Charging and Detecting.

If you rapidly comb your dry hair, on a dry day , the comb and the hair will develop opposite charges.

The comb will be able to pick up small pieces of paper.

          A table showing electrical measurements.

What is Current?

 

There are several methods to get an electric charge. The clouds develop very high charges. Cars often get charged by the contact of rubber tyres on the asphalt road surface. When you comb your hair the comb can also get charged. Click static electricity.

 Fig. 1. Red arrows show the current according to convention while the electrons rush in the opposite direction.

   Although we consider current as a movement of positive charges, it has to be made clear that the protons are not mobile.

   An Ampere is a fundamental unit.

  A Coulomb is the quantity of charge that passes a point when  1 ampere of current flows in one second.

  If a 2 A current passed for 5 seconds   the total charge passed will be =  2 x 5 = 10 C.

                                                              (1 C is approximately equal to the charge of 6.24 x 10¹⁸ electrons.)

Symbol for current is ‘I’ or ‘i’. while the symbol for ampere is ‘A’. 

  As an equation:-  Quantity=  current x time.

            That is :-   Coulombs = amperes x seconds.       

Fig.2. Since the ampere is the transfer of charge in one second, the product of amperes and seconds gives the total charge transferred.

 

What is electric force?

 A force exerted on a positive charge.

 The electric field is the force exerted on a unit positive charge. ( That is for 1 Coulomb)

Arrows above the symbols show that the quantities  are vectors.

‘ E’ without a sign would mean energy.

Some times only a short line is used for this purpose.

 

Fig.3.The direction of the electric field is the same as the direction of force.

Corollaries:-   F = E x q.   and   q=  F / E.

What is Voltage ?

      Fig.4. Voltage measures the work done on a single coulomb. This is the same as   V= J/q   which is  J = V x q.

     But q= I x t

  . 

Fig.5.Then substituting the value of q we get  

             J = Vit.

 

  

Fig. 6. Work done in volts is equal to the product of Electric force and distance moved.

For the charge to move, work has to be done.   

 But Work = Force x distance.      Work , V = F d.

 Fig. 7. Substituting the value of F, which is E. q.

                                                                    we get :-     W = E q d.

 

  In order to convert Work into Volts we have to divide by the no. of charges. That is ‘q’.

   Volts is work done per charge. That means we have to divide  by ‘q’.

 

    Fig. 8. To find the work for a unit charge   above value has to be divided by the quantity moved.

                                 V   =   E d. d’ is the distance moved by the current.

     ‘

Example 1. Find the missing values.

 

       What is Power ?                    

      Power is the rate of using energy. 

       As electrical energy in Joules =  Volt x Current . X time.   To find the rate of using energy it has to be divided by time.

   

  1.0

A 25 Watt bulb , connected to a 230 V in a domestic supply was used for an hour. Find the following:-

    1.1 Energy it is drawing in one second.

                           J / t = W ……….W= 25 J.  

     1.2 The current it is using.

                      P = Vxi. … i = p/v    …… i = 25/ 230

                                                          = 0.11 A.

     1.3 Total energy it would have used in one hour.

                         In one second it uses 25 J

                         In one hour …………..= 25x 60 x60

                                                             = 90000 J

           Another way  J = Vit……..J = 230x 0.1007 x60 x60

                                                            = 90000 J.

Q. 2.0

A car battery of 12 V has a   of 300 CCA (cold cranking amperes) rating; which means that it can give a current of 300A  even  at  0° C  for a maximum of 30 seconds.

 Find the following:-

2.1 The maximum electrical energy it can supply.

   J = Vit   …………………………...J = 12 x 300 x 30

                       =  108,000 J.

   2.2. If a starter motor has a power rating of 1. 2 kW. ,what is the current it will draw to crank the engine?

      P = Vxi ……………………i  =  1200   / 12……..100 A

   2. 3. If a starter motor draws the current  for 2 seconds to start the car, what is the energy it will draw?

       J = Vit …………...J =  12x 100 x 2

                                      =   24 00 J.

   2.4. How many times the car can be started if the recharging system has failed?

              Number of times =  .108,000 / 2400……= 45 times.

                       

    1. V= ir   so r = v/i…….  r= 12/ 300 …….=  0.04 Ω

       2. E=Vit    ………. J = 12x 300 x30 ….   E = 3600…...E = 108000 J

       3.   P - Vxi.   ……..P= 12 x 300 ………...3600  W same as 3.6 kW.