1 Magnetic Effects of Current and Magnetism
A magnet of magnetic moment M is lying in a magnetic field of induction B. W1 is the work done in turning it from 0o to 60o and W2 is the work done in turning it from 30o to 90o.Then 1) W2 =W1/2 2) W2 = 2 W1? 3) W2 = W1 4) W2 = √3W1
2 Kinetic Theory of gases
If the degrees of freedom of a gas are f, the ratio of its two specific heats, CP/CV, is given by: 1) 1 - (1/f) 2) 1 – (2/f) 3) 1 + (1/f) 4) 1 + (2/f)
3 Capacitance
For the circuit shown in figure, which of the following statement is true?
1) with S1 closed V1=15V, V2 = 20 V
2) with S3 closed V1=V2 = 25 V
3) with S1 and S2 closed V1 = V2=0
4) with S1 and S3 closed V1 = 30 V, V2=20 V
charges on the capacitor are
q1=(30)(2)=60 μF
q2=(20)(3)=60 μF
⇒ q1=q2=q (let)
When s3 is closed, due to attraction with opposite charge, no flow of charge take place through s3 Therefore, potential difference across capital plates remain unchanged i.e. v1=30 v and v2 = 20 v
4 Current Electricity
A 240-Volt appliance draws 15A of current for a period of 2 hours. How much energy in kilowatt-hours does it consume?
1) 7.2
2) 2 .6 x107
3) 7200
4) 3600
5 Alternating Currents
Find the value of current through an inductance of 0.7H and negligible resistance when connected to an ac source of 220V and frequency 50Hz. Take π=227. 1) 1 A 2) 4 A 3) 8 A 4) 2 A
For pure inductive circuit, Current lag by π2
6 Laws of Motion
What force, F, will be needed such that the uniform beam , pivoted at its middle point, will balance horizontally? (1) 100N (2) 300N (3) 400N (4) 900N
7 Capacitance
Combination of two identical capacitors, a resistor R and a dc voltage source of voltage 6V is used in an experiment on a (C-R) circuit. It is found that for a parallel combination of the capacitor the time in which the voltage of the fully charged combination reduces to half its original voltage is 10 second. For series combination the time needed for reducing the voltage of the fully charged series combination by half is?
1) 2.5 second
2) 20 second
3) 10 second
4) 5 second
q = q0et/τ
⇒q=q0/2⇒=t=Γln2 t = R(2C)ln2 for first case t = R(C / 2)ln2 for second case ∴ t = 2.5 sec.
8 Current Electricity
Equal potentials are applied on an iron and a copper wire of same length. In order to have the same current flow in the two wires, the ratio r(iron) / r(copper) of their radii must be: [specific resistance of iron = 1.0 ]
1) 2.4 : 1 2) 1 : 2.4 3) 1.2 : 1 4) 1 : 1.2
Solution
To have same current flow, Resistance should be same.
and we know that R= ρl? /πr2
Keeping same resistance r2 ∝ ρ ?r∝ √ρ
rCopper / r ?iron??= √(ρcopper? /ρiron???)
≈ 2.4
9 Thermodynamics
A kettle with 2 litre waters at 27�C is heated by operating coil heater of power 1 kW. The heat is lost to the atmosphere at constant rate 160 J/sec when its lid is open. In how much time will water is heated to 77�C (Sp heat of H2O = 4.2 kJ.mole )
(1)8 min 20 sec (2) 6 min 2 sec (3) 14 min (4) 7 min
10 Current Electricity
Two conductors A and B are joined by a copper wire. If A is positively charged and B is uncharged, the direction of flow of electrons is 1) B to A 2) A to B 3) no flow 4) none of these
The current in the wire is due to the drifting of electrons inside a wire in a direction opposite to the flow of electrons. During their drifting they collide with their atoms vibrating about their mean position and lose some of kinetic energy to the vibrating atoms. The electrons are negatively charged particles and so, the electrons move towards the positively charged conductor A. Hence, the direction of the flow of electrons in the copper wire is from B to A.
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