# 2nd exam, 2007.

(Angfiz2Vizsga2007 szócikkből átirányítva)
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Ha nem tudod, hogyan indulj el, olvasd el a migrálási útmutatót

-- psigy - 2008.01.13.

## I. Problems (2 points each):

#### 1. A current of 10A is flowing in a clockwise direction in a circular loop having a radius a = 2cm. Another current of 10A is flowing in the opposite direction in another, concentric circular loop having a radius b = 4.5cm. Find the magnetic field at the center.

(a) 174T (b) 235T (c) 450T (d) none

$\sum B= |B_1 -B_2|$

$B_1= \frac {\mu_0*I_1}{2*r_1} = 1,3962*10^{-4}$

$B_2= \frac {\mu_0*I_2}{2*r_2} = 3,141*10^{-4}$

$B_2-B_1 = 1,74*10^{-4}$

• a*

#### 2. A long, straight wire carries a current of 10A. At a distance of 1cm from the wire a point charge of 100nC is moving perpendicularly to the wire with 106 m/s instantaneous speed. Find the Lorentz-force acting on the point charge at this point.

(a) 0.02mN (b) 0.1mN (c) 1.65mN (d) none

$\ B= \frac {\mu_0*I}{2*\pi*R}$

$F= Q*v*B$

• a*

#### 3. A current of 2A flows in a cylindrical wire having a radius of 1mm. How far from the axis of the wire (outside the wire) does the magnetic field have the same value as at a distance 0.5mm from the axis?

(a) 1.25mm (b) 1.5mm (c) 2mm (d) other

#### 4. Find the magnetic energy stored in a toroid whose central line is 1m long, the cross sectional area is 10cm2 and the current is 2A. The toroid has 1000 turns.

(a) 0.46mJ (b) 0.91mJ (c) 2.52mJ (d) none

#### 5. 10 turns of wire are tightly wound around a solenoid having a length of 12.56cm and a cross-sectional area of 5cm2. The number of turns of the solenoid is 200. Find the mutual inductance.

(a) 5H (b) 10H (c) 15H (d) none

#### 6. Two slits at a distance of 0.4mm from each other are illuminated with blue light (=400nm). Find the distance between the interference fringes if the screen is 1 meter away from the slits.

(a) 1mm (b) 2mm (c) 10mm (d) none

#### 7. Calculate the ratio between (1) the power incident on a 4mm2 surface at a distance 1 meter from a 25W light bulb, and (2) the power incident on the same surface if it is illuminated by a 1mW laser. The diameter of the laser beam is 1mm.

(a) 1/8 (b) 1/53 (c) 1/126 (d) none

#### 8. An electron is confined to a 1D box in which its ground state energy is 2eV. How much energy is required to excite the electron into the next energy state?

(a) 2eV (b) 4eV (c) 6eV (d) none

#### 9. Determine the number of electrons that can occupy the n=3 shell in an atom.

(a) 9 (b) 18 (c) 36 (d) none

#### 10. Find the smallest possible value of   the angle between the orbital angular momentum vector and an external magnetic field  for an electron in a 3d state of hydrogen.

(a) 35 (b) 45 (c) 55 (d) none

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## III. Theory (3 points each):

#### 5. What is the physical meaning of the square of the absolute value of the wave function?

-

$q_e=1.6*10^{-19}C$

$m_e=9.1*10^{-31}kg$

$\mu_0=4\pi*10^{-7}\frac{Vs}{Am}$

$E_0=8.85*10^{-12}\frac{As}{Vm}$

$h=6.626*10^{-34}Js$