Hi Simon, I got a question regarding electricity and hope you can help me. What is the difference between DC and AC voltage? Which one is more dangerous and can kill us? Is it the watts or amps that can kill ?

Direct current (DC or “continuous current”) is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type.

An alternating current (AC) is an electrical current whose magnitude and direction vary cyclically, as opposed to direct current, whose direction remains constant.

The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the 1880s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mains-frequency AC tends to interfere more with the heart’s electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain. Generally, higher frequency AC current tends to run along the skin rather than penetrating and touching vital organs such as the heart. While there will be severe burn damage at higher voltages, it is normally not fatal.

It is sometimes suggested that human lethality is most common with alternating current at 100–250 volts, however death has occurred from supplies as low as 32 volts and supplies at over 250 volts frequently cause fatalities.

Electrical discharge from lightning tends to travel over the surface of the body causing burns and may cause respiratory arrest.

The voltage necessary for electrocution depends on the current flowing through the body and the duration of the current flow. Using Ohm’s law, Voltage = Current × Resistance, we see that the current drawn depends on the resistance of the body. The resistance of our skin varies from person to person and fluctuates between different times of day. In general, dry skin isn’t a very good conductor having a resistance of around 10,000 Ω, while skin dampened by tap water or sweat has a resistance of around 1,000 Ω.

The capability of a conducting material to carry a current depends on its cross section, which is why males typically have a higher lethal current than females (10 amperes vs 9 amperes) due to a larger amount of tissue. However, death can occur from currents as low as 0.1 to 0.3 amps.

Go Wikipedia and search “electric shock”
http://en.wikipedia.org/wiki/Direct_current

Why do some of the electrical equipment only accepts DC voltage and some need AC voltage? How do they determine the standard?

In 1887 direct current (DC) was king. At that time there were 121 Edison power stations scattered across the United States delivering DC electricity to its customers. But DC had a great limitation — namely, that power plants could only send DC electricity about a mile before the electricity began to lose power.

So when George Westinghouse introduced his system based on high-voltage alternating current (AC), which could carry electricity hundreds of miles with little loss of power, people naturally took notice. A “battle of the currents” ensued. In the end, Westinghouse’s AC prevailed.

DC is commonly found in many low-voltage applications, especially where these are powered by batteries.

Thanks. I am impressed by your knowledge in electricity. I have one question for my project.

At 6 cents poer kiliwatthour, what is the cost of leaving a 60W lamp on for 5 days ? You know how to calculate ?

6 cents per KWH
=$0.06 per 1000WH

5 Days = 24 hours x 5 = 120Hours

60W Lamp x 120 Hours = 7200 Watts Consumed

7200/1000 x $0.06 = $0.432
=Round up to 44 cents for 5 days.

Yes lights are not that bad…the worst are water heater and air conditioner (Air-Con).

If you go to the site at
http://www.cpi.coop/home_energy/billestimator.php

You can see that the national average for Lighting is minimal per month.

Thanks Simon. Air con and water heater indeed consume the most amount of electricity. Do you have any experience with Wheatstone bridge?

A Wheatstone bridge is to measure resistances ranging from 10ohms to 100 ohms. If R1=250ohms and R3=200ohms, calculate the required maximum and minimum resistances for R4.

If you can provide me with the calculation, it will be better. Thanks !

Simon,any solution for me ?

I haven’t been doing electrical calculation for over 10 years!! So correct me if I am wrong…

According to wikipedia…there are R1,R2,R3,Rx

Did you refer R4 as the Rx?

Regards,
Simon

Simon, yes R4 is the Rx. Can you help me figure it out?

There is a missing component somewhere. You cannot calculate this without the voltage input.

http://www.daycounter.com/Calculators/WheatStone-Bridge/WheatStone-Bridge-Calculator.phtml

I think the formula is R2=R1*R4/R3

I find it strange about the question asking for the min and max for R4 when they provided it’s use to measure from 10 ohm to 100ohm.

Isn’t R4 the “measured one” and R2 the variable used to “tune” so that the voltage becomes zero when R1/R2 = R3/R4

Are you looking for Min & Max of R2 instead?

I have to admit I am confused. It’s like asking me an incomplete question. Do you have the full question with diagram and illustration? Scan it and put in some website like http://photobucket.com/ so I can see the whole question instead.

I will try my best to help

I will check with my lecturer about this question again. Maybe there is some information missing. I am doing a topic now on magnetic circuits and is quite difficult.

Simon, what course do you major in?

After graduating from Ngee Ann’s Diploma in Electronic Computer Engineering (ECE) many years back I came out to work for about near a decade liao.

From my email simontay78@gmail.com you can guess my age.

Been off electronics (my worst subject) for too long and without practice it will be hard to pick up again. Now I struggle to find time to re-learn everything from circuits to simple electronics.

Maybe you can even teach me some stuff. I am interested to get some study material from students too…to study and do their calculation to refresh memories.

It’s really been too long…been working in IT (my fav) and sales for all my working life and none requires electronic knowledge haha…

In fact I like IT more but because my English did not get good grades in Secondary School (D7) I am not eligible to get into Computer Science hence chosen ECE instead.