Working of electric motor

ELECTRIC MOTOR

An electric motor is a rotating device which converts electrical energy into mechanical energy.
It means it takes energy from electricity and using this energy the motor system rotates its rotator. The motion of rotator means that it possesses mechanical energy.
It appears so simple but we have to understand the process by which this energy change take place.

Image Credit -Lookang many thanks to Fu-Kwun Hwang and author of Easy Java Simulation = Francisco Esquembre, Ejs Open Source Direct Current Electrical Motor Model Java Applet ( DC Motor ) 20 degree split ring, CC BY-SA 3.0

The basic principle behind the working of motor is that when a current carrying wire is placed in a magnetic field it experiences a force. The direction of this force can be determined by Fleming’s left hand rule.

Thus using electrical energy we setup an electric current in a coil
and in an electromagnet. The electromagnet thus behaves like a magnet. The current carrying coil when placed in magnetic field of electromagnet experiences a force. Using suitable arrangement and designing, the coil can be made to rotate.

Construction
A simple electric motor consists of a rectangular coil ABCD of insulated copper wire placed between two opposite poles magnets as shown in the figure. The ends of the coil are connected to two half of a split ring (S1 and S2) attached to the axle. The split rings are connected to two carbon brushes B1 and B2 as shown in the figure. The carbon brushes are connected to a battery through connecting wire and a key (or switch).

©Udvita.org

Working:
First Half Cycle

Let the plane of coil is initially placed horizontally as shown in the figure. The direction of current in the coil is along ABCD. The direction of magnetic field is from North pole to the south pole.
By applying Fleming’s left hand rule on arm AB, the direction of force on arm AB is downward. Similarly the direction of force on arm CD is upward. Under the action of two equal and opposite will make the coil mounted on an axle to rotate anticlockwise.

Second Half Cycle
After half a rotation, arms AB and CD will interchange its position. The split ring S1 is now in contact with brush B2 and the split ring S2 is in contact with brush B1. The direction of current in the coil is now DCBA, reversed as compared to first half cycle. A device which reverses the direction of current in a circuit is called commutator. In electric motor, split rings acts as commutator.

©Udvita.org

By applying Fleming’s left hand rule on arm AB, the direction of force on arm AB is upward. Similarly the direction of force on arm CD is downward. Again under the action of two equal and opposite will make the coil mounted on an axle to rotate anticlockwise.

Commercial motor
A commercial motor consists of an electromagnet instead of permanent magnets. The current carrying coil consists of a large number of turns (in thousands). A soft iron core is used on which the coil is wound.
The soft iron core along with the coil is called the armature.

Image credit-Abnormaal, Electric motor, CC BY-SA 3.0

Uses of electric motor
Electric motor is used in electric fans, water pumps, mixer, MP3 player, computer etc

Simplest Electric Motor
Watch the Simplest electric Motor video made by our YouTube Channel partner 'Learn n hv Fun'.
The motor is simply made using a copper coil and few neudymium magnets using a 1.5V  electric cell.

Human Eye - Accomodation

HUMAN EYE – ACCOMMODATION

A human eye with normal vision can see nearby objects as close as 25cm clearly and distinctly and far away objects as far as infinity clearly without any strain in eyes. 

To see an object clearly the eye lens must focus the image on the retina. This sharp focusing is achieved by eye lens with the help of ciliary muscles. The ability of eye lens to adjust its focal length to focus objects at different distances on the retina is called accommodation.

How Accommodation is achieved?

An eye lens is a jelly like material made up of protein. The curvature/shape of the eye lens can be changed to some extent by the action of ciliary muscles. When ciliary muscles contract, curvature of eye lens increases. This decreases focal length of eye lens. This enables us to see nearby objects clearly. 

Similarly when ciliary muscles relax, curvature of eye lens decreases. This increases focal length of eye lens. This enables us to see far away objects clearly. 

The following steps would be easier to understand the process.

To view nearby objects:

Ciliary muscles contracts → curvature of eye lens increases → eye lens becomes thicker → focal length of eye lens decreases → image of nearby objects focused on the retina 

The above action of ciliary muscles and eye lens enables us to see nearby objects clearly.

To view far away objects:

Ciliary muscles relaxes in this situation → curvature of eye lens decreases → eye lens becomes thinner → focal length of eye lens decreases → image of far away objects focused on the retina. 

The above action of ciliary muscles and eye lens enables us to see far objects clearly.

The power of accommodation for a normal vision is 4 dioptre.

The power of eye lens when a nearby objects is viewed is 44 D. The power of eye lens when a far away object is viewed is 40 D. Thus the maximum variation in the power of eye lens achieved is 44D – 40D = 4D. 

Least distance of distinct vision (Near point)

The minimum distance at which the object can be see clearly and distinctly without any strain in the eyes is called the least distance of distinct vision or near point of the eye. 

For a person with normal vision this value is 25 cm.

What would happen if the object is closer than 25 cm? Would any image be formed on eye lens?

Yes, the image would be formed but that will be blurred, not well focused. Try focusing on the object. Some of you may be able to focus the object but if would cause strain in your eye after few seconds. 

Try holding a pen or a pencil close to your eye or nose. At first the object will appear blurred but if you try to focus object may get focused to some extent causing strain in your eye. You have to either shift your focus or have to move away the object from you.

Reason:

The curvature of the eye lens can be change up to some extent only and that with the help of cilicary muscles. Thus focal length of eye lens cannot be decreased beyond a certain minimum limit. Hence there is minimum distance at which objects can be seen comfortably. That minimum comfortable distance is 25 cm. This is called near point or least distance of distinct vision.

Far point

The farthest point up to which a human eye with normal vision can see is infinity and this is called the far point of human eye. 

Try focusing a star. These stars are hundred, thousand light years from us. The nearest star Alpha centuri is 4.2 LY away. ( 1 LY = 9.46 X \(10^{15}\) m)

Question of the Day 25 Nov 2017

Neha's father was reading the manual of a camera which was in very small print. He was not able to read comfortably. Neha had two convex lens A and B of focal length 25 cm and 40 cm respectively. She gave the better of the two lenses to her father. He then read the manual comfortably without any strain in his eyes.

Now answer the following question.
(a) Which one of the two convex lenses did she gave to her father and why?
(b) What is the power of the lens A and B?
(c) What is the range of the distance that his father can keep the manual from the lens for comfortable reading?
(d) Unfortunately while handling the better of the two lenses, it got slipped from her father hand and broke into two piece. Will her father still be able to use the broken pieces for magnifying the small prints? 

Question of the day 15 Nov 2107

Question of the day 15 Nov 2017

A student has two resistors A and B of resistance 4 ohm and 6 ohm respectively.  The student was challenged by the Physics teacher to connect them to a 12 V battery so as to draw maximum current from the voltage source.

How would you advice the student to connect the resistors and what would be the current obtained?