Case Study 2 - Electric motor

 Case Study 2 : Electric Motor

(For solution of the case study, click the link: Online case study 2 test. Solution will be declared after attempting online test)

Read the following and answer any four questions from 1 to 5.

 In 1821, Michael Faraday made the first electric motor. It worked using the force of magnetism. He created a simple electromagnet by taking a nail and a wire, wrapping about 100 loops of wire around the nail and connected it to a battery. With this, he had a simple electromagnet with north and south poles. In the middle of the nail, he had made a hole and put a spindle into the hole so that the nail could rotate. He then took a horseshoe-shaped magnet and placed the wire wrapped nail in the middle. He connected the wire of the north pole to the negative pole of the battery and the wire of the south pole to the positive pole. The basic law of magnetism told him what would happen: the north end of the electromagnet would repel the north end of the horseshoe-shaped magnet and would attract the south pole. The same happened on the other side of the nail, with the result that the nail turned. Faraday was not happy with the result of the electromotor, because the motor turned only once. He changed the polarity of the battery and the wire wrapped nail again turned just once. If he changed the polarity every time, when the north pole of the wire wrapped nail is opposite to the south pole of the horseshoe-shaped magnet, then he would have the result he was looking for. The wire-wrapped nail would turn and turn around the spindle (as long as the battery is not empty). 

                                                Design of simple electric motor

1. What type of energy conversion takes in an electric motor? 

(a) Electrical to mechanical 

(b) Mechanical to electrical 

(c) Mechanical to magnetic 

(d) Magnetic to electrical 

2.  On what principle does the working of the electric motor is based? 

(a) Fleming’s left hand rule 

(b) Electric current is induced in a coil when a magnet is moved closer or away from it. 

(c) A current carrying wire experiences a force in a magnetic field. 

(d) Magnetic field lines around a current carrying wire are concentric circles 

3. Faraday was not happy with his invention because his motor turned only once. What modification in his model would correct this defect? 

(a) Using a stronger horseshoe magnet 

(b) Using a high voltage battery 

(c) Using a split ring which can change the direction of current in the wire 

(d) Using nails was a bad idea as it is a magnetic material so using a non-magnetic material such as wood would correct this defect. 

 4. For maximum output of electric motor, What should be the angle between the direction of magnetic field and current carrying arms (AB and CD)?

 (a) 0° 

(b) 90° 

(c) 180° 

(d) it is independent of angle between magnetic field and current carrying arms 

5. In the figure shown with the passage, if arm AB of the coil experiences a force in a downward direction, then what would be the direction of the force on arm CD

 (a) downward 

(b) upward 

(c) same as the direction of the magnetic field 

(d) opposite to the direction of the magnetic field

Case Study 1 - Spherical Mirrors

 Read the following passage and answer the questions that follows:

Anyone who has sat in the driver's or passenger's seat in a car has probably noticed the tiny wording, "Objects in mirror are closer than they appear," that runs along the bottom of the passenger-side rear view mirror. Some may question the logic of making objects seem like they are farther away than they actually are, but the reason is that such mirrors give the driver the best range of vision. Curved outwards spherical mirrors are commonly used as rear-view (wing) mirrors in vehicles because they give an erect, virtual, full size diminished image of distant objects with a wider field of view. These mirrors enable the driver to view a much larger area than would be possible with a plane mirror. 

Q1 Which type of mirror is used as a rear-view mirror in automobiles. 

(a) Plane mirror 

(b) Concave mirror 

(c) Convex mirror 

(d) Distorted mirror 

Q2  A bus of height 3m appears as a diminished image in the rear-view mirror of a car. The driver estimated the approximate image size of the bus to be just 3 cm in the mirror. What is the magnification produced by the mirror? 

(a) 1 

(b) 1/100 

(c) 100 

(d) 0.1 

Q3  The radius of curvature of the rear-view mirror is 400 cm. What is its focal length? 

(a) – 2m 

(b) – 400 cm 

(c) – 200 cm 

(d) 200 cm 

Q4 What is the advantage of using a curved-outwards spherical mirror as a rear view mirror? 

(a) it provides a shorter field of view. 

(b) it provides a much wider field of view 

(c) it provides a full size enlarged image 

(d) it provides a real image thus the driver is sure that the image is not fake.

Q5 The rear-view mirror of Sonali’s car was broken. In a hurry, she replaced the broken rear-view mirror with a concave mirror of the same size and focal length of 20 cm. What she would likely to observe in her mirror while driving? 

(a) She would observe no change because it has the same size as the previous one 

(b) She would observe an inverted image of traffic behind her car 

(c) She would observe an enlarged image of traffic behind her car 

(d) She would observe a virtual image of traffic behind her car

To know the answers of above questions, attempt the online test case study 1