Fundamentals: OHM’S LAWS
Questions:
1. What information does a technician need to determine the resistance of an electric heating element?a) Length and diameter
b) Voltage and amperage
c) Length and voltage
d) Amperage and diameter
2. Which of the following is Ohm's Law?
a) E=1xR
b) R=Ex1
c) R=I/E
d)I=R/E
3. What is the resistance of an electric heating element that is rated at 240 V and draws 30 A?
a) 0.125 Ω
b) 7,200 Ω
c) 8 Ω
d) 72 Ω
4. What is the amperage draw of an electric heating element that has a resistance of 12.8 Ω and is supplied 240 V?
Looking for quick answers on air conditioning, heating and refrigeration topics? Try Ask ACHR NEWS, our new smart AI search tool. Ask ACHR NEWS
a) 12.8A
b) 3,072A
c) 0.05A
d0 18.75A
5. What happens to the amperage of an electric heating element when there is a decrease in the voltage supplied to the circuit?
a) Amperage changes only due to the resistance.
b) Amperage is increased by voltage changes.
c) Amperage is decreased by voltage changes.
d) Amperage is never affected by voltage changes.
6. Should there be an effect on the heating output of an electric heating element when the voltage drops?
a) Yes
b) No
Answers:
1. b) When a technician comes upon a job where the electric heating element has been damaged and needs replacement, it is imperative that a matching element be ordered. To properly order a new element, it may be necessary to supply the wholesaler or manufacturer with a number of details. The details can include the voltage, amperage, resistance, and length and diameter of the heating element. If the element is damaged, as is often the case, then the technician will need to calculate the resistance using Ohm’s Law.
2. a)
Knowing the proper formula for Ohm’s law allows the technician to determine resistance, voltage, and/or amperage of an electric heating element. It is important to remember that the formula uses letter symbols for the actual units of measure as follows:
The formula can be expressed in a number of ways that are all correct.
3. c) Using the formula R = E ÷ I and inserting the actual values (R = 240 V ÷ 30 A), you will get the result of 8 Ω.
4. d) Occasionally, the technician will arrive on a job where he or she needs to know the amperage draw of a heating element. Unfortunately, the data plate may either be missing or impossible to read. In this instance, the technician will have to calculate the amperage draw after taking the resistance reading of the element and knowing the voltage supplied.
5. c) The resistance of a heating element used on a specific job is not going to change unless the element is replaced with an unlike element. Using one of the formulas above, you can determine the amperage draw of the heating circuit. Using the values of 12.8 Ω, 240 V, and 208 V you will calculate that the amperage changes from 18.75 A on a 240-V circuit to 16.25 A on a 208-V circuit.
6. a) Since the resistance of the heating element generally does not change on jobs but the same voltage is not always supplied, we can see that there is a change in the work being done by the element due to the voltage drop and resultant amperage drop. When the amperage drops, so too does the heating output.
Publication date: 03/12/2001