Lab Notes – Ohm’s Law – Physics 2426

Equipment:
·

· Voltmeter

· Ammeter

· 11 ? and 44 ? variable resistors

· Connecting wires

Theory:
If a potential difference ?V is applied across some element in an electrical circuit, free charges will flow through the element and thus a current I is produced in the element. The current is proportional to the potential difference but is limited by a quantity known as the resistance R. The larger the resistance is, the more difficult it is for charges to flow through the element which lowers the current.

The right hand side of the above equation can be considered as the definition of resistance. That is to say, resistance is the ratio of the potential difference across an element to the current passing through it. For some electrical devices that ratio may change as the potential difference and current change, but nonetheless we will still consider that ratio to be the resistance of the device. The units of resistance are therefore Volt/Ampere which is given the name Ohm, named after German physicist Georg Simon Ohm. The capital Greek letter omega (?) was rather cleverly chosen to represent the unit of Ohm. On a somewhat unrelated but amusing note; the reciprocal of resistance is a measure of electrical conductance, the ease with which an electrical current flows. The unit of electrical conductance is therefore ? -1 = ? which is called Mho.

For some electrical devices the ratio of the potential difference across it to the current passing through it is constant under the right circumstances. This means that the resistance is constant. Such devices obey what we call Ohm’s Law and are called ohmic devices.

In order to test if a device obeys Ohm’s Law it is necessary to vary the potential difference (the current will then also vary) and then determine if the ratio of potential difference to current is in fact constant. This will be accomplished by plotting voltage versus current. If the data lie in a straight line, then that is a good indication that the device obeys Ohm’s Law. What will the slope of a fit line through the data points represent?