Numbering jumbled-up in transportation, will be fixed later.

**SUB: PHYSICS, PUC II, midterm preparatory, 16.8.13**

**TIME: 3.15 Hrs MAXIMUM MARKS: 90**

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**GENERAL INSTRUCTIONS:**

**1.****All parts are compulsory****2.****Draw relevant diagram/ figure wherever necessary****3.****Numerical problems should be solved with the relevant formulae.**

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**PART-A**

## I. Answer all the following questions, 1 mark each (1×10)

**Which of the following is a vector?**

A. an electric dipole‘s electric potential ** **B. an electric dipole‘s charge

C. electric current D. electric current density

**Which medium will have a stronger electrostatic force between 2 charges?**

A. medium with ε = 12 B. medium with ε= 102

C. medium with ε= 81 D. free space, ε =ε_{0}.

**3.****Pick only the correct one “Between an electron and a positron”**- the electron has more specific charge.
- the positron has more specific charge.
- Both have equal specific charge.
- For some particles specific charge can’t be determined, so it can’t be said electron or positron has more specific charge.

**Chose the correct one****A.**Kirchhoff’s loop rule is applied to a junction of incoming and outgoing current**.****B.**According to Kirchhoff’s loop rule a junction of incoming and outgoing current carries a potential proportional to the amount of current flowing through the junction.- Kirchhoff’s Junction rule can be applied to a single current carrying path in any electric circuit.
- The potential drop in a loop in any electric circuit is zero and its based on conservation of energy.

**Chose the correct one**- Resultant of capacitance in series in more than individual resistances
- Current division works on the basis of direct proportionality of resistances
- Voltage division works on the basis of direct proportionality of resistances
- Resistors in series will add up

**Ohms’ law corresponds to the following**- Voltage and current are proportional even if Resistance varies.
- Conductivity is a ratio of current density and electric field.
- Resistances must add up in parallel combination.
- Linear relation between Voltage and current can be accommodated to an arbitrary range of resistances.

- If we define a
**system-of-unit**where permittivity of free space is 1 and Electric field intensity E value is 1, which is the correct value for energy density?

A. ½ B. 1/3

C. 2 D. 3

- If we define a
**system-of-unit**where capacitance of a configuration is 1 and potential difference of the same Is 1 what is the electrostatic energy of the configuration?

A. ½ B. 1/3

C. 2 D. 3

- If
**100 electrons**passing through an**unit cross section area****A**constitute a**current of 100 x 1.6 x 10**flowing across it, what’s the^{-19}coul/sec**drift velocity**of the electrons?

A. 1 unit B. 2 units C. 0.5 units D. 5 units

- A capacitor of 18
**µF**is filled with dielectric of**dielectric strength**K = 5. (Or permittivity**ε**). What’s the new value of**capacitance**, if a**Potential Difference V = 1000 Volt**is found on the capacitor?

A. 90 µF B. 18/5 µF

C. 5/18 µF D. indeterminate

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**PART-B**

## II. Answer any 10 out of 12 questions, 2 marks each. (2×10=20)

**1) **Define** “Temperature coefficient of resistance α” **of a metal**. **

**2) **Describe **de-Broglie wavelength** in 1 or 2 lines and write the formula.** **

3) Define **electric current **and** electric current density.**

4) Given **Bohr radius of Hydrogen atom; a**, what would be the radius of the 4^{th} excited orbit (n = 5).

5) Whats the **speed of an electron in a Hydrogen atom** in terms of speed of light c, if its revolving in the **5 ^{th} orbit; v_{n }=v_{5}, according to the Bohr’s model of an atom**?

6) Give example of **2 conservative forces**.

7) Define atomic number, atomic mass and neutron number. What’s mass of an alpha-particle in terms of amu.

8) Write out the expression of **n-th level** atomic spectra {**energy level**} of a Hydrogen atom. What energy will be released if the electron jumps from level = 4 to level = 1. ?

