BASIC ELECTRICITY II

TOPIC:  VOLTAGE, RESISTANCE, INDUCTANCE AND CAPACITANCE 

VOLTAGE (POTENTIAL DIFFRENCE)

Difference between points in an electric field: the work done in moving a unit of electric charge between two points in an electric field. 

 Electric potential: electric potential expressed in volts

It is measured in volts. The measurement instrument is a voltmeter.

RESISTANCE

Resistance is the opposition that a circuit, component, or substance presents to the flow of electricity. Symbol R

Source of resistance: something that is a source of opposition to the flow of electricity, e.g. a resistor. It is measured in ohms. The instrument for measurement is an ohmmeter

INDUCTORS

An inductor is a circuit element that can produce an induced voltage in response to changing current.

A device, usually a compact coil, which opposes any change in a circuit current is called an inductor.  Inductance is a measure of this ability.  

When an electric current is passed through a coil it does generate induced electromotive force ability to do this is called inductance.  The basic unit of inductance is Henry.

The ability of a coil to generate induced electromotive force is therefore called inductance

INDUCED CURRENT

Any electromotive force set up on a conductor by the relative motion between conductors and magnetic field induced electromotive force, the current made to flow from it is called induced current.  It can also be defined as the current generated when a magnet moves in a coil.

EVALUATION

Define (a) inductor (b) inductance (c) induce current

DEFINITION OF CAPACITOR AND CAPACITANCE

A capacitor is a device for storing electrical charges

Capacitor

The capacitance of a capacitor can be defined as the ratio of the electric charge to the potential difference between or across the plates of the capacitor.  The charge-storing ability of a capacitor is also referred to as the capacitance of the capacitor.  It is measured in farad (F)

PRINCIPLE OF OPERATION OF A CAPACITOR

When two flat plates parallel to each other are charged, either positively or negatively with electrical force e.g. cell connected to points A and B.

After charging for some time, the cell is removed.  The plates continue to retain the charges.  This plate can be said to be a bank or an electric stone and this is what a capacitor is.  The plates are usually called electrodes and the gap between the plates is called the dielectric (another name for insulators).  The plates are separated with air, mica, paper, or rubber depending on the value of the capacitance required.

TYPES OF CAPACITOR

We have the following types of (i) air capacitor (ii)paper capacitor (iii)mica capacitor

1.         Air capacitor:   The dielectric is air.  This type of capacitor is commonly used in electric circuits.  Two aluminum plates are used.  One part of the aluminum is fixed while the other can be turned 
2.         Paper capacitor:   This consists of a thin strip of paraffin’s waxed paper rolled between the coatings of thin aluminum foil
3.         Mica capacitor:   This type of capacitor has a solid material as its dielectric constant.  It can be used where many capacitors are required.
4.         Electrolytic capacitor.
5.         Polyethene capacitor. 

The plates and dielectric are rolled up to a convenient size and usually enclosed in a plastic cardboard or metal case.  Capacitors are also classified in terms of ‘fixed’ and ‘variable’ capacitors. 

Capacitors can be used in many electronic circuits e.g. in a motor vehicle, air conduction, and so on.

TRANSFORMERS 

A transformer is a device used to transfer electrical energy from one circuit to another by electromagnetic induction. A typical transformer consists of the primary winding A, the secondary winding B, and the soft iron core, which helps to minimize the flux leakage.

One of the major uses of a transformer is its ability to step up and down a voltage supplied. In Nigeria, we need a step-down transformer to step the 220V to 110 V Such a transformer is shown below:

For example, if an appliance requires 440 volts, the primary windings can have 20 turns. The secondary windings must have 20 turns. This is because, in general, the output voltage about the input voltage is the same as the ratio between the number of turns in the secondary and primary 

Voltage  in primary (Ep)       =     Number of turns in the primary  (Np)

Voltage in secondary (Es)            Number of turns in the secondary (Ns)

WORKED EXAMPLES

The primary voltage of a transformer is 10 volts and the primary winding is 20 turns. If the secondary winding is 100 turns, find the secondary voltage. 

SOLUTION

Given data

Ep = 10 volts 

Es =?

Np = 20 turns

Ns = 100 turns

We use the formula below 

                                   Ep = Np 

           Es     Ns

                              Es = Ep X Ns

                                Np

                          Es = 10 X 100

                                          20  

    Es = 50 volts

Small transformers are used in stepping down voltages for some very low-voltage household appliances 

Very large commercial transformers are used at the substations to step high from the Kanji dam to low voltage for residential houses.

EVALUATION

1.         Define (a) capacitor (b) the capacitance of a capacitor (c) Electrode
2.         Explain the principle of operation of a capacitor