This formula can be used in experiments such as those involving specific heat capacity (Mandatory Experiment 8).The symbol W is usually used to refer to electrical energy; Q is used when dealing with heat energy.The two symbols are interchangeable,so the above formula is often written as
Q VIt
Instead of using a joulemeter to measure energy supplied, an ammeter, voltmeter and a timer can be used.
SCIENCE,TECHNOLOGY & SOCIETY
Advantage of use of EHT in transmission of electrical energy EHT, Extra High Tension, means high voltage.
In accordance with Joule’s law, heat produced in a conductor is proportional to current squared. It is therefore desirable to keep the size of the current small so that the energy loss will be reduced.The energy carried per second (the power) transmitted through the wire is necessarily high. Based on the formula the current will be reduced.
P IV , it follows that if the voltage is increased Higher voltageQsmaller currentQless energy wasted as heat
Extra High Tension (EHT) transmission lines
Generating station
Transformer 1 Fig 18.2: EHT transmission lines
Transformer 2
SAMPLE PROBLEM 18B
Calculate the power lost (as heat) as 80A of current flows through EHT wires when the potential drops from 50 kV to 45 kV from the first pylon to the last.
SAMPLE ANSWER 18B The first step is to calculate potential difference between the pylons:
V 50 103 45 103 5 103 The potential difference between the pylons is 5 103 V . The rate at which (electrical) energy is lost (as heat) is found using the formula P IV :
Electrical energy is converted to heat at a rate of 400 kW. P IV (80)(5 103) 4 105 W
