Electrical Engineering Principles

Next topic  >>  OPEN and CLOSE circuit 

2  

Ohm's Law 

Basic Circuits  

Basic circuit and ohm's law work together, therefor basic circuit must be CLOSE circuit formation. If any of the circuit lines is broken or open then current will not flow and no load will operates.   

Refer to figure 2 is complete close loop of the circuit will the following electrical elements.  

For more information about basic OPEN/CLOSE circuit theory click here .

Figure 1

Close circuit  in Ohm's Law principles

Ohm's Law  formula

This is the law stating that electric current is proportional to voltage and inversely proportional to resistance. 

 When current is increase in the amperes value, this will affect the voltage directly.  But the resistance is then expected to have lower ohm value. 

       I ↑      V  ↑      R (Ω)  ↓   P  ↑

Likewise, when current is lower in amperes value, the voltage will too decrease. Thus the resistance readings expected to be high ohm value.


           I ↓      V  ↓      R (Ω) ↑   P  ↓ 


Figure 2  Ohm's Law formula.

Note:  If any incorrect calculated values will results in damaging the circuit (or load) may suffer burn or cable burn due to cable under size.

Example : Voltage calculation 

1)   Given the current is 2.5 ampere and a resistors is 40 Ω, calculate the voltage, V ?

         Answer:       Apply the formula     V = I x R     

                                   V  = 2.5 A x 40 Ω  = 100 v 


2)   Power circuit for an electric table lamp 80 watt, the lamp resistance is 35 Ω, find the voltage ? 

        Answer:      Apply the formula    V = √(P x R)    

                                 V  =  √(80 w x 35 Ω)  = 52.91 v 


3)   Aloud speaker resistance is 530 W and given current draws from it is 5.8 ampere, find the voltage ?

        Answer:      Apply the formula     V = P ÷ I     

                                  V  =  530 w ÷ 5.8 A  =  91.37 v 

Example : Current calculation 

1)   A torchlight rated 20 watts measure resistance value of 32.5 Ω, calculate the  current I?

         Answer:      Apply the formula     I = √ P ÷ R     

                             I  =   √ (20 w ÷ 32.5 Ω) = 0.784 A


2)   A grinder tools rated 520 watts uses 230 volts AC , find the current I ? 

        Answer:      Apply the formula     I = P ÷ V     

                             I  =  520 w ÷ 230v  =   2.26 A


3)   A resistance is 530 Ω and supply voltage is 230 volts, find current I?

        Answer:      Apply the formula     I = V ÷ R     

                             I  =  230v  ÷  530 Ω =   0.433 A

Example : Resistance calculation 

1)   Given load draws the current about 2.5 ampere and voltage applied 24v DC, calculate the resistance, R ?

         Answer:      Apply the formula     R = V ÷ I     

                                 R  =  24 v ÷ 2.5 A  =  9.6 Ω


2)   A vacuum cleaner rated 420 watts and the current measured 8.5 A, find the resistance, R ? 

        Answer:      Apply the formula    R = P ÷ I²     

                              R  =  420 w ÷ 8.5 A²  =   5.813 Ω


3)   An old television rated 395 watts  and connected to the supply 220 volts, find resistance, R ?

        Answer:      Apply the formula     R = V² ÷ P     

                             R  =  220v ² ÷ 395w  =   122.53 Ω

Example : Power calculation 

1)   A florescent lamp operates on 220 volts AC and draws current measured 1.85 A , find the power rating, P ?

         Answer:      Apply the formula     P = V x I

                             P  =   220 v x  1.85 A = 407.0 w


2)   A heater with resistance value of 485 Ω and draws 230 volts AC , find the power rating, P? 

        Answer:      Apply the formula     P = V² ÷ R     

                            P  =  230 v² ÷ 485Ω  =   109.07 w


3)   A coil resistance measured 125 Ω and taking 2.85 ampere , find the power rating, P?

        Answer:      Apply the formula     P = I² x R     

                             P  =  2.85 A² x 125 Ω =   1015.31 w