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.

( 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

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.)

Fig.3 Symbols used in electrical circuits.

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.

Fig.5 The structure of filament lamps.

Find out. Courtsey-How stuff works.

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.

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.

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

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

Resistors in Parallel and in Series.

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.

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

V/i- is a constant.

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

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/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

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

2x10- 20 Marks.

Q.4.0