E=ir
 

I know this stuff, but I have a physicist arguing with me about it. I think he's all wet but he's really smart and now I'm worried I'm wrong.

Using the basic electrical equation E=IR:

If you put 12 volts across a 10 ohm resistance the current is some value ( X ).
If you double the resistance the current becomes ( X/2 ) and the voltage remains the same, correct? He SWEARS the voltage is affected and the current is not. I cannot understand his logic.
edit:
This is in the context of an automotive circuit, so the max voltage in the entire circuit is 12.

E = I R
Suppose R x100 means E x100 and I unchanged
W = E I
So W x100
Thus by merely raising the resistance, you can get arbitrarily more voltage E and power W from the source. With sufficiently high resistance, I can get millions of volts and millions of watts from a AAA battery.
Very cool, if it were true.

E=IR or R=E/I

Therefore 2R=2E/I

Ian

Theoretically you're right but in practice there may a small voltage drop when applying double the resistance but you would defiantly get way more current.

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Just for info, I use this formula for determining the value of a resistor for using LEDS.

When using a Light Emitting Diode (LED) as an indicator, use the following formula to determine series resistance for various voltages:

R = (E - 1.7) x 1000/I,

Where R is the resistance in ohms, E is the supply voltage (DC) and I is the LED current in milliamps.
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The household water analogy can be used.
-Voltage is the consistant water pressure(alternator/regulator) supplied from the street.
-Total resistance is the pipe diameter(wires) along with restrictions in valves(resistors/rheostats) and connections.
-Amperage is the total flow of water molecules as a result.

Your buddy is wrong.

If you double the current in your car and the voltage goes down it is likely due to the source impedance which is another resistor

Nevermind. I figured out what he was talking about. Thanks.

You can get very high voltage from a low voltage source, but not that way.

You hook up a battery to an inductor. The current becomes stable and it creates a magnetic field. If you suddenly open the circuit, the magnetic field breaks down which creates a voltage across the switch. This energy in the coil needs to go someplace. The voltage will increase until it "jumps" the gap. And that is usually a big spark. This is why there are little resistors in parallel with most coils, to bleed down the energy in the coil.

Kids used to make surprise sparkers with a box (usually an old cigar box, something with a hinge) some insulated wire, a battery, a good sized nail or bolt, some aluminum foil and some tape.

You wind the wire around the nail, which makes the inductor, then use a little tape to keep it in place. Tape the battery to the bottom of the box. Cut a couple of pieces of narrow strips of foil. Tape one piece to the box top and the other to the box side, lined up to create the switch. Now tape one end of the coil wire to the foil on the side and the other coil wire to one end of the battery. Finally connect the foil on the top side to a spare piece of wire and then connect and tape that wire to the other end of the battery.

Close the box and hand it to someone. When the open the box, a big spark will cross the foil contacts. Hilarious! SmileWavy

Oh year, E is usually ENERGY. V is used for voltage. V = I*R, at least in DC circuits.

Assuming 2 resistors connected in series voltage supplied to the 1st resistor will be greater than voltage supplied the 2nd resistor. Connected in parallel voltage supplied to both resistors will be equal to source voltage. Current is determined by the total resistance of the circuit. I = V/R(total) where R(total) = R1 + R2 in series and 1/R(total) = 1/R1 + 1/R2 in parallel.

I also think of V for voltage, but in electrical context, they sometimes use E (electromotive force). Irritating, but there you are.

Yes, that is what I was taught in school I = E/R (back in the 70's), but soon thereafter, E was substituted with V.

I have never seen someone use "E" for emf/voltage. E is used for Energy.

was thinking the same.

He assumes the circuit is fed by a constant current source, instead of a constant voltage source (alternator with regulated voltage).

E and V are the same (voltage)

I= Current. (Voltage = Current X Resistance)

It is a math equation you alter the resistance (E=IR so 12= 2*6 for instance)

double the resistance but keep voltage the same (12= 1*12) then the CURRENT is altered.

In DC- Voltage is determined by your source, only the CURRENT can be changed.

on an AC circuit the CURRENT and Voltage can be changed through the use of a step up or step down transformer. That's how you alter the current and voltage on AC circuits.

DC- battery power (cannot alter V unless you have more than one battery) AC= non storable power (can alter V and I with transformers)

In the context of AC voltage he is correct- assuming step up or step down transformers.

There.

rjp

EDIT- congratulations, tell your nerd friend you are correct. For once :cool:

It's not an E, its a capitol Epsilon. Or at least that is how I was told to represent Electromotive force. 12V battery has a greater electromotive force than 12V but the internal resistance brings it down at the terminals. Anyway, they are the same thing.