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Electric Circuits.

posted Jul 11, 2016, 1:54 PM by Upali Salpadoru   [ updated Aug 30, 2017, 11:15 AM ]


Fig.1. Diagram of a torch. 

A circuit means a round trip. The runner must come back to the starting point after the journey.. Similarly for an electrical current to exist there must be a complete circuit for the electrons to flow .

 Fig.2 

A- circuit using a cell to light a bulb.

B- A circuit using the main domestic supply.

The arrows indicate the direction of current. 


( Electrons really flow in the opposite direction)



  Question If the current starting from a battery comes back, how does a battery run down? High light to read the answer. Answer. The current is produced as a result of a chemical reaction. The cell will stop once it has used up the required chemical.

AC and DC

There are two forms of electric current. 'Alternating current' and 'Direct current'.
What we get from chemical cells is a direct current where the flow is only in one direction.

Domestic electrical supply does not have positive and  negative terminals. As that current changes the direction 50 times in one second. The terminals in house wiring  are called Phase or 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 a car.)







These are the common symbols used in drawing circuits.


Fig.3 Symbols used in electrical circuits.

In our DC circuit diagrams, we use red for the positive connection and blue for negative connection.

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 electromagnetic 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.

Domestic bulb






Fig.5 The structure of filament lamps.

Find out. Courtsey-How stuff works.

 Quesion

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


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 in 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.

 An interesting task. Draw a circuit diagram to light two bulbs together and separately using only the following.
  • 2 Torch bulbs attached to holders.
  • 3 switches. 1- Cell, and connecting wires.


Series and Parallel connections.

 Series connection.  Parallel connection.
 When objects such as bulbs, resistors, meters or cells are connected in such a way so that the current has to go through every item we say that they are in series.  When an object is connected in such a way so that the current can bypass another the two are parallel.


The two methods are compared in this experiment.


Home Experiment 2  

Find out what happens to bulbs under given conditions.

Condition

    Blue         bulb

      Red               bulb

1.S1 and s2 closed.

Lights

Lights

2.s1 and s2 open.

off

off

3 s1 open s2 closed.

off

off

4.s1 closed s2 closed V removed.

Lights

Lights.

 High light to verify the answers.


Condition

Red bulb.

Blue bulb.

comparing Ammeters.

1.s1,s2 ands3 closed.

lights

lights

A2= A1+A3.

2. s1,s2 closed

s3 open.

lights

off

A2=A1

A3=0.

3. s1,s3 closed

s2 open.

off

lights

A2=A3.

A1=0.

4.S1open

s2,s3 closed.

off

off

All= 0.

Home Experiment.



Electrical Measurements.

1.Ammeter.

In order to measure the current, the total current has to pass through this instrument. Therefore this has to be connected in series. The unit is the Ampere. (A) If you connect it parallel, only a part of the current, that will pass through it can be measured.


2. Voltmeter.

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 (V)’.

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


3. Resistance meter.
This is the opposition to the flow of current in a conductor. Multi meters usually permit you to measure this directly. If not it has to be calculated from the measurements of current and potential difference across the conductor. The conductors that have a high resistance are called Resistors.
If a material does not pass any current , or pass only an insignificant amount that substance can be called an insulator.  Some of the best insulators are Air or {all gases) , Pure water, Rubber, Plastics and Glass.

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

4.Galvanometer.

This can measure extremely weak currents and also indicate the direction.










Resistors in Parallel and in Series.

B 5.jpgB 6.jpg

Fig.A. Series connection.     Fig. B. Parallel connection.

In Fig A, the current has to overcome the resistance of the blue bulb to reach the red one.This reduces the electrical pressure. (Voltage) . In Fig.B. Both bulbs get the maximum pressure. So they will be brighter.

Voltmeters are connected parallel while the ammeters have to be in such a way to get the entire flow; that’s in series.

Formulas to find the sum of resistors.if there sre 3 resistors.
When in series  R = r1 + r2 + r3.
When parallel 1/R = 1/r1  + 1/r2  + 1/r3.


Electrical Resistance  - Ohm's Law.

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:-

 Fig.  A Resistor is in series with a variable resistor.

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.

Volts (V)

0.5

1.00

1.40

1.60

1.80        

 2.0           

Amps (i)

0.05

0.10

0.14

0.16

0.18

0.20

V/I

Resistance

Here is the graph according to the above results.

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

Interpretation


Conclusion.

V/i- is a constant.

This constant value is the resistance of the tested conductor or the resistor.

Ohm's Law and the Ohm - Unit of measuring resistance.
  Ohm's Law. Ohm. Ω.
 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. Some objects defy Ohm's 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.

(symbol)



Example 1

R 1.jpg


Calculate the current shown in the ammeter.


Working.

As the bulbs are in series the resistance is equal to the sum of the resistors.

That is R = r1  + r2.

           R = 3 + 2 …….= 5Ω.


Using V = ir.

We get   12 = ix 5

Therefore i = 12/5……..= 2.4A.


Example 2.


    Find the reading on the voltmeter.

Working:-

R 3.jpg
 Using the formula for parallel resistors.

1/R = 1/r1   +  1/r2

1/R =  ⅓  +  ½

1/r =  (2 +3)  /6    

Therefore R =  6/5  
= 1.2Ω.
  

 Finding the voltage.
V=ixR
V=  2x 1.2
V=  2.4 


Highlight to get the answers.

Q.1.0

Are these  electrical conductors , resistors or insulator

  conductors Resistors Insulators
 I. Copper 2.  Mica 3.  Salt solution-  4.  Butter ,5. Gold- ,   6.  Nichrome alloy-7. Carbon (Graphite)-,    8. Fuse wire-9. Nitrogen-,    10. Leather-Copper
Salt solution
Gold

 Nichrome Fuse wireMicaNitrogen Leather Butte

10 Marks.

Q.2.0

Identify the numbered parts and give the direction of current at the point marked. (To left or right)

B 4.jpg2x5=10 Marks.



Q.3.0

Complete this table

Source of Electricity

Potential difference

AC  / DC

AA.PNG

1.5 VDC

1.5 V

DC

12 VDC

Car dynamo Car dynamo.PNG

!2 VDC

Main domestic current.

230 VAC.

2x10- 20 Marks.

Q.4.0

Q.4


No

Circuit

State of fuse.

Bulbs ON /Off ?

                      

.1.

OK

Blue

On.

Red

Off.

2.

OK

On

On

3.

OK .

Off.

On

4.

Melts.

Off.

Off.

5.


OK .

Off.

Off.



2 x 15=30.

Q.5. .

R 5.jpg

Find the following:-

  1. Resistance.  R1.

R-V/I

R=12/3  =   4 Ω

  1. Resistance   R 2.

R=V/I

R= 12/2  = 6 Ω

  1. Combine resistance of R1 හ& R2

1/R  =  1/r1  + 1/r2

1/R = ¼   +  ⅙

1/R =  3+2

            12

R=  12/5   = 2.4 Ω

  1. Reading on A..

I=V/R

I= 12/2.4  

I= 5 A


      5.  If S’ s open the current on A.

I=V/R  

I= 12/3

I=4. A

30 marks

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