Ohm's Law

 

I ampere = I A

I milliamp = I mA           1/1000 A = 10^-3A.

I microamp = I μA          = 1/1 000 000 A =10^-6 A

 

I volt = I V

I kilovolt = I kV =1 000 V            =10³ V

I millivolt = I mV = 1/1000 V = 10³ V

 

1 ohm = 1Ω

1 kilohm = 1KΩ =10³ kΩ

1 megohm        = 1 MΩ = 1 000 000 Ω = 10⁶Ω

 

 

 

 

 

Find the potential difference needed to send a current of 2 μA through a resistor of 4 kΩ

 

                                    V=IR                 V = p.d. in V

I = current in A = 2μA = 2 x 10^-6A

                                                     R =  resistance in Ω = 4kΩ = 4000Ω

                                    V          = 2 x 10^-6 x 4000

= 8 X 10^-3 V or 8 mV

 

 

 

1 Find the current through a resistor of 20 Ω if the potential difference across it is 0.5 V.

 

2 When a p.d. of 2 V is applied across a resistor the current is 0.1 A. What is the value of the resistor?

 

3 A p.d. of 100 V is applied across a resistor of 2 kΩ. What is the current?

 

4 What p.d, will produce a current of 2 mA through a resistor of 4 kΩ?

 

5 Find the value of a resistor if a p.d. of 2 V produces a current of 0.5 μA.

 

6 Find the current through a 1.5 MΩ resistor when a p.d. of 4.5 V is applied.

 

7 A lamp operated by a 240 V supply takes a current of 0.625 A. Find the resistance of the lamp.

 

8 A coil of very fine copper wire is found to take a current of 0.75 A when a p.d. of 4.5 V is applied. If the wire has a resistance of 1.5 Ω per metre, what length of wire is in the coil?