9) Write out the expression of **n-th level** atomic spectra {**energy level**} of a Hydrogen atom. What energy will be released if the electron jumps from level=5 to level = 2?

10) What’s the power of a lens whose **object distance is 15 cm** and whose **image distance is 55 cm**?

11) What’s the focal length of a convex lens if **object distance is 20 cm** and **image distance is 30 cm**.

12) What’s the **resistance** of a wire that obeys Ohm’s law if it carries a **current of 2.5 Amp** when a **Potential Difference of 230 volts** is applied across its ends?

**PART-C**

## This question is Compulsory (5×1)

**Question 1**

Define **electric current** and **electric current density**. State if these two quantities are scalar or vector quantities and explain why. State and explain **Ohm’s law**. Derive the Ohm’s law in vector form from the scalar form.

## Answer any six from Question 2 – 8. (5×6)

**Question 2 **

Derive the **Electric potential V **for the **Dipole Vector** **P,** V_{||}; in a **direction of alignment with the Dipole Vector**, also called **End-On configuration. **Show that the electric field due to a dipole vector is zero along a line perpendicular to it.

**Question 3 **

Derive the magnetic field due to a torroid by using **Ampere’s circuital law**.

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**Question 4 **

Define **Electric flux ****F (Phi)**. State and explain **Gauss Law** in terms of charge and flux. Apply the Gauss law to the case of an infinitely straight distribution of charge with linear density of charge λ. Mention 4 properties of the lines of forces. What is signified by the fact that lines of forces are densely packed? Why lines of forces must not intersect?

**Question 5**

Derive the expressions for combination of 3 capacitors in **series** and **parallel**. What’s the capacitance of a combination of two capacitors each with capacitance **12 µF** connected in **series**? What’s the value if these are **connected in parallel**? Derive the **energy** associated with a capacitor.

**Question 6 **

Derive the expression for the capacitance of a configuration where two plates are kept distance **d** apart when a material of thickness **t **with dielectric constant **K** is inserted in-between.

**Question 7**

Derive the **magnetic field B** of a Solenoid by using **Ampere’s circuital law**.

**Question 8 **

State the **Ampere circuital law** and define **magnetic flux**. State the Maxwell’s cork screw rule. From the Lorentz Force expression derive the force acting on a current element of length **l** by a magnetic field **B**.

## This question is Compulsory (5×1)

**Question 9**

Describe the mechanism of a cathode ray tube with a neat and labeled diagram. Write a short history of how electrons were discovered. What is X-ray emission? What are K_{a} and K_{b} lines?

## Each carry 5 marks, answer only two from Questions 10 – 12 (5×2)

**Question 10 **

Describe the Bohr model for atom and point out its drawbacks. Write the nth level energy of a Hydrogen atom. Calculate the emission energy of a Hydrogen atom if an electron jumps from n=8 to n=1 level. Explain the origin of **K**** _{a}** and

**K**

**lines through electronic transitions.**

_{b}

**Question 11 **

Describe the **Millikan’s experiment for **electron charge e**.** **Derive **in necessary steps how **e** would be determined by this set up from various known parameters. Explain your steps with the concepts.

**Question 12**

Threshold wavelength of Tungsten is 2400 Angstrom. Tungsten is illuminated by a light of wavelength 1600 Angstrom. Whats the value of **work function**, maximum **kinetic energy of electrons** and **stopping potential? **Describe the ideas of Photoelectric effect.

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**PART-D**

## Answer both questions (5×2)

**Question 13.**

What is Photoelectric effect? Explain what is **work function**, maximum **kinetic energy of electrons**, **stopping potential **and** threshold frequency in Photo electric effect. **Describe the Thomson’s e/m experiment with neat diagrams and obtain electron speed v = E/B.

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**Question 14. **

Draw a simple and neat diagram showing a **convex lens, optical center and focal point** {Focus}. Now define **power of a lens** and obtain an expression for it. If u = 10 cm and v = 15 cm, whats the power of the lens?