Explanation Based on the equation for resistance of a wire we get:
R rl A Qr RA
Ra l
pd2 4 b
l Rpd2 4l
Precautions Subtracting the resistance of the ohmmeter and leads from the total resistance gives the resistance of the wire on its own.
Use a micrometer with a slip-screw so that the nichrome wire is not compressed, leading to inaccurate measure- ments of diameter.
Avoid kinks in the wire as it will lead to inaccurate measurements of length. Take care to avoid parallax error when using the metre stick. Ensure that the temperature of the wire remains constant as resistance is related to temperature.
Safety Precaution Safety glasses should be worn as the wire could snap when stretched.
Conduction in Metals In metals the charge carriers are electrons
Current through a metallic conductor is the flow of electrons from the negative terminal to the positive. Conventional current is said to flow from positive to negative, even though it is actually electrons moving in the opposite direction that cause the current in a metal.
Effect of Temperature on a
Metallic Resistor As the electrons travel through the metal, they are impeded by the atoms in the lattice structure described in Chapter 15.The electrons give up some of their energy to the atoms, causing the atoms to increase their vibration, i.e. the electrons heat up the metal.The net effect of this is that some of the electrical energy is converted to heat as the current flows through the conductor. As the vibration of the nuclei increases, so does the resistance, because there is greater impedance to the flow of electrons. Thus resistance increases as temperature increases for a metallic conductor.
V / V (a)
V / V (b)
Fig 18.18: I–V graphs for conduction in (a) metal at constant temperature and (b) a filament bulb
