At the end of this lesson, the students should be able to:
• Explain the concepts of current and electron flow
• Define electric current, potential difference, electromotive force (e.m.f) and resistance
• State multiples and submultiples of electric quantities
1.4 CONCEPT OF CURRENT AND ELECTRON FLOW
At any instant in time, the electrons in a conductor are in random motion. However, if a directional force e.g. electromotive force EMF from a battery is applied to the conductor as shown in Fig 2.1 below, then end A of the conductor is positive while end B is negative. This results in directed flow of electrons.
The directional movement of free electrons is referred to as current flow and the conventional current flow is in the opposite direction of electron flow as can be seen in the diagram. Normally, conventional current flow is generally used.
1.5 ELECTRIC CURRENT, POTENTIAL DIFFERENCE, ELECTRO-MOTIVE FORCE
AND RESISTANCE
Electric Current
Electric current is the rate of flow of electric charges round a circuit. It has a symbol I and is measured in amperes (A).
Potential Difference (P.d.)
The potential difference between two points is defined as the number of joules of electrical energy transferred from one side of the points to the other with the passage of one coulomb from one point to another. It is measured in volts (V).
Electromotive Force (e.m.f)
Electromotive force is the force that gives rise to electric current in a circuit; also it’s the force that makes flow of electric current in a circuit. This force arises from many effects including chemical (e.g. battery cells) and magnetic, as (e.g. generator). The unit of electromotive force is the volt, symbol E.
Resistance
The resistance can be defined as an opposing force experience by the flow of charge through a material. The opposition is due to the collision between electrons and other atoms in the material, and it converts electrical energy into heat energy. The unit of resistance is Ohm (Ω), and has a symbol R.
1.6 MULTIPLES AND SUBMULTIPLES
Since it is often necessary to describe quantities that exist in large multiples or submultiples of a unit, therefore standard prefixes are used to denote powers of 10 of SI units. See below table;
Table 2.1. SI prefixes
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