Magnetism Moving Coil Galvanometer in Physics from HelpWithAssignment.com

Magnetism Moving Coil Galvanometer

Torque on a current carrying loop suspended freely in a magnetic field.

When a loop is suspended freely in a magnetic field and a current is passed through it, we find that the resultant force on the loop is zero but the resultant torque is not zero.

Suppose, at any instant during the rotation of the loop normal drawn on the plane of loop or axis of the loop makes an angle θ with the direction of magnetic field. Hence moment of couple at this instant is given by

T = Force F₁ × Perpendicular distance

= ilB × b sin θ

= i(l × b) B sin θ

T = iAB sin θ

Where A = l × b = area of the current loop.

Important points

The above formula for torque is valid for any shape of closed loop i.e., circular and rectangular or any other. This can be easily justified by considering any loop to be divided into a large number of small adjacent rectangular loops.

If instead of a single current loop, we have a coil or solenoid consisting of large number of loops, then a couple of lAB sin θ will act on each loop. Thus, the torque or couple acting on a coil consisting of N turns is given by

T = NlAB sin θ

T = NlA × B = M × B

Where M = NlA = magnetic dipole moment of the current carrying coil.

Torque will be minimum = 0 when sin θ = min = 0, i.e., θ = 0˚ or 180˚ the place of the coil is perpendicular to magnetic field, i.e., normal to the coil is perpendicular to the field.

Torque will be maximum when sin θ = max = 1. i.e., θ = 90˚, i.e., normal to the coil is perpendicular to the field.

By analogy with electric or magnetic dipole in a field, in case of a current carrying coil in a field.

U = – M.B

W = MB (1 – cos θ)

Instruments such as electric motor, moving coil galvanometer and tangent galvanometers are based on the fact that a current carrying coil in a uniform magnetic field experiences a torque.

Moving Coil Galvanometer:

In case of a moving coil galvanometer the deflecting torque due to current in the coil BiNA (by making the field radial θ become 90˚) is balanced by the restoring couple due to elasticity of spring supporting the coil. So, if C is the restoring couple per unit twist and φ is the direction of the coil;

BiNA = C φ

Where, (K = C/NAB) is a constant for a given galvanometer called as Reduction factor of the galvanometer.

In this expression, N = number of turns in the coil, A = area per turn of the coil, B = magnetic induction of the radial magnetic field.

Elastic torsional constant C depends upon the three factors: 1. Modulus of rigidity of the material of suspension strip η. 2. Length of the suspension strip and. 3. radius of suspension strip. The dependence on above three factors is according to relation

C = π ηr⁴/2l

Sensitivity – sensitivity of a galvanometer can be expressed as

S = φ/I = 1/K = NAB/C

Higher is deflection produced in galvanometer even for a very small current, more sensitive is the galvanometer, ie higher is the value of (φ/i) higher will be the sensitivity of the galvanometer.

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Magnetism in Physics from HelpWithAssignment.com

Magnetism

A magnet is an object that produces a magnetic field. This magnetic field is responsible for attracting ferromagnetic materials such as iron and attracts or repels other magnets.

Magnets are of two types:

Natural Magnets and Artificial Magnets

• Natural Magnets are those magnets which are naturally found in mines. Due to their odd shape and weak attracting power, natural magnets are rarely used.
• Artificial Magnets are those magnets which are artificially prepared. These magnets exist in various shapes and sizes like a bar magnet, horse-shoe magnet or a magnetic needle.

The places in a magnet where its attracting power is maximum are called poles and the place where the attracting power is minimum is called neutral region. The distance between the poles along the axis of a magnet is called its effective or magnetic length. The line joining the two poles of magnet is called magnetic axis and the vertical plane passing through the axis of a freely suspended or pivoted magnet is called magnetic meridian.

Pole Strength: the strength of a magnetic pole to attract magnetic materials towards it is known as pole strength.

Pole Strength = Magnetic force / Magnetic Induction

Greater the number of unit poles in a magnetic pole, greater will be its strength. The unit of pole strength is ampere-meter. A pole of a magnet attracts the opposite pole while repels the similar. However, a sure test of polarity is repulsion and not attraction, as attraction can take place between opposite poles or a pole and a piece of unmagnetised magnetic material due to ‘induction effect’.

At the poles of the magnet the magnetic field is stronger because the lines of force there are crowded together and away from poles the magnetic field is weak. Therefore, the magnetic field intensity is proportional to the number of lines of force.

Magnetic field: The space around a magnet in which a net force acts on a magnetic test pole is known as magnetic field or the space around a magnet in which a torque acts on a magnetic needle is known as a magnetic field.

Magnetic Flux: The number of magnetic lines of forces passing through unit normal area is defined as magnetic induction whereas the number of lines of force passing through any area is known as magnetic flux.

Properties of Magnets:

• If a magnet is dipped into iron fillings, the fillings cling to it, maximum at the ends and least in the middle.
• The regions at the ends of the magnet, where the attraction of the iron filings is maximum and hence the magnetism is maximum are called poles.
• A bar magnet freely suspended through its centre of gravity always stays in the north-south direction.
• The end of the magnet pointing to geographic north is called ‘North Pole’ and the end pointing south is called ‘South Pole’.
• Like poles repel each other and unlike poles attract each other.
• A magnet induces magnetism in magnetic materials such as in a piece of iron and steel.
• An isolated magnetic pole does not exist, they always come in pairs.
• A magnet can loose its magnetic properties by beating, mechanical jerks, heating and with lapse of time.
• The pole strength of a magnet’s two poles is same.

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