Lesson 3: Current Electricity

Unit 1: Definition of electric current, potential difference and resistance
A current is any motion of charge from one region to another in an electric circuit. In this section, we shall discuss current in conducting materials (wire) in an electric circuit which may consist of a cell (source of e.m.f) connected by copper wires to one or more resistors or other components which will be described later. The cell provides an electromotive force which sets up potential differences across various circuit components and drives the current through them. The resistors offer opposition to the free flow of current.

Unit 2: Carriers of Electric Charge
Charge carriers can either be electrons, positive ions (cation), negative ions (anion) or holes. The flow of a particular type of electric charge depends on the type of material medium.

 Types of material Charge carriers Metal electrons (-) Gases electrons and ions (+) Semi-conductors (like silicon or germanium) electrons (-) and holes (+) Liquid (salt and acid solution) ions (+ and -)

Unit 3: Electric Current
From the nature of charge carriers, it can be said that an electric current ¡s the quantity of an electrical charge flowing past a given point in a conductor per second.

i.e. current (I) = $\frac{Quantity\,&space;of\,&space;charge}{time}$

i.e. I = $\frac{Q}{t}$

The unit of electric current is Coulomb per second or Amperes (A). Smaller units of current are

Milliampere (mA) 10-3 A
Microampere ( μA) = 10-6 A.

Example:
A current of 4.5A flows through a car headlight. How many coulombs of charge flow through it in 2hrs?

Solution:
Q = ?; I = 4.5A; t = 2 x 60 x 60

Current (I) = $\frac{Q}{t}$

i.e. Q = I x t = 4.5 x 2 x 60 x 60 = 32,400C = 3.2 X 104C

The instrument used in measuring current is the ammeter. It is placed along the direction of flow of current. Other types of ammeter used are
(i) Microammeter
(íi) Milliammeter or
(iii) Galvanometer

Galvanometer is also used to detect the passage of current, it measures a thousand times less than a microampere

In any electric circuit, there is a need to measure:

(i) Current, measured in amperes (A);
(ii) electromotive force and potential difference both measured in volts (V);
(iii) resistance, measured in ohms (Ω)

(i) Current, measured in amperes (A)
It should be noted that the ammeter must always be placed in a circuit so that the current to be measured flows directly through it. In the figure below, for example, the current flows through the ammeter A1, and through the ammeterA2 placed at P and Q; both will record the same current (I). Such ammeters are said to be placed in series in the circuit.

Ammeters have low resistance so that it introduces negligible resistance into the circuit.

In the figure below, similar ammeters A1 and A2 at R and S respectively are connected in parallel with each other will indicate different currents while the ammeter A3 at T will record the main current flowing from the battery. The current I = I1 + I2

(ii) Electromotive force and potential difference both measured in volts (V)|
The potential difference (p.d.) between any two points in a circuit is defined as the work done, in joules, when one coulomb of electricity moves from one point to another.

The instrument for measuring p.d. is known as a voltmeter and is graduated in volts or millivolts. The circuit symbol is as shown in the figure below:

Since it measures the difference in potential between two points, it is therefore connected in parallel. Thus in the figure above, the voltmeter V1 measures the p.d. across the resistor P connected between A and B. The voltmeter measures the p.d. across the resistor Q connected between C and D. Total p.d. across the resistances sum of individual p.d. = V1 + V2

(iii) Resistance, measured in ohms (Ω)
We have seen that different materials have different conducting abilities and are accordingly known as good or poor conductors or insulators. In current electricity, we think in terms of the ability of a substance to oppose or resist the flow of electricity through it.

A good conductor is therefore said to have a low resistance and a poor conductor, a high resistance. Very good insulators may be regarded as having infinite resistance. Hence no current can pass through an ideal insulator. Silver offers the least resistance to the currents; however it is too expensive for normal use and so copper is generally used as connecting wires and cables in electric circuits.

