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Magnetic effects of electric current


Compass needle
A compass needle is a small magnet. Its one end, which points towards north, is called a north pole, and the other end, which points towards south, is called a south pole.

Relation between Electricity and magnetism (Oersted’s Experiment)

The first evidence of any connection between Electricity and magnetism was established by Hans Christian Oersted. He accidentally discovered that as he laid a wire carrying an electric current near a magnetic compass needle, it got deflected as if acted upon by a magnet.
This observation led to the discovery that when current passes through a conductor, magnetic field is produced around it.

Magnetic field

            The space around a magnet or a current carrying conductor, in which the force of attraction or repulsion can be experienced, is called a magnetic field.

Demonstration of magnetic field lines (Iron-filings pattern)

1.  Take a bar magnet and placed it on a cardboard.
2.  Sprinkle some iron-fillings around the magnet.
3.  Tap the cardboard gently.
4.  Iron fillings arrange themselves in a pattern as shown in figure.

Conclusion: This pattern demonstrates that under the influence of magnetic field, the fillings align themselves along the magnetic field lines.

Tracing of magnetic field lines of a bar magnet

1.  Place a bar magnet on a sheet of paper.
2.  Bring the compass near the north pole of the magnet.
3.  The needle will deflect such that its south pole points towards North Pole of the bar magnet.
4.  Mark the position of two ends of needle.
5.  Move the compass so that its south end occupies the position previously occupied by the north end.
6.  Again mark the new position of ends of needle.
7.  Repeat step 5 and 6 till you reach the south pole of the magnet.
8.  Join the points marked to get a smooth curve, which represents a field line.

Magnetic field lines
Magnetic field lines are the imaginary lines used to represent a magnetic field. A field line is the path along which a hypothetical free north pole would tend to move. The direction of the magnetic field at a point is given by the direction that a north pole placed at that point would take.

Properties of Magnetic field lines
1.   Outside the bar magnet, the magnetic field lines originate from the north pole of a magnet and end at its south pole.
2.   Inside the bar magnet, field lines move from South Pole of the magnet to the North Pole.
3.   Magnetic field lines always form closed curves.
4.   The regions, where field lines are closer, the field is strong and the regions, where the field lines are farther apart, the field is weak.
5.   The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it.
6.   The magnetic field lines never cut each other. In case, two field lines intersect each other at a point, then it will mean that at the point of intersection, the magnetic needle would point in two different directions, which is not possible.

Magnetic field around a current carrying straight conductor
1.   Take a straight wire AB and pierced it through a horizontal cardboard such that wire AB is vertical.
2.   The ends of the wire AB are connected to a battery.
3.   Place some iron fillings on the cardboard.
4.   Switched the key on.
5.   Gently tap the cardboard.
6.   The iron fillings arrange themselves in concentric circles around the wire.
7.   This shows that magnetic field lines are concentric circles. The circles become larger and larger as we move away from the wire.

Direction of field lines- Right Hand thumb rule
Imagine that you are holding a current carrying wire in your right hand such that the thumb is stretched along the direction of the current, then, the fingers will wrap around the conductor in the direction of the field lines of the magnetic field.

Factors on which the magnetic field at a distance from the straight wire depends on

1.                  Directly proportional to the current flowing in the wire.
2.                  Inversely proportional to the distancefrom the wire.

Using Right Hand thumb rule, the magnetic field lines at every point of the circular wire are in the form of concentric circles with wire as the center. These magnetic field lines become larger and larger as we move away from the wire. Just at the center of the coil, magnetic field lines are almost straight. By applying Right hand rule, it is easy to check that every section of the wire contributes to magnetic field lines in the same direction within the loop.

Factors on which the magnetic field at the centre of the circular loop depends upon

1.                  Directly proportional to the current flowing in the loop
2.                  Inversely proportional to the radius of the circular wire.
3.                  Directly proportional to the number of turns of the circular loop. This is because the current in each circular turn has the same direction, and the field due to each turn then just adds up.

A coil of many circular turns of insulated copper wire wrapped closely in the form of a cylinder is called a solenoid.
The magnetic field due to a solenoid is very much similar to that of a bar magnet. The patternof the magnetic field lines around a current-carrying solenoid is similar to that of bar magnet.Just like a bar magnet, one end of the solenoidbehaves as a magnetic north pole, while the other behaves as theSouth Pole. The field lines inside the solenoid are in the form ofparallel straight lines. This indicates that the magnetic field isthe same at all points inside the solenoid. That is, the field isuniform inside the solenoid.

A strong magnetic field produced in a solenoid can be used to magnetize a piece of magnetic material when it is placed within the coil, which is carrying electric current.

1.         Take a bunch of iron nails.
2.         Wrap a coil of insulated copper wire on it.
3.         Connect the coil to a battery through a switch.
4.         As the current is passed through the coil, the bunch of nails, which acts as a core inside the solenoid, gets magnetized.
The magnet so formed is called an electromagnet.

            An electromagnet consists of a long coil of insulated copper wire wound on a soft iron core.

            If the core of the solenoid is taken of soft iron and electric current is passed through the solenoid, the soft iron core is temporarily magnetized which means when the current is switched off soft iron loses its magnetic properties. An electromagnet is a temporary magnet.

            If the core of the solenoid is taken of carbon steel, chromium steel, cobalt and tungsten steel and certain alloys like Nipermag (alloy of iron, nickel, aluminium and titanium) and ALNICO (alloy of Aluminium, nickel and cobalt) and a strong electric current is passed through the coil then these materials become permanently magnetized.
Uses: Such permanent magnets are used in microphones, loudspeakers, electric clocks, ammeter, voltmeter and speedometer, etc

Any current carrying conductor when kept in magnetic field experiences a force. The direction of force is given by FLEMING’S LEFT HAND RULE.

1.         Take a small aluminium rod AB.
2.         Suspend it horizontally with the help of connecting wires from a stand.
3.         Place a strong horseshoe magnet in such a way that the rod is between the two poles with the field directed upwards.
4.         When current is passed in the rod from B to A, the rod gets displaced towards left.
5.         On reversing the direction of the current, the rod gets deflected towards right.
The deflection in the rod is caused by the force acting on the current carrying rod when placed in a magnetic field.
The displacement of the rod is largest (or the magnitudeof the force is the highest) when the direction of current is at right anglesto the direction of the magnetic field. In such a condition we can use asimple rule to find the direction of the force on the conductor.

Stretch the forefinger, the central finger and the thumb of your left hand mutually perpendicular to each other. If the forefinger shows the direction of the field and the central finger that of the current, then the thumb will point towards the force or direction of motion of the conductor.

A current carrying conductor experiences a force when placed in a magnetic field. As current is simply flowing of charges, it implies that moving charged particles also experiences a force in a magnetic field.
The direction of the force on a moving positive charge is given by Fleming’s Left hand rule.

Application of force experienced when placed in a magnetic field
Devices that use current-carrying conductors and magnetic fieldsinclude electric motor, loudspeakers, microphones and measuring instruments.


Magnetic effects of electric current (ppt)

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