Electronics Online

When current flows in a resistance, heat is produced because friction between the moving free electrons and the atoms obstructs the path of electron flow. The heat is evidence that power is used in producing current. This is how a fuse opens, as heat resulting from excessive current melts the metal link in the fuse.

The power is generated by the source of applied voltage and consumed in the resistance in the form of heat. As much power as the resistance dissipates in heat must be supplied by the voltage source; otherwise, it cannot maintain the potential difference required to produce the current.

Any one of the three formulas can be used to calculate the power dissipated in a resistance. The one to be used is just a matter of convenience, depending on which factors are known.

In many circuits, some components are connected in series to have the same current, while others are in parallel for the same voltage. When analysing and doing calculations with series-parallel circuits you simply apply what you have learnt from the last two readings.

In the circuit of figure 1 below, we could work out all the voltages across all of the resistances and the current through each resistance and then total resistance. For now I am just going to walk through the simplification of this circuit to a single resistance connected across the 100 V source.

Keep in mind that any circuit (resistive) can be reduced to a single resistance. This is particularly useful when we come to do transmission lines and antennas.

For now let’s have a go at simplifying the circuit of figure 1. There are many ways to go about this problem. The method I prefer is to start at the right hand side and work my way back to the source, simplifying the circuit as I go.

Figure 1

On the right hand side we see R3 and R4 in parallel and each 12Ω. Do you remember the short cut method when parallel resistances are all the same value? Divide the value of the branch by the number of branches: 12Ω / 2 = 6Ω.

A common method of producing an emf is by the chemical action in a battery. Without going into the chemical reactions that take place inside a cell, a brief outline of the operation of a Leclanche cell is given here.

Consider a torch cell. Such a cell (two or more cells form a battery) is composed of a zinc container, a carbon rod down the middle of the cell, and a black, damp, paste-like electrolyte between them. The zinc container is the negative terminal. The carbon rod is the positive terminal. The active chemicals in such a cell are the zinc and the electrolyte.

The materials in the cell are selected substances that permit electrons to be pulled from the outer orbits of the molecules or atoms of the carbon terminal chemically by the electrolyte and be deposited onto the zinc can. This leaves the carbon positively charged and the zinc negatively charged. The number of electrons that move is dependent upon the types of chemicals used and the relative areas of the zinc and carbon electrodes. If the cell is not connected to an electric circuit, the chemicals can pull a certain number of electrons from the rod over to the zinc. The massing of these electrons on the zinc produces a backward pressure of electrons, or an electric strain, equal to the chemical energy of the cell, and no more electrons can move across the electrolyte. The cell remains in this static, or stationary, 1.5 V charged condition until it is connected to a load.