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131 votes

How do I prove to my physics teacher that adding a battery in parallel doesn't double the current?

Just ask her what the voltage across the resistor is
Dirk Bruere's user avatar
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97 votes

How do I prove to my physics teacher that adding a battery in parallel doesn't double the current?

Method 1 simulate this circuit – Schematic created using CircuitLab Figure 1. A simple practical experiment. Performing an experiment with the circuit of Figure 1 would demonstrate that ...
Transistor's user avatar
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71 votes

How can 4 resistors of 30ohms each be connected so you end up with a resistance of 18ohms?

Clearly to get 18 ohms, you need some of the resistors in parallel. So, what do you need in parallel with a 30 ohm resistor to get 18? Answer: 45 ohms. Now, you have a simpler problem: how to make ...
alephzero's user avatar
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66 votes
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Why don't we use low voltage Power Sources for high wattage applications?

You are right in that power is the product of voltage and current. This would indicate any voltage x current combination would be fine, as long as it comes out to the desired power. However, back in ...
Olin Lathrop's user avatar
65 votes
Accepted

Why does my 120V 40W lightbulb only have 26 ohms across it?

Filaments heat up to produce light and the tungsten they're made of will change resistance as this happens. The temperature change is let's say 3000K, the temperature coefficient of resistance for ...
vir's user avatar
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58 votes
Accepted

Is Ohm's law really accurate?

Ohm's Law says that the current through a resistance is the voltage across it divided by the resistance. The key point is that it is the voltage across the resistance, not the voltage of your supply, ...
GodJihyo's user avatar
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46 votes
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How did scientists deal with electronics' problems before Kirchhoff and Ohm's laws?

This is a bit like asking how Aztecs built cars without the wheel: they didn't. There was a chain of invention by scientists in the early 1800s building off each others work. Prior to then there was ...
pjc50's user avatar
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42 votes

How do I prove to my physics teacher that adding a battery in parallel doesn't double the current?

He said that each battery has a current of 2A if hooked up to the resistor on its own, and so they both have 2A of current through them Right. Both circuits have 2A through them. simulate this ...
followed Monica to Codidact's user avatar
40 votes
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Does a diode really follow Ohm's Law?

This really is not a black and white question and many folks will argue it does not follow "Ohm's Law", and depending how you argue it, they can be right. However, the truth is the resistance of a ...
Trevor_G's user avatar
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39 votes

Should I take Ohm's law for granted?

Yes, you should take Ohm's law seriously. You do, though, need to keep in mind that it applies only to simple resistors and conductors. Ohm's law is a simplification of some complicated math. It ...
JRE's user avatar
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38 votes
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Why is a 220 Ω resistor for this LED suggested when Ohm's law seems to say much less is required?

LEDs are not very fussy about the current you apply, but will be dimmer with lower currents. The 25 mA you mention is around the Absolute Maximum rating for common LEDs. If you exceed the Absolute ...
Peter Bennett's user avatar
35 votes

How do I prove to my physics teacher that adding a battery in parallel doesn't double the current?

Others have already abundantly pointed out the teacher's wrong reasoning. I want to mention another part of this where there also seems to be some confusion. We all understand now that the current ...
Olin Lathrop's user avatar
35 votes

Why does voltage drop across resistance contradict Ohm's law?

The problem with your test setup is that you are not measuring the voltage across the resistor you are swapping out. You are measuring the drop across the multimeter's internal resistance. You have a ...
Yet Another Michael's user avatar
34 votes
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Component requires specific voltage and current but the math doesn't add up

Many tubes are designed to run on a nominal 6.3V for the heater. The reason you are reading a very low resistance is that the heater is cold. As it heats up, the resistance increases. If in doubt, ...
Simon B's user avatar
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33 votes

How can 4 resistors of 30ohms each be connected so you end up with a resistance of 18ohms?

There is a technique I once found in a graph theory book by Béla Bollobás of all places. Imagine having a resistor network where the schematic can be draw without any crossings. Then for a given ...
Kyle Miller's user avatar
31 votes
Accepted

Is the electric field in a wire constant?

The key to understanding how the electric field follows Ohm's law in a good but non-perfect conductor, is: surface charges. When the conductor is connected to the supply, it is initially subject to ...
Sredni Vashtar's user avatar
31 votes
Accepted

Is power proportional to V or V^2?

It depends on the circumstances. Without knowing anything else, we don't know whether power is proportional to \$V\$, \$V^2\$, or neither. If a variable voltage source is connected to a resistor, then ...
Tanner Swett's user avatar
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29 votes
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Does a generator produce voltage or current?

Internally, a generator produces voltage proportional to its speed. However, that doesn't mean a constant voltage always comes out of a generator run at a fixed speed. The windings and other parts ...
Olin Lathrop's user avatar
29 votes

Is Ohm's law really accurate?

All models are wrong. Some models are useful. First, you should calculate the resistance of the wires (Whether thick or thin) and include that in your model. Provided the thin wire isn't too thin, the ...
The Photon's user avatar
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29 votes
Accepted

Does Ohm's law always apply at any instantaneous point in time?

No, Ohm’s law only applies when considering constant-value resistive elements in a lumped-element circuit model. Maxwell’s equations apply always in all situations. But that requires vector calculus, ...
MarkU's user avatar
  • 14.7k
29 votes

Reasons to use combinations of resistors (parallel or in series): Looking for educational examples

Examples from the real world: Sticking to the "E series". The calculated component value you need might not be available in one of the standard "E series", which in turn means ...
Lundin's user avatar
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26 votes

Ohm's law doesn't seem to be working for this electric motor

Have you ever played around with an electric motor connected to something like a light bulb or another motor? If you spin the motor, the motor acts like a generator and spins the other motor or lights ...
C_Elegans's user avatar
  • 2,901
25 votes

Why does my 120V 40W lightbulb only have 26 ohms across it?

The resistance of a lightbulb is not constant. Becomes the lightbulb resistance is not constant but will change with the applied voltage. See the graph of resistance of a \$100W/230V\$ light bulb in ...
G36's user avatar
  • 14.9k
25 votes

Why is a 220 Ω resistor for this LED suggested when Ohm's law seems to say much less is required?

25mA is enough to almost blind someone with a modern blue LED, it can throw shadows across a dimly lit room which is really irritating. It will also cause the LED to have a truncated life span. These ...
Spehro Pefhany's user avatar
24 votes
Accepted

How does a transformer increase voltage while decreasing the current?

Ohmls Law states V = IR. That means when we increase voltage we must also increase the current (I). That is true when feeding a resistor. But transformer increases the current while decreasing the ...
Transistor's user avatar
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22 votes
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Powering a 5v relay from a 12v power supply

Your relay is rated for 5 V and you know its DC resistance is 400 Ω. That means it draws (5 V)/(400 Ω) = 12.5 mA. Yes, a resistor would work to drive this relay from 12 V. The resistor ...
Olin Lathrop's user avatar
21 votes

Why don't we use low voltage Power Sources for high wattage applications?

Combine $$ P = V \cdot I $$ with Ohm's law $$ V = R \cdot I $$ to obtain: $$ P = I^2 \cdot R $$ where \$P\$ is the power dissipated on the supply wires, \$I\$ is the current flowing through the ...
Andre Holzner's user avatar
21 votes

Amps / current doesnt add up to the expected figure - Ohm's law failure on my part!

2.9 ohms (measured with power disconnected through circuit) As the lamp heats, its resistance rises sharply. Ohms law is never wrong except when misapplied.
Andy aka's user avatar
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