## PHYS208 Fundamentals of Physics II

### Hint for Ch. 27, 5E

Convert diameter to area and then calculate current density. This only takes a few minutes with a spreadsheet program or programmable calculator!

### Hint for Ch. 27, 27E

Since the mass and density of the material may be assumed to not change, the volume remains the same. Accordingly, how does the area then change when the length is increased by a factor of three?

### Hint for Ch. 27, 35P

Look up the thermal coefficient of resistivity for tungsten and apply Eqn. 27-17.

### Hint for Ch. 27, 41P

a. Divide the "truncated right-circular cone" into a series of cylindrical disks of infinitesimal thickness dx and radii that vary accordingly from a to b. Span the series of disks by integrating over the location variable x.

The solution in CD-Physics 2.0 puts the assumption above (infinitesimal disks in series) on firmer ground.

b. Let a = b.

### Hint for Ch. 27, 55P

a. Remember that energy is power x time.
kW-hr is a unit of energy, 1000 watt-hour.

b. Solve (c) first and then apply Ohm's law.

c. Recall that electrical power = current x voltage.

### Hint for Ch. 27, 62P

a. Elimate current from the formula for power by incorporating Ohm's law.

b. Resistance will decrease at the lower operating temperature caused by the lowered applied voltage.

"http://www.physics.udel.edu/~watson/phys208/exercises/hints/0218.html"
Last updated Feb. 19, 1998.
Copyright George Watson, Univ. of Delaware, 1997.