KFUPM-Physics102-032
Dr.Aljalal
Sec07
Sec08
Sec09

Recitation ch 27
Answers at the end

Question 1
A cylindrical wire of radius R = 2.0 mm has a uniform current
density J = 2.0*10**(5) A/m**2. What is the current through
the portion of the wire between radial distances R/3 and R/2?
(see figure 1)


      a   1.95 A.
      b   3.73 A.
      c   5.31 A.
      d   0.35 A.
      e   9.11 A.
==============================================
Question 2
A heater element of resistance 10**3 Ohm is constructed
to operate at 110 V. How much thermal energy is produced
in one hour by the heater?

      a   4.4*10**4 J.
      b   2.2*10**7 J.
      c   5.1*10**2 J.
      d   6.2*10**5 J.
      e   1.9*10**5 J.
==============================================
Question 3
A current of 0.300 A is passed through a lamp for 2.00 minutes
using a 6.00-V battery. The energy dissipated by the lamp during
the 2.00 minutes is:

      a   216 J
      b   20.0 J
      c   36.0 J
      d   1.80 J
      e   12.0 J
==============================================
Question 4
A copper wire of cross sectional area 3.00*10**(-6) m**2
and length 4.00 m has a current of 2.25 A uniformly
distributed across that area.
How much electric energy is converted to thermal energy
in 10.0 minutes ?
[The resistivity of copper is 1.68*10**(-8) ohm*m]

      a   0.50 J
      b   68.0 J
      c   13.6 J
      d   21.7 J
      e   30.2 J
==============================================
Question 5
If 4.7*10**(16) electrons pass a particular point in a wire
every minute, what is the current in the wire?

      a   2.9*10**(-5) A.
      b   1.3*10**(-4) A.
      c   4.7*10**(-3) A.
      d   2.9*10**(-3) A.
      e   9.1*10**(-3) A.
==============================================
Question 6
An electric device, which heats water by immersing a
resistance wire in the water, generates 153 J of heat per
second when an electric potential difference of 12 V is
placed across its ends. What is the resistance of the
heater wire?

      a  0.81 Ohms
      b  0.58 Ohms
      c  0.94 Ohms
      d  2.10 Ohms
      e  0.48 Ohms
==============================================
Question 7
At T = 20 degrees-C, the length of a wire is 10 m, and its
cross sectional area is 0.50 mm**2. A potential difference
of 1.0 V is maintained across the ends of the wire. The
resistance of the wire changes by 0.30 ohms for a temperature
change of 50 Celsius degrees.
what is the temperature coefficient of resistivity of the wire ?
At T = 20 degrees, the resistivity is 5.0*10**(-6) ohm*m.

      a   15*10**(-3) degrees**(-1)
      b   15*10**(-4) degrees**(-1)
      c   15*10**(-5) degrees**(-1)
      d   6.0*10**(-5) degrees**(-1)
      e   6.0*10**(-4) degrees**(-1)
==============================================
Question 8
Conduction electrons move to the right in a certain
wire. This indicates that:

      a   the current density points to the right but
         the electric field points to the left.
      b   the current density and the electric field both
         point to the right.
      c   the current density points to the left but
         the electric field points to the right.
      d   the current density points to the left but
         the direction of the electric field is unknown.
      e   the current density and the electric field both
         point to the left.
==============================================
Question 9
A metal wire of resistance R is cut into three equal
pieces that are then connected side by side to form a
new wire the length of which is equal to one-third the
original length. What is the resistance of the new wire ?
      a   R
      b   R/3
      c   9*R
      d   R/9
      e   3*R
==============================================
Question 10
A copper wire of cylindrical shape has resistance R.
What is the resistance of a second wire, made of the
same material, that is twice as long and has twice the
diameter ?

      a   R
      b   4R
      c   2R
      d   R/2
      e   R/4
==============================================
Question 11
A copper wire "1" has a length L1 and diameter d1. Another
copper wire "2" has a length L2 and diameter d2. At constant
temperature, the second conductor has smaller resistance if:

      a  d2 = d1 and L2 > L1.
      b  d2 < d1 and L2 < L1.
      c  d2 < d1 and L2 = L1.
      d  d2 > d1 and L2 > L1.
      e  d2 > d1 and L2 < L1.
==============================================

Answers
1     d
2     a
3     a
4     b
5     b
6     c
7     d
8     e
9     d
10     d
11     e