- polarity can be changed by reversing direction of current in solenoid
uses of electromagnet
- lifting and transporting heavy iron scrap, girders, plates.
- loading furnaces with iron
- separating iron pieces from debris and ores, where iron is impurity
- removing iron pieces from wounds
- scientific research, to study the magnetic properties of a substance in a magnetic field
- electric bell, telegraph, microphone
Lorentz force
a charge moving in a magnetic field, in a direction other than the direction of the magnetic field, experiences a force named Lorentz force. this can produce motion in a conductor
experimentally, what is observed during Lorentz experiment
- when no current flows in wire, no force acts on wire and wire does not move
- when current is passed through the wire, force acts on the wire in a direction perpendicular to both the direction of current and direction of magnetic field. conductor moves towards the observer/ normal to the plane of paper in outward direction
- if direction of current through wire is reversed by interchanging the terminals of battery, direction of movement of wire is also reversed
- on reversing direction of magnetic field, direction of movement of wire is reversed
during Lorentz experiment, what if wire is short
when length AC is in a direction such that current in it is in the direction parallel to direction of magnetic field, no force acts on the wire and it does not move
what does magnitude of force acting on current carrying wire placed in magnetic field, in direction perpendicular to its length, depend on
- force is directly proportional to current I flowing in wire
- force F is directly proportional to strength of magnetic field B
- fore is directly proportional to length of wire within magnetic field
unit of magnetic field B
N/ A.m
OR
tesla (T)
OR
weber/meter^2 (Wb/m^2)
Fleming’s left hand rule
stretch forefinger, central finger, and thumb of your left hand mutually perpendicular to each other. if forefinger indicates direction of magnetic Field, and central finger indicates Current direction, thumb will indicate direction of motion of conductor
define electric motor
a device which converts the electrical energy into the mechanical energy
principle of electric motor
on passing electric current through a conductor placed normally in a magnetic field, a force acts on the conductor as a result of which the conductor begins to move and thus mechanical energy is obtained
when is deflecting couple on coil of dc motor max
when plane of coil is parallel to the direction of magnetic field, while the deflecting couple is zero