Magnetic Effects Of Electric Current

The direction of force acting on a current-carrying conductor kept in a magnetic field is given by Fleming’s left hand rule.

Fig. : Fleming’s left hand rule

The relative strength of magnetic field is represented by the degree of closeness of the magnetic field lines.

If the magnetic field lines are close then, the strength of magnetic field is strong.

If the magnetic field lines are far away, then the magnetic field is poor in strength.

Check the diagram given below:

Earthing is the safety method used for protecting home appliances short circuiting or overloading in domestic electric fittings.

Check the diagram below:

On applying Fleming’s left-hand rule, the direction of positive charge will be vertically out of the page due to its interaction with the magnetic field.

If two magnetic field lines cross each other and a magnetic needle is kept at that point, it will show two directions of the magnetic field at that point. It is impossible for a magnetic field to have two or more directions at a point, so two magnetic field lines cannot cross each other.

An electric generator is based on the phenomenon of electromagnetic induction.
The phenomenon of electromagnetic induction states that whenever there is relative motion between a coil and the magnetic field around it, an emf is produced in the coil.

The slip rings of an AC electric generator should be replaced with split rings of the electric motor for it to become a DC generator.

An electric appliance having metallic body can have current leak and current may flow through its body. It can be fatal if a person touches its current carrying metallic body. So, it must be connected to an earth wire to pass the extra current, flowing through its body, to the earth plate.

The statement (i) is correct as the magnetic field produced around the conductor is proportional to the current flowing through it. More the magnetic field, more is the force exerted by it.
The statement (ii) is incorrect as it is against the laws of magnetism.

a.

b.

c.

Magnetic field lines originate from North pole and merge at the South pole outside magnet. Inside the magnet, they direct from South to North pole. So, they form a closed curve.

A particle is deflected by the field if it has charge. Since neutron is an uncharged particle,the field does not affect its direction of motion and it passes undeflected through the field.

An electric generator is based on the phenomenon of electromagnetic induction. In an electric generator, mechanical energy is converted to electrical energy.
The principle of electromagnetic induction states that whenever there is relative motion between a coil and the magnetic field around it, an emf is produced in the coil.

The wire of 20 A is thicker than that of 5 A. We know that the resistance of a wire is inversely proportional to the area of cross-section of the wire. Thus, the wire having higher current through it should have lesser resistance.

a. An electric fuse works on the heating effect of electric current. When a current higher than the rating of the fuse passes through it, the fuse melts. As the fuse is connected in series to the household appliances just after the main switch, it disconnects all the appliances from the main switch by melting. In this way, a fuse protects the appliances from excess current.

b. The frequency of A.C. in India is 50 Hz. An alternating current is advantageous over DC for long range transmission of electricity because it can be transmitted over long distances without much loss of electric power as compared to direct current.

(i) Whenever the key is plugged in or out, a momentary deflection is shown in the galvanometer. It is because of the phenomenon of electromagnetic induction.
(ii) Whenever the key is plugged in or out the magnetic field linked to the coil 2 changes and a current is induced in it.

The conditions required to obtain permanent electromagnet when a current carrying solenoid is used are:

● The rod inside solenoid should be made of a magnetic material like steel, soft iron etc. This type of material can retain the magnetic properties for a long time after magnetization.

● Direct current should be used for solenoid.

● The turns should be closely packed and large in number for producing a uniform strong magnetic field

Fig. : A current carrying solenoid

A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid.
The divergence of magnetic field lines near the ends of a current-carrying straight solenoid indicate a decrease in the magnetic field strength near the ends of the solenoid.

Fig.: Magnetic field lines around a current carrying solenoid

i. The magnetic field is directly proportional to the magnitude of current flowing through a current carrying coil. There is increase in the magnetic field strength with the increase in current.

ii. The distance of point P is inversely proportional to the strength of magnetic field at P. Thus, more the distance, lesser is the magnetic field strength.

iii. The number of turns is directly proportional to the magnetic field strength. More the number of turns, more is the magnetic field strength.

a.

In this setup, the magnetic field around a current carrying solenoid is shown. The field lines inside a solenoid are in the form of parallel straight lines. It indicates that the magnetic field is uniform inside the solenoid. One end of solenoid behaves as North pole and the other as South pole.
b.

The magnetic field lines are circular around a current carrying wire. These concentric circles become larger as we move away from the wire indicating decrease in the decrease in magnetic field strength with increase in the distance.
c.

The magnetic field due to current carrying loop is shown here. The magnetic field due to a current carrying straight wire is inversely proportional to the distance from it. They form concentric circles around the wire having increase in radius with distance. Similarly, at every point of the loop, the concentric circles would become larger as we move away from the wire. At the centre of loop, the arcs of these big circles appear as straight lines.

Fig. : AC generator
The axle of an AC generator is rotated such that one arm AB moves up and other CD moves down. The change in the magnetic field linked to the coil induces current in the coil. From Fleming’s right hand rule, the current flows from AB to CD in the coil during half rotation and from B₂ to B₁ in the external circuit.
After half rotation, the arm CD starts moving upward and AB downwards. So, the current starts flowing from CD to AB by Fleming’s Right hand rule. As a result the current starts flowing from B₁ to B₂ in the external circuit after half rotation.
In this way, the direction of induced current changes after each half rotation in an AC generator.

Fig. : Fleming’s right hand rule.

a. The bar magnet kept at the centre of board has its magnetic field around it. The iron filings sprinkled on the board experience a force on them due to the magnetic field of bar magnet. So, when the student taps the board the iron filings align themselves according to the magnetic field lines of the bar magnet.
b. The relative strength of magnetic field is shown by the degree of closeness of magnetic field lines. The iron fillings are crowded near the poles of the bar magnet. This shows that the magnetic field due to the magnet is maximum near the poles of the magnet.
c. The force exerted on the iron filings align themselves according to the magnetic field lines of the magnet. Thus, the lines along which the fillings align represent the magnetic field lines.
d.
e. The properties of magnetic field lines are:

● They are closed curves. They originate from North pole and merge at the South pole outside magnet. Inside the magnet, they direct from South to North pole.

● No two magnetic field lines can cross each other. If they do, it means that there are two directions of the field at that point which is impossible.

a. A magnetic field exerts force only on moving charges. So, a stationary charge kept in a magnetic field does not experience any force until it starts moving.
b. The direction of force experienced by a current carrying conductor placed in a magnetic field depends on:

● the direction of the current in the conductor

● direction of the magnetic field applied

c. The force experienced by a current carrying conductor placed in a uniform magnetic field is maximum when the current makes right angle with the magnetic field.

d. The force experienced by a current carrying conductor placed in a uniform magnetic field is maximum when the current makes right angle with the magnetic field.

i. When key K is pressed on, there is momentary deflection in the galvanometer. The needle of galvanometer instantly jumps to one side and quickly returns to zero. It shows momentary current through the coil 2.
ii. When current in the coil C₁ is switched off, there is momentary deflection in the galvanometer. The needle of galvanometer instantly jumps but to the opposite side and quickly returns to zero. It shows momentary current through the coil 2 in the opposite direction.
iii. When the current is passed continuously through coil C, there is no deflection in the galvanometer.
iv. The phenomenon of electromagnetic induction is responsible for the above observations. According to this phenomenon, whenever there is relative motion between a coil and the magnetic field around it, an emf is induced in the coil.
Fleming’s right hand rule is used to determine the direction of current in the above phenomena.