Resistance:

The ability of material to resist the flow of current through it is called resistance. The resistance of the wire depends upon:

1. Length of wire
2. Area of wire
3. Temperature of wire
4. Nature of wire

Length of wire:  Resistance of wire is directly proportional to the length of wire

Area of wire: Resistance of wire is inversely proportional to the area of wire

Temperature of wire:  Resistance of wire is directly proportional to the temperature of wire

Nature of material:  Resistance of wire depends on the materials of which it is made. Copper, silver has low resistance because of the free electron present in them.

Resistivity and Condutivity:

 Resistance of a conductor is directly proportional to the length (L) of the conductor, i.e. : R ∝ L, and

Resistance is inversely proportional to its area (A) i.e., R ∝ 1/A. 

so, we get R  ∝ L / A

Now, R = ρ (L / A)  where, ρ is the proportionality constant called Resistivity.

And Conductivity = 1 / Resistance

Conductivity, σ is the reciprocal of the resistivity. That is 1/ρ and has the unit of siemens per metre, S/m. Conductivity ranges from zero (for a perfect insulator) to infinity (for a perfect conductor).

 

Earth’s Magnetic Field

Terrestrial Magnetism

The earth’s magnetism is called terrestrial magnetism. Declination, dip and the horizontal component of earth’s magnetic field are elements of terrestrial magnetism which helps to locate correct direction.

Evidences of terrestrial magnetism are as follows:

• A freely suspended magnet always point N-S direction by the influence of terrestrial magnetism on it.
• Some iron ores possess magnetism, which may be due to the effect of earth’s magnetism.
• Angle of declination and angle of dip are due to the effect of earth’s magnetism.
• Neutral points are obtained on the magnetic field of a magnet which is due to the combined effects of its magnetic field and earth’s magnetism.

Earth's Magnetic Field

A freely suspended bar magnet always comes to rest in a north-south direction. This shows that earth behaves as a huge magnet. The n-pole of a compass needle points towards geographical north showing that there is earth’s magnetic South Pole. Similarly, S-pole of the compass needle points towards geographical south showing that there is earth’s magnetic North pole.

The possible causes of earth’s magnetic field are given below:

• There is a permanent magnet inside the earth. This idea is ruled out. This is because the earth’s interior is at very high temperature and in this state nothing is magnetic.
• Earth’s crust contains magnetic substances, which may have produced the earth’s magnetism. This idea was also rejected because the earth’s crust does not contain sufficient magnetic substances.
• According to the latest theory, the magnetic field of the earth is caused by the magnetic current circulating in its core due to convection currents, arising from radioactive heating inside the earth.

Magnetism meridian and Geographic meridian:

The vertical plane passing through the axis of a freely suspended magnet at a place is called the magnetic meridian.

The vertical plane passing through a place and the geographical north and south poles is called the geographic meridian at that place.

Magnetic elements of earth’s magnetic fields

The quantities that are required to specify the magnetic field of the earth completely is called the magnetic elements of the earth.

Elements of earth's magnetic field are given below:

1. Angle of declination: The angle between the magnetic meridian and geographic meridian at a place is called angle of declination at that place.
2. Angle of dip or inclination: Angle of dip is the angle made by a freely suspended magnet at rest with the horizon.