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Physics FactsheetJanuary 2001 Number 07
Electrical Current, Voltage and Resistance
What is current? Finding current and average current from a graph
Electricity can seem very abstract and difficult to understand. The key to Q Q
grasping the subject, like so many in Physics, is to build up a picture of tangent
what is happening and follow the concepts through logically. The aim of Q1
this Factsheet is to help you do just that by explaining in simple terms Q1
what current, voltage and resistance are and how they all play a part in an
electrical circuit.
t1 t t1 t
An electric current is nothing more than a net movement or flow of In both of the above graphs, we need to find the current at time t1
charge in a certain direction. In a conducting metal the charge carriers are In both cases, we use the gradient of the graph to find the current.
free electrons; these electrons originate from the rigidly bonded metal
atoms that form the structure of the conductor. Their outer electrons are • In the first graph the gradient is constant and so therefore is the
only weakly bonded to the atom and so many escape and are free to move current. So dividing Q1 by t1 will give the right answer as you are
throughout the structure of the metal. As the metal atoms have lost taking the gradient of the graph.
electrons they are no longer neutral but are now positively charged. Good
metallic conductors include silver and copper. • In example 2 dividing Q1 by t1 will give the average current up
untilthat point. The gradient has been falling so the current at the
Metals are not the only materials that conduct; semiconductors are a time t1will be less than the average value. So instead we must use a
group of materials whose resistance lies somewhere between that of metallic tangent to the line at t1. Watch out for the distinction between average
conductors and insulators. A semiconductor is made from covalently bonded and instantaneous values.
materials. Electrons in the outermost orbits only have a small ‘jump’ to
make to move to the next orbit a little further away. When an electron does
this it has two implications. As there are not many electrons in these Drift velocity
higher levels it can jump into a vacant site in the adjacent atom and in so All free electrons move around due to their thermal energy even if there is
doing move through the element. Secondly, when it jumps up it leaves a no current present. However as this motion is completely random then the
vacant site below it known as a hole, these holes act as though they were net effect is no overall movement. To be part of a current they have to
positively charged and move the opposite way through the metal, so we exhibit a drift velocity in a given direction
get double the current we would expect. Silicon and Germanium are Consider the two electron paths below:
examples of semiconductors.
Liquids can also conduct as they long as they contain charged particles.
For instance, impure water will conduct as the impurities in it exist in the
form of ions, which move through the liquid. Pure water will not as the displacement
H2O molecules are neutral. Both display random motion as their thermal kinetic energy causes them to
move, colliding with the fixed positive ions that make up a metal’s structure
as they go. The first electron shows very little change in displacement, the
Current is the rate of flow of charge second has moved about the same distance as the first but shows a definite
If the current is constant, we have: displacement to the right - it has a drift superimposed on its random motion.
Q
I= t Therefore we can say the second electron is probably part of an electrical
I = current (amps, A) Q = charge flowing past a point (coulombs, C) current and the first is not.
t = time taken for the amount of charge Q to flow (seconds, s).
When the overall effect on all the electrons is taken into account this small
This formula will also give average current if the current is variable.
drift shown by each electron provides a current, whilst when all electrons
in the first example are considered the net movement of charge is zero.
If the current is not constant, then the gradient of a charge (y-axis)
against time (x-axis) graph would give the current at a particular time
Current can be calculated from:
I = nAQv
Current and electrons I = current (amps, I)
We now know that current is rate of flow of charge. The flow of charge in n = number of free charge carriers per m3
solids depends on the movement of charge carriers; these are generally Q: Charge on each charge carrier. (coulombs, C)
electrons. A: Cross-sectional area (m2)
v : Drift velocity (ms-1)
Current therefore depends on the number of free (meaning "able to move")
charge carriers in the material and how quickly they move - the more
Exam hint: Many students lose marks by not using the correct units in the
charge carriers there are and the faster they move, the higher the current
equation above. In particular, note that cross-sectional area must be in m2.
will be.
To convert cm2 to m2, divide by 1002 = 10 000.
To convert mm2 to m2, divide by 10002 = 1000000
1
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