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5. In a CRT tube, electrons are accelerated by a 20,000 V potential difference between the electron gun (the cathode) and the positive metal mesh 5.00 cm away. a. What is the electrons speed when it reaches the positive wire mesh? b. How much time does it take the electron to reach the wire mesh?

6. A horizontal wire carries 15 A current is used to suspend a second horizontal wire 10.0 cm below it. If the second wire is made of copper and has a diameter of 1.50 mm, how much current (and in what direction) should it carry so it can be suspended without any other support? (mass density of copper is 8900 kg/m3).

7. A simple dc-motor is made up of a single coil rotor placed between two permanent magnets. The magnetic field between the magnets is 0.100 T. The cross-sectional area of the coil is 25 cm2. How many turns should the coil have so that the motor can supply a torque of 0.100 N-m?

8. A 40-turn, 4.0 cm diameter coil with R = 0.40 ohm surrounds a 3.0 cm diameter solenoid. The solenoid is 20 cm long and has 200 turns. The 60 Hz current through the solenoid is I = Io sin(2? ft) What is Io if the maximum induced current is 0.20A?

Explanation / Answer

5)a)

electron speed = sqrt((20000*1.6*10^-19)*2/(9.1*10^-31)) = 8.39*10^7 m/s

b)

time taken = sqrt(2*0.05/((20000/0.05)*1.6*10^-19/(9.1*10^-31))) = 1.19*10^-9 s

6)The F of attraction between the wires must support the weight. Thus

F/l = uII/2pi(r)

For the weight, we need the area of the wire

A = pi(r^2)

Since the diameter is 1.5 mm, the radius is 7.5 X 10^-4 m, so...

W/l = (8900)(9.8)(pi)(7.5 X 10^-4)^2

W/l = .154 N/m

.154 = (4pi X 10^-7)(15)(I)/(2pi)(.1)

I = 5133 A

The direction is the same direction as the 15 A current

7)orque = BINA

So, N = torque/BIA = 0.1/(0.1*25*10^-4*I)

I = current

)torque(T)=0.100N-m

cross sectional area of coil(A)=25cm^2

the magnetic field between magnets(B)=0.100T

torque(T)=N*A*B

number of turns(N)=0.100/0.100*25*10^-4=400turns

8)magnetic field at the center of solenoid (B)
B =

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