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## Magnetism Exam2 and Problem Solutions

**Magnetism Exam2 ****and Problem Solutions**

**1.**

**a) **Find whether the transformer is step down or step up.

**b)** Find V_{1}/V_{2}

**c)** Find whether secondary voltage depends on R or not.

**)** **Since the number of turns in primary side is larger than the secondary side, transformer is step down.**

**b) V _{1}/V_{2}=N_{A}/N_{B}=5N/N=5**

**c)** **V _{1}/V_{2}=N_{A}/N_{B}**

**V _{1}=V_{2}.N_{A}/N_{B}**

**Thus, V _{1} does not depend on resistor R.**

**2.** If the system given below is in equilibrium, find the poles of magnets.

**B attracts C and repels A. Thus;**

**If B is N, then C must be S and A must be N**

**If B is S, then C must be N and A must be S**

**3.** Force F acting on the AC wire, placed in a magnetic field B, is shown in the picture. Find the force acting on CB wire having length L.

**We find direction of force by right hand rule.**

**Direction of force is outward.**

**F _{AC}=B.I.2L=F**

**F _{CB}=B.2I.L=F**

**So, force acting on CB wire has magnitude F and, it is in -X direction.**

**4.** Three wires A, B and C are given below. If force acting on the one part of B exerted by A is F. Find resultant forces acting on wire C.

**Since directions of currents on A and B are opposite in directions, A repels B.**

**F _{AB}=2k.(2I.I/d).L=F**

**A attracts C, B repels C**

**F _{AC}=2k.(2I.I/2d).L**

**F _{BC}=2k.(I.I/d).L**

**F _{AC}=F_{BC}**

**Resultant force acting on C is zero.**

**5.** If the magnetic field at point O is zero, find the ratio of r_{1}/r_{2}.

**Magnetic field of circle having radius r _{1};**

**B _{1}=1/2.(2π.k.I/r_{1}) inward**

**Magnetic field of circle having radius r _{2};**

**B _{2}=1/4.(2π.k.I/r_{2}) outward**

**Since resultant magnetic field at point O is zero;**

**B _{1}=B_{2}**

** 1/2.(2π.k.I/r _{1})=1/4.(2π.k.I/r_{2})**

**r _{1}/r_{2}=2**

Author: Mrs. Şerife (Erden) SARICA

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