Physics 3rd year final exam 3

To increase the sensitivity of galvanometer, we use .....................?

• Two straight magnetic poles

• Uniform scale of equal divisions

• Spiral springs of high torsion

• Jeweled bearings

An observer is seeing the setup of an aluminum ring (B) facing an electromagnet (A) . The current (I) through (A) can be altered, so which statement is correct ?

• (B) is independent of an increase or decrease of (I)

• If I increase, (A) will attract (B)

• If (I) increase, (A) will repel (B)

• If (I) decrease, (A) will repel (B)

Primary coil is connected with (DC) source and is placed inside secondary coil. As a primary coil carrying a direct current is taken out from a secondary coil? The induced current will be:

• Forward induced current

• Backward induced current

• Alternating current

• Direct current

A.C. current produced from a dynamo, it reaches half the maximum value from zero position in a time (3sec) , then the required to reach the maximum value is:

• 24 sec

• 6 sec

• 12 sec

• 9 sec

By comparing. The number of turns of the primary coil and of the secondary coil in each of step-up transformer and in step-down transformer.

• A

• B

• C

• D

An ideal step-up transformer the ratio between number of turns of primary coil to that of secondary coil (1 : 3) and its secondary coil is connected with lamp work on (60V) , to operate the lamp, the voltage of primary coil must be:

• 10 V

• 20 V

• 30 V

• 40 V

In faraday's Experiment for electro- magnetic induction, the induced emf in the coil increases when:

• Keeping the magnet still inside the coil

• Speeding up the magnet motion relative to the coil

• Connecting a galvanometer to the coil

• Increasing the spacing between the coil turns

Two solenoids have same length, same radius and Same permeability. If the number of turns of the first solenoid is double , the number of turns of the second solenoid, the ratio between the self-induction coefficient of the first to the self-induction coefficient of the second is:

• 0.25

• 0.5

• 1.0

• 4

A bar magnet is released into a copper ring which is directly below it. Knowing that the acceleration due to gravity is (g). What about the acceleration of the magnet?

• Greater than (g)

• Equal to (g)

• Less than (g)

An air cored coil of (600 turns) has a self inductance of (90mH). If the number of turns is reduced to (200 turns) without changing the area of the coil, the self inductance will be:

• 30mH

• 270mH

• 180mH

• 90mH