Ch 34: Electric Current
Electric Field
Electric field is a vector quantity whose direction is defined as the direction that a positive test charge
would be pushed when placed in the field. Thus, the electric field direction about a positive source charge
is always directed away from the positive source. And the electric field direction about a negative source
charge is always directed toward the negative source.
1. Which way is the electric field around a positive charge
2. Which way is the electric field around a negative charge
Electric Potential Difference
Consider the task of moving a positive test charge within a uniform electric field from location A to location B as
shown in the diagram at the right. In moving the charge against the electric field from location A to location B, work
will have to be done on the charge by an external force. The work done on the charge changes its potential energy to
a higher value; and the amount of work that is done is equal to the change in the potential energy. As a result of this
change in potential energy, there is also a difference in electric potential between locations A and B. This difference
in electric potential is represented by the symbol ΔV and is formally referred to as the electric potential difference.
By definition, the electric potential difference is the difference in electric potential (V) between the final and the initial
location when work is done upon a charge to change its potential energy.
The standard metric unit on electric potential difference is the volt, abbreviated V and named in honor of Alessandro
Volta. One Volt is equivalent to one Joule per Coulomb. If the electric potential difference between two locations is 1
volt, then one Coulomb of charge will gain 1 joule of potential energy when moved between those two locations. If the
electric potential difference between two locations is 3 volts, then one coulomb of charge will gain 3 joules of potential
energy when moved between those two locations. And finally, if the electric potential difference between two
locations is 12 volts, then one coulomb of charge will gain 12 joules of potential energy when moved between those
two locations. Because electric potential difference is expressed in units of volts, it is sometimes referred to as the
voltage.
1
Define voltage.
2
Unit for voltage.
Electric Current
If the two requirements of an electric circuit are met, then charge will flow through the external circuit. It is said that
there is a current - a flow of charge. Using the word current in this context is to simply use it to say that something is
happening in the wires - charge is moving. Yet current is a physical quantity that can be measured and expressed
numerically. As a physical quantity, current is the rate at which charge flows past a point on a circuit. The current is
simply the ratio of the quantity of charge and time.
Current is a rate quantity. There are several rate quantities in physics. For instance, velocity is a rate quantity - the
rate at which an object changes its position. Mathematically, velocity is the position change per time ratio.
Acceleration is a rate quantity - the rate at which an object changes its velocity. Mathematically, acceleration is the
velocity change per time ratio. And power is a rate quantity - the rate at which work is done on an object.
Mathematically, power is the work per time ratio. In every case of a rate quantity, the mathematical equation involves
some quantity over time.
Note that in an equation the symbol I will represent the quantity current.
The standard metric unit for current is the ampere. Ampere is often shortened to Amp and is abbreviated by the unit
symbol A. A current of 1 ampere means that there is 1 coulomb of charge passing through a cross section of a wire
every 1 second. 1 ampere = 1 coulomb / 1 second
1
Define Current.
2
Unit for Current.
Resistance
An electron traveling through the wires and loads of the external circuit encounters resistance. Resistance is the
hindrance to the flow of charge. For an electron, the journey from terminal to terminal is not a direct route. Rather, it
is a zigzag path that results from countless collisions with fixed atoms within the conducting material. The electrons
encounter resistance - a hindrance to their movement. While the electric potential difference established between the
two terminals encourages the movement of charge, it is resistance that discourages it. The rate at which charge flows
from terminal to terminal is the result of the combined effect of these two quantities. Resistance is a numerical
quantity that can be measured and expressed mathematically. The standard metric unit for resistance is the ohm,
represented by the Greek letter omega - Ω. An electrical device having a resistance of 5 ohms would be represented
as R = 5 Ω .
1. Define Resistance.
`1
2. Unit for Resistance.