IB 12
A Mechanism for Electric Current
Electrical conduction is possible in:
Conduction in Metals
1)The metal atoms in a solid are bound together by the metallic bond.
2)When a metal solidifies from a liquid and
the bonds form, electrons are donated
from the outer shells of the atoms to a
common sea of electrons that occupies
the entire volume of the metal.
3)Around the positive ions in fixed positions
is the sea of free electrons called
conduction electrons. These are
responsible for the electrical conduction.
4)The electrons interact with the vibrating
ions and transfer their KE to them.
5)This transfer of energy from electrons to
ions accounts for the phenomenon that we
call “resistance”.
So how do electrons being moving throughout the circuit?
In a metal in the absence of an electric field, the free electrons move at random and at average speeds close to the speed of sound in the material. However, when an electric field is present, then an electric force will act on the electrons with their negative charge, causing them to drift along the conductor.
Conduction in Gases and Liquids
Electrical Breakdown:
Examples:
KIRCHOFF’S LAWS
Junction or Node:
Kirchoff’s 1st Law (Junction Rule): for a given
Junction or node in a circuit, the sum of the
currents entering equals the sum of the currents
leaving. This law is a statement of conservation
of……
For any junction:
Kirchoff’s 2nd Law (Loop Rule): around any closed loop
in a circuit, the sum of the potential differences across all
elements is zero. This law is a statement of conservation
of…..
Any charge that starts and ends up at the same point with
the same velocity must have gained as much energy as it lost.
For any closed loop in a circuit:
In the illustration, each box denotes a circuit element, so
the loop rule tells us
0 = (Vb - Va) + (Vc - Vb) + (Vd - Vc) + (Vd - Va) .
Starting at any point in the loop continue in the same direction noting the direction of all the voltage drops, either positive or negative, and returning back to the same starting point. It is important to maintain the same direction either clockwise or anti-clockwise or the final voltage sum will not be equal to zero.
Example: Find the current across the 10 V battery.
Electric Cells
Cells –
2 Main Types of Cells
1) Primary Cells – Cells that are used until they are exhausted and then disposed of; the original chemicals have completely reacted and been used up and cannot be recharged
Ex:
2) Secondary Cells – rechargeable cells; when the chemical rxn’s have finished, the cells can be connected to a charger, reversing the chemical rxn and reforming the original reactants
Ex:
Two cells with the same chemistry will generate the same emf as each other. However, if one of the cells has larger plates than the other and contains larger volumes of chemicals, then it will be able to supply energy for longer when both cells carry the same current.
Capacity of a Cell –
1)If a cell can supply a constant current of 2 A for 20 hours, then it is said to have a capacity of 40 Ah (Amp-hours). How much current could it supply for 1 hour? For 4 hours? For 400 hours?
Practical cells do not necessarily discharge in such a linear way.
Shown below is a typical discharge curve in which the terminal potential difference of the cell is plotted against time since the discharge current began (at different temperatures).
Important features of this graph are that:
1)
2)
3)
4)
Cycle Performance Graphs
Nickel-cadmium
In terms of life cycling, nickel-cadmium is the most enduring battery. Figure 1 illustrates the capacity, internal resistance and self-discharge of a 7.2V, 900mA pack with standard NiCd cells. Due to time constraints, the test was terminated after 2,300cycles. The capacity remained steady; the internal resistance stayed low at 75mWand the self-discharge was stable. This battery receives a grade “A” rating for almost perfect performance.
Figure 1: Performance of standard NiCd(7.2V, 900mAh)
This battery receives an “A” rating for a stable capacity, low internal resistance and moderate self-discharge over many cycles.
Theultra-high-capacitynickel-cadmium offers up to 60 percent higher specific energy compared to the standard version, however, this comes at the expense of reduced cycle life. In Figure 2 we observe a steady drop of capacity during 2,000 cycles, a slight increase in internal resistance and a rise in self-discharge after 1,000 cycles.
Figure 2: Performance of ultra-high-capacity NiCd(6V, 700mAh)
This battery offers higher specific energy than the standard version at the expense of reducedcycle life.
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