
Experiment to verify that resistors
obey Ohm's law and to verify the formulae for calculating the effective
resistance of resistors connected in Series and in Parallel
1.
Preparation: 
a) Learn
Ohm's law. 

b) Note that
Ohm's law applies to metals. Most resistors used in electronic
circuits are not made of metal. What are they made of? How does
their behaviour differ from that of resistors made of metal? 

c) Learn
the "series and parallel resistors formulae". Try to
"prove" these formulae, starting from Ohm's law and the law
of conservation of charge (Kirchhoff's current law) and the law of
conservation of energy (Kirchhoff's voltage law). 
2. 
Obtain
results from which you could plot graphs of voltage (horizontal)
against current for (at least) two resistors separately and then
for the same two resistors connected i) in series and ii)
in parallel with each other. Plot the graphs and use them to
calculate the resistances, R_{1}, R_{2} and R_{e}
(series) and R_{e} (parallel). 

A graph of voltage
against current for a component is called the characteristic
of the component. To obtain the electrical characteristics
of a component we need a variable voltage supply. The
simplest way to produce a variable voltage supply from a
fixed voltage supply is by using a rheostat (variable
resistance) as a variable potential divider. 

. 
The circuit for obtaining
the characteristics of a component is shown in the circuit diagram
below (figure 1). Before starting the experiment, complete figure 2. 

figure 1 

figure 2 

A variable
potential divider circuit is useful in this and many other
similar experiments but it should be noted that it is NOT a
very stable voltage source. To see this, try setting the
sliding contact S to give a voltage of 3V with the
resistor, R, removed from the circuit. Now
connect the resistor back in the circuit; what happens to
the reading of the voltmeter?
Can you explain this change?