Unit 4: Electric Cells and Production of Electric Current
A steady current is produced through continuous flow of charge. Such a continuous flow of charge can be generated from:

(a) chemical energy
(b) mechanical energy
(c) heat energy
(d) solar energy

(a) Electricity from chemical energy
Electricity is produced from chemical energy through the use of electric cells which convert chemical energy into electrical energy

(b) Electricity from mechanical energy
Most of the world’s electricity is produced from the conversion of mechanical energy to electrical energy using electric generators. Such generators produce electricity by the movement of coils of insulated wire (the armature) that cut lines of force in the magnetic field between the poles of powerful magnets.

The induced electric current on the wire is picked up from the copper splitting commutators by means of graphite carbon brushes. The bicycle dynamo is an example of this mechanism which generates electricity for the headlamp of the bicycle.

(c) Electricity from heat energy
An electric cm-rent can be produced when the ends of two different metal wires (e.g. copper and iron) are joined end to end. One junction is placed in hot water (hot junction) and the other in a beaker of ice chips (cold junction).

Electricity flows around the circuit formed by the two wires. Such a device, known as thermocouple produces electricity by the thermoelectric effect. The greater the difference in temperature between the two junctions, the greater is the electric current. A micro galvanometer is connected in one arm of the thermocouple to indicate the current flow as in the figure below:

Thermocouples are used for measuring high temperatures particularly when constructed from metals or alloys with high melting point (e.g. platinum and platinum- iridium).

(d) Electricity from solar energy
When sunlight falls on a photosensitive surface (e.g. surface of potassium) electrons are produced whose movement constitutes a current. A photocell or photoelectric cell consists of a photosensitive surface as a cathode and a wire ring as the anode. If visible light falls on this surface, electrons are emitted by a process called photoelectric effect and the flow of these electrons can be detected by a micro ammeter as shown in the diagram below:

Unit 5: Electric Cells
A cell is a device for converting chemical energy to electrical energy. A cell consists of two electrodes (dissimilar metals) placed in a container in which there is a solution of acid or salt called the electrolyte. The positive electrode is the anode while the negative electrode is the cathode. Some examples of electrolytes are dilute tetraoxosulphate (VI) acid or a strong solution of ammonium chloride in water. Examples of electrodes are rods of aluminium, carbon (graphite), copper, iron, lead arid zinc. There are two main types of cells -primary and secondary cells.

(a) Primary Cells
A simple primary cell consists of a copper rod and a zinc plate immersed in é container filled with dilute sulphuric acid (tetraoxosuiphate (VI) acid). When the copper rod and zinc plate are connected by a wire, the zinc slowly dissolves in the acid and bubbles of hydrogen gas are formed on the copper. As a result of chemical reaction, electrons flow from zinc to copper through the wire as shown below:

Copper is the anode (+ve electrode) and zinc is the cathode (-ve electrode). When a bulb is connected between the tent terminals, it lights up indicating that current is flowing in the external circuit from copper to zinc. If a voltmeter is connected across these terminals, it will register about 1.0V. The current supplied is for a short time due to some defects; and both the electrolyte and the electrodes have to be replaced.

Defects of a Simple Primary Cell
There are two defects of a simple cell. These are:
(i) Polarization (ii) Local action

(i) Polarization:
This is due to the production of hydrogen gas bubbles around the copper plate of the cell, This is due to the reaction of zinc and dilute tetraoxosulphate (VI) acid. This reaction sets up a back e.m.f which reduces the current in the external circuit. Polarization can be reduced by brushing the plates or by the use of a depolarizer- such as manganese dioxide or potassium dichromate which oxidizes hydrogen to form water and so removes the hydrogen bubbles.

(ii) Local action:
This is due to impurities in the zinc plate which results in the wearing off of the electrode. These impurities (iron and carbon) set up tiny cells around the zinc surface producing bubbles of hydrogen. Zinc plate is being dissolved and washed since no current is supplied to the external circuit. Local action can be prevented by amalgamation, i.e. cleaning the zinc with tetraoxosulphate (VI) acid and then rubbing it with some mercury which covers up the impurities and prevents their contact with the electrolyte

(b) Leclanche cell
This is a primary cell and it is of two types; wet and dry types.

