Exam 5: Current and Resistance

Jadeen says that you can increase the resistance of a copper wire by hammering the wire to make it narrower and longer.Arnell says that you can increase its resistance by heating the wire.Which one, if either, is correct, and why?

What is the resistance of 1000 m of 4-mm diameter copper wire? ( $\rho$ Cu = 1.7 *10-8 $\Omega$ .m)

A wire (length = 2.0 m, diameter = 1.0 mm) has a resistance of 0.45 $\Omega$ .What is the resistivity of the material used to make the wire?

An aluminium wire of cross-sectional area 4.0 mm2 is carrying a current of 8.0 A.The density of aluminium is 2.7 g/cm3, and its molar mass is 27 g.Assuming one free electron per aluminium atom, what is the drift speed of the electrons in this wire?

A cook plugs a 500 W crockpot and a 1000 W kettle into a 240 V power supply, all operating on direct current.When we compare the two, we find that:

A 4.0- $\Omega$ resistor has a current of 3.0 A in it for 5.0 minutes.How many electrons pass through the resistor during this time interval?

A copper cable is to be designed to carry a current of 300 A with a power loss of only 2.0 watts per meter.What is the required radius of the copper cable? (The resistivity of copper is 1.7 * 10-8 $\Omega$ .m.)

An electric heater is constructed by applying a potential difference of 110 V across a wire with a resistance of 5.0 $\Omega$ .What is the power rating of the heater?

A conductor of radius r, length and resistivity $\rho$ has resistance R.What is the new resistance if it is stretched to 4 times its original length?

The temperature coefficient of resistivity of iron is 5.0 * 10-3 / $\degree$ C and that of carbon is -0.50 * 10-3 / $\degree$ C.When an iron wire and a carbon rod, each having the same 10 $\Omega$ resistance at 20 $\degree$ C, are cooled from that temperature to -80 $\degree$ C, the new ratio of the resistance of the carbon rod to the resistance of the iron wire at the lower temperature is:

The electron density in copper is 8.49 *1028 electrons/m3.The electron charge is e = -1.60 *10-19 C.When a 1.00 A current is present in a copper wire with a 0.40 cm2 cross-section, the electron drift velocity, in m/s, with direction defined relative to the current density, is:

What maximum power can be generated from an 18-V emf using any combination of a 6.0- $\Omega$ resistor and a 9.0- $\Omega$ resistor?

A conductor of radius r, length and resistivity $\rho$ has resistance R.It is melted down and formed into a new conductor, also cylindrical, with one fourth the length of the original conductor.The resistance of the new conductor is: