Reverse Polarity
Reverse Polarity in a circuit?
I have been searching for hours through my physics book and online for something about emf and reverse polarity 
please help me! i would greatly appreciate it
Suppose the polarity of E2 in the figure is reversed
Suppose the three resistors in this circuit have the values R_1 = 9.45 Omega, R_2 = 7.18 Omega, and R_3 = 15.5 Omega, and that the emf of the batteries are E1 = 16.2 V and E2 = 11.7 V. What is the direction (clockwise or counterclockwise) of the current in this case?
Calculate the magnitude of the current in this case.
I can’t post the pic but it is a square circuit, the left side of the square has battery E1, top of the square starting from the left there is R1 and next to it is battery E2, the right side of the square has R2 and the bottom of the square has R3
in this type of questions circuit diagram is a must to assess the problem correctly. anyhow i will try to answer your question.
since you didn’t mention about the battery initial polarity, i suppose them to be connected in series, then
total emf E = 16.2 +11.7 = 27.9 volt
and total resitance R = 9.45 + 7.18 + 15.5 = 32.13 ohm
so that current i = E / R = 27.9 / 32.13 = 0.852 amp, and the direction of such current may be clockwise or anti-clockwise depending on the circuit arrangement.
furthet, when the polarity of E2 is reversed, the net emf in the circuit E = 16.2 – 11.7 = 4.5 volt
so that current in the circuit is i = E / R = 4.5 / 32.73 = 0.137 amp.
next time, when you ask such questions, try to give circuit diagram.you can get correct help. good luck.
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Finding the difference between reverse and straight polarity
When you see the letters AC/DC on your welder, do you think of one of your favorite rock and roll bands? Seriously though, the difference is quite important to the final quality of your welds.
AC (Alternating Current) and DC (Direct Current) is used to describe the polarity of the electric current that the welder generates and in what direction it travels. If you use the wrong polarity for a certain welding rod, your weld strength will not be very good.
The general terms associated with polarity are reverse polarity and straight polarity. These are common to the welding trade. Another way to describe the two terms are electrode positive and electrode negative. Electrode positive is the same as reverse polarity. Electrode negative is the same as straight polarity. Hence the + and the – written on your welder where the cables connect to it.
Any type of welding rod you buy will be labeled as to what polarity should be used for welding with it. Using the correct polarity will ensure the proper penetration and the over all look of the final bead.
If you use the wrong polarity you can tell by the signs. There will be an excessive amount of spatter, you will have bad penetration, and you will have less control of your arc.
Some welding machines have a switch to adjust the polarity. If your welder doesn’t have one you will need to switch the welding cables around where they plug into the machine. If you want reverse polarity, you need to make sure the electrode holder is plugged into the + terminal.
The easiest way to tell if you are using the wrong polarity is by the sound and the feel of the weld you are laying down. If you don’t have much experience with stick welding you will have a little more trouble determining the difference. I have seen guys weld all day long with the wrong polarity. Then I would grab their welder to use for couple of minutes and I could tell right away. It all comes down to experience.
If you don’t have much experience stick welding, you will need to double check the way the machine is set up. Follow what the welding rod package tells you to do and do it. If you are using 7018 rods, you will want to make sure it is set up for reverse polarity.
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