Electric Motor

Electric Motor

An electric motor is a rotating device that converts electrical energy into mechanical energy.

 

Uses of an Electric Motor

Electric motor is used as an important component in electric fans, refrigerators, mixers, washing machines, computers, MP3 players etc. 

Construction of an Electric Motor

An electric motor consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively, as shown in the Fig.

Working of an Electric Motor

Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y. Notice that the current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB. On applying Fleming’s left hand rule for the direction of force on a current-carrying conductor in a magnetic field. We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle O, mounted free to turn about an axis, rotate anti-clockwise. At half rotation, Q makes contact with the brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA. A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator. The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle.

The commercial electric motors use 

(i) An electromagnet in place of permanent magnet; 

(ii) Large number of turns of the conducting wire in the current- carrying coil; and 

(iii) A soft iron core on which the coil is wound. The soft iron core, on which the coil is wound, plus the coils, is called an armature. This enhances the power of the motor.

Class 10 Physics Chapter wise Synopsis

Light: Reflection and Refraction

• Light seems to travel in straight lines.

 

• Mirrors and lenses form images of objects. Images can be either real or virtual, depending on the position of the object.

 

• The reflecting surfaces, of all types, obey the laws of reflection. The refracting surfaces obey the laws of refraction.

 

• New Cartesian Sign Conventions are followed for spherical mirrors and lenses.

 

• Mirror formula, 

1/v + 1/u = 1/f

, gives the relationship between the object-distance (u), image-distance (v), and focal length (f) of a spherical mirror.

 

• The focal length of a spherical mirror is equal to half its radius of curvature.

 

• The magnification produced by a spherical mirror is the ratio of the height of the image to the height of the object.

 

• A light ray travelling obliquely from a denser medium to a rarer medium bends away from the normal. A light ray bends towards the normal when it travels obliquely from a rarer to a denser medium.

 

• Light travels in vacuum with an enormous speed of 3×10⁸ m s⁻¹. The speed of light is different in different media.

 

• The refractive index of a transparent medium is the ratio of the speed of light in vacuum to that in the medium.

 

• In case of a rectangular glass slab, the refraction takes place at both air-glass interface and glass-air interface. The emergent ray is parallel to the direction of incident ray.

 

• Lens formula, 

1/v - 1/u = 1/f

, gives the relationship between the object-distance (u), image-distance (v), and the focal length (f) of a spherical lens.

 

• Power of a lens is the reciprocal of its focal length. The SI unit of power of a lens is dioptre.

The Human Eye and the Colourful World

• The ability of the eye to focus on both near and distant objects, by adjusting its focal length, is called the accommodation of the eye.

 

• The smallest distance, at which the eye can see objects clearly without strain, is called the near point of the eye or the least distance of distinct vision. For a young adult with normal vision, it is about 25 cm.

 

• The common refractive defects of vision include myopia, hypermetropia and presbyopia. Myopia (short-sightedness – the image of distant objects is focussed before the retina) is corrected by using a concave lens of suitable power. Hypermetropia (far-sightedness – the image of nearby objects is focussed beyond the retina) is corrected by using a convex lens of suitable power. The eye loses its power of accommodation at old age.

 

• The splitting of white light into its component colours is called dispersion.

 

• Scattering of light causes the blue colour of sky and the reddening of the Sun at sunrise and sunset.

Electricity

• A stream of electrons moving through a conductor constitutes an electric current. Conventionally, the direction of current is taken opposite to the direction of flow of electrons.

 

• The SI unit of electric current is ampere.

 

• To set the electrons in motion in an electric circuit, we use a cell or a battery. A cell generates a potential difference across its terminals. It is measured in volts (V).

 

• Resistance is a property that resists the flow of electrons in a conductor. It controls the magnitude of the current. The SI unit of resistance is ohm (Ω ).

 

• Ohm’s law: The potential difference across the ends of a resistor is directly proportional to the current through it, provided its temperature remains the same.

 

• The resistance of a conductor depends directly on its length, inversely on its area of cross-section, and also on the material of the conductor.

 

• The equivalent resistance of several resistors in series is equal to the sum of their individual resistances.

 R = R₁ + R₂ + R₃

• A set of resistors connected in parallel has an equivalent resistance R given by

1/R = 1/R₁ + 1/R₂ + 1/R₃

• The electrical energy dissipated in a resistor is given by

 

• W = V × I × t

 

• The unit of power is watt (W). One watt of power is consumed when 1 A of current flows at a potential difference of 1 V. 

• The commercial unit of electrical energy is kilowatt hour (kWh).

 1 kW h = 3,600,000 J = 3.6 × 10⁶ J.

Magnetic Effects of Electric Current

• 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.

 

• A magnetic field exists in the region surrounding a magnet, in which the force of the magnet can be detected.

 

• Field lines are 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. Field lines are shown closer together where the magnetic field is greater.

 

• A metallic wire carrying an electric current has associated with it a magnetic field. The field lines about the wire consist of a series of concentric circles whose direction is given by the right-hand rule.

 

• The pattern of the magnetic field around a conductor due to an electric current flowing through it depends on the shape of the conductor. The magnetic field of a solenoid carrying a current is similar to that of a bar magnet.

 

• An electromagnet consists of a core of soft iron wrapped around with a coil of insulated copper wire.

 

• A current-carrying conductor when placed in a magnetic field experiences a force. If the direction of the field and that of the current are mutually perpendicular to each other, then the force acting on the conductor will be perpendicular to both and will be given by Fleming’s left-hand rule. This is the basis of an electric motor. An electric motor is a device that converts electric energy into mechanical energy.

 

• The phenomenon of electromagnetic induction is the production of induced current in a coil placed in a region where the magnetic field changes with time. The magnetic field may change due to a relative motion between the coil and a magnet placed near to the coil. If the coil is placed near to a current-carrying conductor, the magnetic field may change either due to a change in the current through the conductor or due to the relative motion between the coil and conductor. The direction of the induced current is given by the Fleming’s right-hand rule.

 

• A generator converts mechanical energy into electrical energy. It works on the basis of electromagnetic induction.

 

• In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. One of the wires in this supply is with red insulation, called live wire. The other one is of black insulation, which is a neutral wire. The potential difference between the two is 220 V. The third is the earth wire that has green insulation and this is connected to a metallic body deep inside earth. It is used as a safety measure to ensure that any leakage of current to a metallic body does not give any severe shock to a user.

 

• Fuse is the most important safety device, used for protecting the circuits due to short-circuiting or overloading of the circuits.

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