Ohm's Law

Ohm's law states that at constant temperatures the current (I) going through a load (R) is proportional to
the potential difference or Voltage (V) across it. 

This law can be summarized as a simple but  very important equation:

 V= I  x R

Where:
 V is the voltage measured in volts,
 I is the current measured in amperes and
R is the resistance measured in ohms.

To verify this simple law we can conduct an experiment.
 Using the above circuit we will use a voltage source (applied voltage) of 10 Volts and a resistance of 1 ohm.  Then we will measure the current flowing through the resistance using the ammeter and the voltage drop across the resistance using the voltmeter.   We record the values of voltage and current in a chart.  We repeat the same experiment several times using the same voltage source but replacing the resistor with a 2 ohm resistor first and finally with a 5 ohm resistor.

We Repeat the experiment again with two different voltage sources; 2.0 Volts and 5.0 Volts and record our findings.

Observations Table:

Results:

We note that the given resistance value is equal to the calculated resistance value.
 This confirms Ohm's Law V = I x R

Graphical Analysis:

If we plot the values of voltage (on the y-axis) vs. current  (on the x-axis) for each resistance we obtain the following set of graphs

Conclusions:

1. The slope of each line is the resistance value of the given resistor

slope = rise/run

                         = voltage/current

               = resistance

2. Another way of stating OHM's LAW is by calculating the value of the slope on the V vs. I
    graph for a given resistor

3. The relationship between voltage and current for an "ideal" load is that the voltage is directly proportional to the current.

4. An ideal resistor or load is known as an "ohmic" resistor

Numerical Examples:

Using the equation   V= I  x R, try the following questions:

Question 1:

 A filament lamp passes a current of 60 mA (0. 06 A) when the voltage is 6V What is the resistance of the filament?

 Answer R=V/I = 6V/ 0.06 A = 100 ohms ( 100 Ω)

Question 2:

 What is the current flowing through a 30 ohm fixed value resistor when it is connected to a 9V battery?

Schematic diagram (In the following diagrams we used electron current flow -- negative to positive).

 Solution:

 I = V/R

  = (9 V / 30 Ω)

 = 0.3 A

  = 300 mA

 Question 3:

 What is the voltage across the 2.2 kΩ (2200Ω) in the circuit below?

 Solution:

V= I x R

= 2.2kΩ x 4 mA

= 2200Ω x 0.004A

= 8.8 V

 Review Assignment

1.     Complete the following chart
 

 2.     Draw schematic diagrams for the following circuits using proper symbols

a.  A series circuit with a 9.0 volt battery a light bulb a resistor and a diode.

b.   A parallel circuit with two resistor, a battery, and a light bulb

 3.  Calculate the resistance of the following resistors using the colour code chart.

     For each resistor indicate the tolerance (high/low value) expressed as a percentage.

a)     brown, black, green, gold

b)    violet, red, red, silver

c)     blue, yellow, orange, gold
 

4. Give the proper symbol and amount for the following prefixes:

Example: centi – symbol “c�; amount 0.01 (or 1/100)

a)     Giga

b)    Mega

c)     Kilo

d)    milli

5. Complete the following charts