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### Electric Circuits.

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

## B- A circuit using the main domestic supply.

The arrows indicate the direction of current.

## 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.

## 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.

## Domestic bulb Quesion

## 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 closeds3 open. lights off A2=A1A3=0. 3. s1,s3 closeds2 open. off lights A2=A3.A1=0. 4.S1opens2,s3 closed. off off All= 0.

## 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.

## 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.  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.

Conclusion.

## 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

## (Ω symbol)

Example 1 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:- Using the formula for parallel resistors.1/R = 1/r1   +  1/r21/R =  ⅓  +  ½ 1/r =  (2 +3)  /6    Therefore R =  6/5  = 1.2Ω. Finding the voltage.V=ixRV=  2x 1.2V=  2.4

### 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- CopperSalt solutionGold Nichrome Fuse wire MicaNitrogen 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) 2x5=10 Marks.

Q.3.0

## Complete this table

 Source of Electricity Potential difference AC  / DC 1.5 V DC 1.5 V DC 12 V DC Car dynamo !2 V DC Main domestic current. 230 V AC.

2x10- 20 Marks.

Q.4.0

Circuit

## 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.

## Find the following:-

R-V/I

R=12/3  =   4 Ω

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

I=V/R

I= 12/2.4

I= 5 A

I=V/R

I= 12/3

I=4. A

30 marks