Chapter 17 Current and Resistance
• Electric charge is an intrinsic property of proton and electron; only two types of charge have been discovered, positive and negative.
• I will use a symbol q for a charge. The unit of charge q is C (coulomb).
• An electron carries charge -e and an proton carries charge +e.
e = 1.60 x 10-19 C
• The charge on an electron or a proton (e) is the smallest amount of free charge that has been discovered (quantized). Thus, any charge of magnitude of q is an integer multiple of e; q = Ne.
• A battery provides a potential difference.
• An electrical device needs electrical energy to operate. The energy source (the battery) and the energy-consuming device (the cassette player) are connected by conducting wires, and the transfer of energy takes place via an electric current.
17.1 Electric Current
•
The current, I, is
defined as the amount of charge (
) per unit time (
) that passes through a surface that is perpendicular to the
motion of the charges. (See Figure
17.1)
I 
(17.1)
• The SI unit for current is referred to as an ampere (A) and is a coulomb per second (C/s): 1 A = 1 C/s
• When charges flow, they can be positive, negative, or both. However, the conventional direction of the current is the same direction as the flow of positive charge (from the positive terminal to the negative terminal).
• Examples.
17.2 Current and Drift Speed
• The current in a conductor is related to the motion of the charge carrier by
= 
(17-3)
where
is the number of
mobile charge carriers per unit volume, 
is the charge on each
carrier, 
is the drift speed of
the charges, and 
is the
cross-sectional area of the conductor.
• Examples.
17.3 Resistance and Ohm’s Law
• The current that a battery can push through a wire is analogous to the water flow that a pump can push through a pipe.
The ratio V/I is a constant, where V is the voltage applied across a piece of materials (such as a wire) and I is the current through the material (Figure 17.5):
= R = constant or V = IR (17.4
& 5)
R is the resistance of the piece of material.
SI unit of Resistance: volt/ampere (V/A) = Ω (ohm)
• Examples.
17.4 Resistivity
• For a wide range of materials, the resistance of a piece of material of length L and cross-sectional area A is given by
R = r
(17-6)
where is r a proportionality constant known as the resistivity of the material. The unit of resistivity is the ohm-meter (Ω-m).
• Each material has a characteristic resistivity that depends on its electronic structure and on temperature. (Table 17.1)
• Examples.
17.5 Temperature Variation of Resistance (Reading Assignment)
(Note) Does resistance change with temperature? What is the parameter used for describing the dependence? What are the applications?
17.6 Superconductors (Reading Assignment)
17.7 Electrical Energy and Power
• If a battery is used to establish an electric current in a conductor, chemical energy stored in the battery is continuously transformed into kinetic energy of the charge carrier. This kinetic energy is quickly lost in the form of thermal heating due to the collisions in the conductor.
When there is a current I in a circuit as a result of a voltage V, the electric power P delivered to the circuit is
P = IV (17-9)
SI unit of Power: watt (W)
= I2R
= V2/R
• Examples
17.8 Voltage Measurements in Medicine (Reading Assignment)
(Supplementary) The Measurement of Current and Voltage
• An ammeter is an instrument that measures current, it must be inserted in the circuit so the current passes directly through it (series). An ideal ammeter would have zero resistance.
• A voltmeter is an instrument that measures voltage between two points in a circuit, it must be connected between the points (parallel). An ideal voltmeter would have infinite resistance.