Electric Circuits.

Fig.1. Diagram of a torch. 

The journey of a current may start from a battery, a generator or from a plug point. In every case one wire will take the current and another will bring it back These are diagrammatically illustrated in Fig.2.As it is difficult to draw equipment such as cells and bulbs we use symbols . The common symbols are given in a chart below

.
Domestic electrical supply does not have a positive and  negative terminals. As that current changes direction 50 times in one second. The terminals in house wiring  are called Live and Neutral. And usually marked L and N.  Sometimes there is another wire, green and yellow attached to appliances, which carry any leaked out current to earth. ( Earth may be the metal body of the apparatus, such as in a torch or a car.)

 A ---Circuit to light a bulb using a single cell.  Direct current  (DC)

 B--- A circuit using a current from some other source.  Alternating current  (AC)

• The arrow head indicates the direction of current. ( Electrons really flow in the opposite direction)
  

 Home Experiment 1

Aim:    To examine what happens to a thin   wire when an electric current passes.
Method:-
Connect two or three cells in a circuit leaving a gap as shown. Then take a thin
strand of steel wool or any other metal (not copper) and connect it across the gap. 

Results
What happened?..................................................

Why did that happen?........................................................

Electricity Generates Heat.

Whenever an electric current flows through a solid a certain amount of energy gets transferred to thermal energy. When matter gets very hot they give out electro magnetic waves such as infra red, and  the other colours of the light spectrum depending on the temperature and the nature of the heated molecules.

Electric lamps or bulbs are made using this principle. Let us examine a  household bulb and a torch bulb now.
Joseph Swan had already tried out electric lights. [1828 – 1914, UK.] Thomas Alva Edison [U.S.],[1847 – 1931] wanted to do better. He tried out over 2000 materials to be used as a filament. Ultimately on 21st October 1879 a length of sewing thread , partly burnt, produced a steady glow for over forty hours. The biggest problem he encountered was the burning out of the filament when it gets heated. He overcame this by removing the air inside the bulb.

Fig.4. Thomas Alva Edison.1847 - 1931 . US

Thomas Edison died At 9 P.M. On Oct. 18th, 1931 in New Jersey. He was 84 years of age. Shortly before passing away, he awoke from a coma and quietly whispered to his very religious and faithful wife Mina, who had been keeping a vigil all night by his side:  "It is very beautiful over there..."

Courtsey:  Edison's Home page.

domestic bulb

A torch bulb

 Fig.5 The structure of filament lamps.

Find out.

• Could a torch bulb light if you change the positive and negative of a battery? 

An Interesting task

• Draw a circuit diagram to light two bulbs together and separately using only the following.

List of things to be used.

Two torch  bulbs {better with holders}, One cell., Three switches, Connecting wires.

 
Series and Parallel connections.

The method of connecting cells in a torch has been termed ‘Series connection’. The other method of connecting is the 'Parallel method of connection'.
The two methods are compared in this experiment.

Home Experiment 2  
Aim:             To find out the difference between connecting cells in series and parallel.
Method:   
The circuit diagrams of connecting are illustrated below. Observe  the brightness of the bulbs and what happens if one cell gets disconnected. 

Connection of cells	Series	Parallel
Circuit diagram.     Test
1. Does the bulb light when   switched on?
2.  Is the light dim or bright?
3.  What happens when one cell is disconnected?

 Home Experiment  3
Aim:   To  Investigate the differences between connecting of bulbs in series and parallel.
Method:     Carry out the tests given in column one and record the results. If a bulb lights mark with a tick.; if it does not light mark with a cross.

Connection of bulbs	Series		Parallel
Circuit diagram
Test	Lights or not					Lights or not.
Main switch	Bulb 1	Bulb 2	Main switch	Switch 1	Switch 2	Bulb 1	Bulb 2
M.S. On			On	On	On
M.S. Off			On	Off	On
			On	On	Off
			Off	On	On

 Connecting Ammeters and Voltmeters.
 

Ammeter is in series. It measures the current.	Voltmeters are connected parallel. V1 measures the Maximum pressure difference the two cells can give. [Electromotive force

The instrument used to measure current is the Ammeter and the unit for measurement is the Ampere.  A circuit has to be opened up to connect an ammeter. In other words an ammeter has to be in series to measure the current in a circuit.
Voltmeter is the instrument  which can measure the pressure difference between any two points in a circuit. The entire current need not pass through that. The pressure difference is usually called the Potential Difference.  [PD]. The unit for this measurement is the ‘volt’.

Electrical Resistance  - Ohm's Law.

 When a certain potential difference  is applied to the two ends of a conductor, a current flows. The current will depend on the conducting ability of the conductor. If it does not allow much current to flow it should be called a resistor. The ability to resist can also be measured. The unit for this measurement is the 'Ohm' and usually denoted by the  Greek letter  Omega 

Electrical resistance is the opposing pressure offered by a substance against the flow of an electric current. George Ohm conducted experiments to determine this in 1825.  What he determined and subsequently established as the Ohm’s law can be determined by a simple laboratory experiment.
Aim: 
To investigate the relationship between the current and potential difference
List of apparatus and material:-
Ammeter,  Voltmeter,  Variable resistor, Cell / battery,  Connecting wires.

Method:-

Assemble the circuit shown. (If you cannot get a resistor you may use a suitable bulb)  Note down the readings in the two meters and tabulate as follows and complete the table shown below.
Then change the resistance of the variable resistor and take the readings.  Do this several time after allowing the resistor to cool in case it gets heated.
Complete a chart for your results and plot a graph similar to the ideal example given Your results may not be perfect,  due to unavoidable experimental errors.
An Ideal Example.
 

An Ideal Example.

Your graph from an actual experiment may not be perfect like this due to un avoidable experimental errors.

Interpretation

V/R is a constant. This constant value is the resistance of the tested conductor or the
resistor.
The current flowing through a conductor is directly proportional to the potential
difference applied at the two ends, ( provided the conditions such as temperature do not
change.).
This essentially is  the Ohm’s Law.   It should be kept in the mind that there can be
objects that will not obey this law.
Ohm is the unit of resistance. If a current of 1 amp. Is produced when a potential
difference  of 1 volt is applied across the two ends the resistance is 1 Ohm. (Ω symbol

)

X'mas lights

There are two main methods of connecting bulbs.  They are termed parallel and series. When two bulbs are parallel it is possible to have two separate switches and a main switch. In the figure, two bulbs are connected parallel. Numbers indicate the suitable points, where circuit may be opened up to have switches. Lets try how we can fix them.  Main switch at 1,2,3 or 4 ?  Switch for the top bulb at 1,2,3 or 4 ? Lower bulb 1,2,3 or 4 ?   A switch at 1 will work on the upper bulb. Switch at no. 2 will suit the lower bulb. A switch at no. 3 or 4 only will act as a main switch. Fig. Two bulbs are fixed in series. In this type of a connection there is only one circuit. So if you open up anywhere, it will become a main switch. No separate switches are possible.