Lesson Notes By Weeks and Term - Junior Secondary School 1

SUBJECT:  BASIC TECHNOLOGY           

CLASS:  JSS1

DATE:

TERM: 3RD TERM

REFERENCE MATERIALS 

• MELROSE, Basic Science and Technology Book 1
• NERDC, Basic Technology for JSS, Book 1

WEEK NINE AND TEN

TOPIC:  VOLTAGE, RESISTANCE, INDUCTANCE AND CAPACITANCE 

CONTENT

• Voltage (Potential Difference)

• Resistance

• Inductor 

• Inductance 

• Induced current 

VOLTAGE (POTENTIAL DIFFRENCE)

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

Electric potential: electric potential expressed in volts

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

RESISTANCE

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

electricity 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 a ohmmeter

INDUCTORS

An inductor is a circuit element which has the ability to 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 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 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 circuit.  Two aluminum plates are used.  One part of the aluminum is fixed while the other can be turned 
2. Paper capacitor:   This consists of 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 in   transferring 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 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 440volts, the primary windings can have 20 turns. The secondary windings must have 20 turns. This is because, in general, the output voltage in relation to 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 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

READING ASSIGNMENT

Read more on inductors & capacitors (Intro Basic for JSS1- pg 109-110, Basic Tech for JSS1 –pg78-79)

GENERAL EVALUATION

1. Mention three (3) types of scale drawing.
2. State and  give examples of the types of wood.

WEEDEND ASSIGNMENT

1. The gap between the capacitors plate is called ___ (a) electric (b) dielectric (c)capacitance (d) Inductance
2.               The above symbol represents __(a)cell (b)switch (c)capacitor  (d) resistor

3. Capacitors cannot be used in one of the following (a) air-conduction (b) motor vehicle (c) boiling ring (d) television
4. The SI unit of inductance is ___ (a) voltage (b) ampere (c) Henry (d) ohms
5. ___ is a circuit element which has the ability to produce in response to charging current (a) resistor (b) inductor (c) capacitor (d) cell

THEORY

1. Define (a) Potential Difference (b) Inductance (c) Resistance
2. What is capacitor? State the types of capacitor