Ohm's Law

Resistance

In a previous section, the concept of Electrical Current was covered. The flow of Charge around a circuit can be seen as similar to the flow of water through pipes. Using this analogy, Resistance can be thought of as the size of the pipe; The wider the pipe, the easier the water can flow through the system. The diagram below shows another analogy for Resistance of traffic being reduced from 3 lanes to 1 (the 'Pinch Point' represents the Resistor):-

Resistance is a measure of how hard it is for Current to flow. The larger the Resistance, the lower the Current. Resistance is caused by components not being perfect conductors; when current flows through them, heat is generated, causing energy to be lost.

Resistance has the symbol R and is measured in Ohms (Ω).


Measuring Current and Voltage

In order to measure Current and Voltage within a circuit, the meters must be placed within the circuit in the correct positions:-

Measuring Resistance of a fixed Resistor

The experiment below shows a method for calculating the Resistance of a Resistor:-

By varying the power supply and measuring the Current and Voltage the following graph could be obtained:-

The gradient of the graph above gives the Resistance of the Resistor.


Ohm's Law

In 1827, Georg Ohm published his complete theory of Electricity, part of which focussed on Electrical Resistance. By performing the above experiment using the newly invented Galvanic Cell, Ohm concluded that the Current flowing through the circuit was directly proportional to the Voltage applied across it.

This is written more formally as Ohm's Law:-

V = I R

Where:-

V = Voltage in Volts (V)

I = Current in Amperes (A)

R = Resistance in Ohms (Ω)

Example 1 -

A Circuit is set up as shown below:-

If the readings on the meters are as shown, what is the value of the unknown Resistor?

V = I R

R = V / I

R = 10 / 0.1

R = 100 Ω


Variable Resistors

In the section above, the focus was upon Resistors of a fixed value, however, there are several other types of Resistor that will be used within this unit:-

The above diagram shows the symbol for a variable Resistor. This type of Resistor can be set to a range of Resistances, generally by turning a dial on the component. This is therefore a manually controlled Resistor.

The above diagram show the symbol for a Thermistor. This type of Resistor changes its Resistance due to the Temperature of the component. As the Temperature increases, the Resistance decreases.

The above diagram show the symbol for a Light Dependent Resistor (LDR). This type of Resistor changes its Resistance due to the light level on the component. As the light level increases, the Resistance decreases.