No matter what your engineering discipline - you would have learnt about Ohm’s Law at some time or other (even at school). Herewith a simple application question which tests your conceptual knowledge. I posed this a few years ago and have brought it back somewhat modified.

Dear Colleagues,

No matter what your engineering discipline - you would have learnt about Ohm’s Law at some time or other (even at school). Herewith a simple application question which tests your conceptual knowledge. I posed this a few years ago and have brought it back somewhat modified.

Ohm’s Law

As we all (should) know, Ohm’s Law states that the current through a conductor between two points is directly proportional to the potential difference across the two points with the following formula: 

I = V/R

Where I is the current through the conductor in units of amperes (A), V is the potential difference measured across the conductor in units of volts (V), and R is the resistance of the conductor in units of ohms. (Thanks Wikipedia).

Do you Understand Ohm’s Law?

An electrician opens the control panel of a 3-phase resistive electric furnace while it is in operation and accidentally receives a shock from one of the 400 Volt lines inside the panel, while the furnace is drawing current, of say 100 amps. This is situation A.
The furnace is now switched off; but the electrician (being an idiot) accidentally touches the same place before (situation B) and gets another shock from the lines feeding the control panel. Assume everything is the same for both situations (skin resistance/point of contact/humidity). 

Which was the more intense shock for the electrician? Situation A or B?

 

 

 

 

The Answer

The answer has to be Situation B where the voltage has gone higher due to lack of load (no current being drawn) of the furnace. The 100A current in the first situation A has no direct relevance (apart from being a red herring).

A few Parting Comments

Thanks to John Reid for posing the question.

As Elbert Hubbard wryly observed: You can lead a boy or girl  to college but you can't make them  think. 

Yours in Engineering Learning,

Steve

Mackay’s Musings – 21st April’15 #561
125, 273 readers – www.idc-online.com/blogs/stevemackay