10th Class Science Electricity and Circuits Electrical Power

        Electrical Power

 

When electric current flows through the circuit it use up the electrical energy to do certain amount of work. The rate of doing work by the electric current is called the electrical power. It is given by

 

\[\text{Power }=\frac{\text{Electrical Work Done}}{\text{Time Taken}}\]

                Or,          \[P=\frac{W}{T}\]

The SI unit of power is watt or joule per second. The power of one watt is defined as the rate of doing work of one joule per second. One kilowatt is equal to the 1000 watt. We can also define the electrical power as the rate of consuming electrical energy.

Since we have, \[P=\frac{W}{T}\]

But work done is equal to the

\[B=V\times I\times T\]

Therefore, \[P=\frac{V\times I\times T}{T}\]

\[\Rightarrow \,\,P=V\times I\]

Where V is potential difference and I is the current flowing through the conductor.

But, \[V=I\times R\]

Putting in the above equation we get,

\[P=I\times R\times I\]

\[\Rightarrow \,P={{I}^{2}}\times R\]

Again, \[I=\frac{V}{R}\]

Therefore, \[P=\frac{{{V}^{2}}}{R}\]

Where R is the resistance of the conductor.

 

 

            Electrical Energy

Electrical energy is defined as the product of power and time. The unit of electrical energy is kilo watt hour.

\[E=P\times t\]

One kilowatt hour is the amount of electrical energy consumed when an electrical appliance of one watt power is used for one hour.

1 Kilo Watt = 1000 watt

Therefore, 1 kilowatt hour = 1000 x 3600

\[=3.6\times {{10}^{6}}\,\text{joules}\] .

 

 

          Heating Effect of Electric Current

The heat is produced when an electric current is passed through the wire of high resistance. The resistance in the wire offers resistance to the flow of current. Hence, work must be done by the current to keep itself flowing. If W be the work done by the current I flowing through the conductor for time t and Q be the total charge flow during this time against the potential difference V, then

\[W=Q\times V\]

But, \[Q=I\times t\] and \[V=I\times R\]

Therefore, \[W=I\times t\times I\times R\]

\[\Rightarrow \,W={{I}^{2}}\times t\times R\]

Assuming that all the electrical energy consumed during this work done is converted into heat we have,

Heat produced \[(H)={{I}^{2}}\times t\times R\] Joules

Thus, according to joules law of heating:

• Heat produced is directly proportional to the square of current flowing through the conductor
• Directly proportional to the resistance of the conductor
• Directly proportional to the time for which the current flow through the conductor.

  

There are many application of heating effect of the current.

 

          Some of the Heating Effect of the Current are

 

1. The appliances’ such as electric oven, room heater, electric iron etc uses electrical energy and converted it into heat energy.
2. The heating effect of electric current is used in electric fuse for the safety purposes.
3. It is used in electric bulb for producing light.

 

 

 

 

        Identify the series connection in the given figures

(a)    

(b)  

(c)    

(d)  

  

(e) None of these

 

Answer: (d)  

 

 

        Calculate the equivalent resistance of the circuit given below:

(a)                                                                

(b)

(c)                                                                 

(d)

(e) None of these

 

Answer: (b)  

 

 

       Which one of the following is a safety device in the household electric circuit?

 

(a)             

 

(b)

 

(c)                          

                    

(d)  

 

 

Answer: (a)  

 

 

     

Calculate the electrical energy consumed by a electric bulb of 100 watt in 40 hours.

(a) 3 kwh

(b) 2 kwh       

(c) 4 kwh             

(d) 6 kwh

(e) None of these

 

Answer (c)  

 

 

      Which one of the following appliances produces the maximum heat when electric current flows through it?

(a)    

(b)   

(c)    

(d)     

 

(e) None of these

 

Answer: (b)  

 

 

• A typical window left open overnight in winter will waste enough energy to drive a small car over 35 miles"
• A PC monitor switched off overnight saves enough energy to microwave six dinners"
• The first windmills were developed in Persia in about 600 B.C.

 

 

• The flow of electron through a conductor constitute electric current.
• Potential difference of a conductor is directly proportional to the current flowing through the conductors.
• The force which opposes the flow of charges through the conductor is called resistance.
• The resistivity of a conductor is defined as the resistance of the wire of unit length having unit area of cross section.
• The resistance of the wire is directly proportional to the length of the wire.
• Fuse, earthing and main switch are the three main safety device in the house hold circuit.
• The house hold circuit is connected in parallel so that each appliance works independently.