(i) Wet Leclanche cell
The cell consists of a negative zinc rod dipped into a solution of ammonium chloride (salammoniac) and a positive carbon rod ¡n a mixture of magnanese (IV) oxide (MnO2) and carbon in a porous pot.

1. The cell is not suitable for giving current of long duration because the rate at which the hydrogen is being oxidized by the depolarizer (MnO2) is slow compared to the rate at which the hydrogen is formed.
2. The electrolyte evaporates with time and as such requires occasional addition of water.

1. The cell is useful in cases where intermittent current of short duration ¡s required e.g. for ringing bells.
2. The chemicals are easily available and cheap.

Note: The addition of powdered carbon helps to ensure continuous flow of charged electron since the manganese(IV) oxide (MnO2) is not a conductor

(ii) Dry Leclanche Cell
The dry Leclanche cell was invented to overcome the second defect mentioned above. In the dry cell, the electrolyte is a jelly-like material containing ammonium chloride; instead of the liquid solution. The positive electrode is a carbon rod surrounded by a packed mixture of manganese dioxide and powdered carbon, inside a zinc container which is the negative electrode. The working is similar to that of the wet cell.

(1) It is portable (i.e. it is easily carried about)
(2) There is no liquid spillage in the course of carrying it from one place to the other.
(3) It gives greater amount of current than the wet type.

(1) Slight polarization still occurs because of the slow rate of oxidation of hydrogen bubbles formed by the depolarizer.

(c) The Daniell cell
The Daniell cell is another type of primary cell invented to counter the problem of polarization.

As in the Leclanche cell, the zinc rod is the negative electrode but the positive electrode is a copper container. The electrolyte is dilute tetraoxosulphate (VI) acid contained in a porous pot around the zinc rod, and the depolarizer is copper tetraoxosulphate (VI) in the surrounding copper container. The action is similar to that of a Leclanche cell but depolarization is much more efficient. The e.m.f of a Daniel cell does not, therefore, vary very much and has a constant value of 1.08V

Secondary Cells
In primary cells, once their energy is used up, it cannot be restored by recharging, but only by the addition of fresh electrolyte. In secondary cell, when an accumulator is receiving current through a source of electricity, it is being charged. When delivering a current, it is being discharged. There are two main kinds of secondary cell; the lead-acid accumulator and the alkaline or Nife accumulator.

This is more common than the Nife accumulator. It consists of lead oxide as the positive electrode, lead as the negative electrode and tetraoxosulphate (VI) acid as the electrolyte. During discharge, when the cell is giving out current, both electrodes gradually change to lead tetraoxosulphate (VI) while the acid gradually becomes more dilute and the density decreases. When fully charged the relative density is about 1.25 and the e.m.f. of the cell is 2.2V, but when discharged relative density is about 1.15 and the e.m.f. less than 2.0V. The relative density which should not be allowed to drop below 1.15 and is measured using a special hydrometer,

A voltmeter connected across a battery will also show either the car battery is well charged or in need of charging. When newly charged, the voltage is well over 12V, but it soon falls to 12V when the battery is used. It stays constant throughout use, until the battery has run out of usage, when the voltage drops suddenly well below 12V.

(ii) The alkaline or Nife accumulator (nickel-iron)
The name Nife comes from Ni and Fe, the chemical symbols for nickel and iron. The cell has a positive electrode made of nickel hydroxide while the negative plate is iron or cadmium. The electrolyte is potassium hydroxide dissolved in water. Alkaline cells last much longer than lead-acid cells, keep their charge longer and they require less maintenance.

They are used for emergencies in factories and hospitals. They are, however, more expensive and bulky. The e.m.f. of a Nife cell is relatively small, about 1.25V.

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