Physics The electric current &Ohm's law review 2 ثانوية عامة

Physics| The electric current &Ohm's law - review 2
The electric current &Ohm's law (review)

Give reasons for the following:

1 – The home electric devices are not connected in series.
If the electric devices are connected in series, the total resistance of the home circuit increases, and the total current decreases, so it can't operate any device, and the potential diff. across each device is not constant and all devices must be worked in the same time.
2 – If the electric circuit is switched off the potential diff. between the two poles of the electric source equal its e.m.f.
Bec. V = VB – Ir. , when the circuit is opened . there is no current (I = 0) , so V = VB
3 – when the length of the wire increases its resistance increases.
When the length of the wire increases, the number of points connected in series increases, the resistance increases in case of series connection.

4 – When the length of the wire increases its resistance increases.
Some materials conduct electricity, bec. they contain more free electrons, while others are considered insulators, bec. They do not contain free electrons.
5 – Work must be done to transfer electric charges from one point to another.
To overcome the resistance of the conductor.
6 – If the filament of a lamp in the house is cut off, the other lamps still lightening.
Bec. The electric devices in the home circuit are connected in parallel.

7 – When the total power of the electrical instruments used in houses exceeds a certain value, the electric current intensity flowing through the fuse increase .
Bec. Pw= V I , and the electric devices are connected in parallel , so potential diff. is constant , so Pw∝ I , when the total power increases, the current intensity increases.
8 – the electrical conductivity of different materials is different.
Bec. The electrical conductivity depends on the number of free electrons and the atomic structure of the substance and these properties are different in different materials.

9 – It is preferable to use copper wires in making electrical wire taps.
Bec. Copper has low resistivity to increases the electric current intensity and decreases the electrical energy lost in the wires.
10 – The resistance of metal wires increases by increasing the temperature.
When the temp. of the conductor increases , the kinetic energy of the molecules & the free electrons increases, so the rate of collision of the free electrons increases and the resistance increases.


Physics| revision ch1

What is meant by:
1 – Electric current intensity = 0.3A?
It means that the quantity of electricity passes through a given cross-sectional area per unit time is 0.3 C.

2 – potential difference between two points is 3V:
It means that 3J is the work done to transfer 1C between two points.
3 – E.M.F. of a source is 1.5 volt:
It means that the potential diff., between the two terminals of the source is 1.5 volt when the circuit is opened.
4 – the resistance of a conductor = 10 Ω:
It means that the ratio between the P.d (in volt) across this conductor to the electric current intensity (in amp.) is 10.