**PHY1161
Exam #2
October 7, 2002
**

High: 80

Mean: 41

Low : 5

1.What is theof a series RCL circuit with a 15 resistor, a 250 µF capacitor, and a 200 mH inductor?resonance frequency

Whatwill flow in the circuit if it is connected to a 20 V supply at this resonance frequency?current

Whatwill flow if thecurrentfrequencyis increased by 30%?

^{ }

2.A 25–turn, circular coil of wire, with an area of 225 cm^{2}is placed in a 0.075 T magnetic field. The magnetic field is perpendicular to the coil and points into the page as shown in the figure above. The magnetic field strength increases uniformly so that in 5.0 seconds it has a value of 0.125 T.

What is the area of the coil in units of m^{2}?

What-- or emf -- is induced in this coil?voltage

In whichdoes the induced current flow?directionWe all

knowthat 100 cm = 1 mThe "trouble" starts when we need to convert

areasinstead of merelydistances!A = 225 cm

^{2}A = 225 cm

^{2}[^{1 m}/_{100 cm}]^{2}Be sure the

whole expressionis squared!A = (

^{225}/_{10 000}) m^{2}A = 225 x 10

^{ - 4}m^{2}A = 0.0225 m

^{2}

3.A twelve–turn current coil carries 0.60 A of current in a region where the magnetic field is 0.450 T. The coil is a circle of radius 2.5 cm. Find theexerted on the coil when it is rotated so the normal to the coil makes an angle of 37° with the magnetic field.torque

(Hint:Be careful of the units -- find the area of the coil in m^{2}).

The “normal to the coil” is the directionperpendicularto the coil.

4.What is the magnetic field strength B at a distance of 0.1 m from a very long, straight wire that carries a current of 2.5 A?

**Concept Questions —
i)** What

**ii)** Consider a transformer that will be
used to change regular 120-volt AC electricity into 12-volt AC electricity
to power some electronic device. The “primary” winding, connected
to the 120-volt source has 500 turns. How many turns should there be in the
“secondary” winding which will provide the 12-volt output electricity?

**iii)** Consider an R-C-L series circuit. Can the total impedance ever
be __less than__ the resistance R?

**iv)** Consider an R-C-L series circuit. Can the total impedance ever
be __more than__ the resistance R?