Seth Johnson
Electric Current
Electric current is the rate of charge flow past a given point in an electric circuit, measured in
Coulombs/second which is named Amperes. In most DC electric circuits (direct current), it can
be assumed that the resistance to current flow is a constant so that the current in the circuit is
related to voltage and resistance by Ohm's law. The standard abbreviations for the units are 1 A
= 1C/s.
Ohms law
A law relating the electric current I through some conducting medium to the potential difference
V applied across the ends of the conductor. Ohm’s law states that current is proportional to
potential difference.
V velocity
R resistance
I current
Resistance
Since voltage and current are proportional, the ratio of voltage to current is a constant
for a particular piece of ohmic material. Thus another way to express Ohm’s law is to
say that the ratio of V to I is a constant. We call this ratio the electrical resistance of the piece of
material and denote it by R; R = V/I
Resistivity
Different kinds of materials can give radically different values of resistance. For
example, you may apply a 1 V potential difference both across the ends of a copper
wire and across the ends of a strip of rubber. Resistivity is how easy or hard it is for the movement of currents
through a given material. Every material has a different resistivity which is used to help calculate the resistance
caused by different things.
Resistors
All kinds of electric devices—light bulbs, electric heaters, electric motors, and so on—
have resistance, represented in circuit diagrams by the symbol below . The same
symbol is used to represent the resistance of resistors, small devices designed to
provide electrical resistance.
Electric Power
Electrical energy is used for a variety of purposes, for example, to provide mechanical
energy to an electric motor or to provide thermal energy to an electric heater, an
electric stove, or a toaster. Consider the process that occurs as current flows through a resistor. We can think of this
current as being produced by positive charges flowing from the higher potential
end to the lower potential end.
Electric Power Loss
The electric power loss P is the rate of loss* of electrical potential energy and is
obtained when the expression above is divided by the time interval deltat: P = IV
Batteries
A current-carrying resistor continuously uses electrical energy and so must be
connected to a source that continuously produces electrical energy. A battery is a
common source of electrical energy.
This diagram shows a simple battery charging a circuit. The battery is the main power source.
EMF
A battery is quantitatively described by its “emf” value.* The emf, denoted by E, is
defined to be the energy per unit charge provided by a source as charge crosses
the source’s terminals. Since emf is an energy per unit charge, the SI unit of emf is joules per
coulomb, or volts, which is the same as the unit of electric potential.
Voltage Drop Across a Scource emf.
In describing electric circuits, we often need to calculate the voltage drop across the
terminals of a source.
Alternating Current Source
Batteries provide an energy source for devices such as flashlights, portable radios, and
calculators. The current through a battery flows in one direction. We call this direct
current, or “DC.” The electrical energy provided by electrical outlets in your home
produces alternating current, or “AC,” which means that the current alternates in
direction back and forth.
(similar to the diagrams we drew in class).
Electric Current and Moving Charges
First we shall consider the various factors that determine the value of electric current
when charges flow through a region of space. For simplicity, suppose that identical
positive charges q are uniformly spread over some region of space and are all moving
with the same velocity.
Voltmeter
A voltmeter is a meter used to measure potential difference. The reading of the meter
is the potential difference, or voltage drop, between its two terminals. We find the
potential difference between two points in a circuit by connecting the voltmeter’s
terminals to these points, so that the voltmeter is connected in parallel with other
circuit elements connected between the two points
Series Circuit
an alternating current source connected across a series combination
of a resistor, a capacitor, and an inductor. The source produces a voltage drop v across
the combination, where
Microscopic Ohm’s Law
A small section of a metal consists of atoms that form a regular array, called a “lattice.”
In forming the lattice, each atom loses one or two electrons, and so the lattice consists
of positive ions rigidly bound in place, whereas the electrons given up are free to move
throughout the lattice. These free electrons behave much like gas molecules,
moving in random directions and colliding with positive ions.