# Questions tagged [ohms-law]

A well known relationship that describes the relationship between Voltage and current through a device's resistance expressed mathematically as V= IR. This formula says that voltage across the device is equal to the current through the device multiplied by the resistance.

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

simulate this circuit – Schematic created using CircuitLab My physics teacher said that the current through the resistor is 4A because each battery has a current of 2A if hooked up to the ...
7answers
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### Why don't we use low voltage Power Sources for high wattage applications?

Super nooby question involving Ohm's law, but this has been on mind this morning. Say I have a 60W device, and I want to power it. Usually this calls for a 120V source or something. However, why not ...
7answers
23k views

### Where does U for voltage come from?

I believe in Europe the letter U is commonly used for voltage in (eg.) Ohm's law $U = I × R$. I think I understand where the letter V came from, commonly used in North America. But what's the story ...
4answers
55k views

### accounting for LED resistance

I'm doing a simple lab (I'm a hobby EE) to reinforce my ohm's law math and learn a little about how to do proper measurements with a multimeter. I have simple circuit with a 2.2k ohm resistor ...
12answers
11k views

### Does a diode really follow Ohm's Law?

Does a diode really follow Ohm's Law? Ohm's law states that the current through a conductor between two points is directly proportional to the voltage across the two points. Introducing the ...
7answers
54k views

### Why do we need resistors in led

I've researched and it says that resistors limit the current flowing through the LED. But this statement confuses me because we know that in a series circuit, the current is constant at every point, ...
4answers
77k views

### How can a resistor affect current AND potential at the same time?

Though the resistor is always introduced as one of the most simple components it is the one which makes least sense to me. Ohm's law defines the resistance as$$R = \frac{V}{I}$$ this means that the ...
3answers
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### How did scientists deal with electronics' problems before Kirchhoff and Ohm's laws?

Both physicists developed really powerful laws which still nowadays rule the electronic behavior of circuits. These help us every day to solve problems, calculate circuit variables… but how did ...
7answers
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### Ohm's law doesn't seem to be working for this electric motor

I'm a beginner in this field so please forgive me if I'm confusing with my question. There is a component that I can't understand with Ohm's law which is a washing machine drain pump. Washing machine ...
4answers
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### What are the consequences of oversizing resistors?

When building a circuit to power an LED, we use Ohms law to calculate the required resistance, then to calculate the required wattage of the resistor. Suppose this formula dictates a 1/8 W resistor, ...
6answers
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### Why are these readings violating ohm's law? (Are they?)

I'm brushing up on my high school electronics knowledge and I decided to experiment with a small acquarium pump I had lying around. I took some measurements with a multimeter and the results confuse ...
1answer
3k views

### Powering a 5v relay from a 12v power supply

I have a relay which is rated for 5v. My power supply is 12v. I have measured the resistance across the coil as 400 ohm. If I place the relay in series with a resistor that is 560 ohm, would it ...
5answers
40k views

### How do I calculate the necessary resistance for a voltage divider?

I’m self-taught, and this is a little bit of a thought experiment for me to understand Ohm’s Law better. I have a very simple voltage divider. Given a 15V DC input, each of three 4.7KΩ resistors ...
3answers
16k views

### Why do diodes have a voltage drop?

Is resistance what makes a diode have a voltage drop? Or is it impedence? If it is electrical energy being converted to light, please tell me what this effect is called.
6answers
35k views

### Why does an LED have a maximum voltage?

When powering a simple LED circuit (DC power source, LED, resistor), does the supply voltage matter, as long as the correctly calculated current limiting resistor value is used? In other words, is ...
4answers
18k views

### Why do LEDs not obey Ohm's law?

In a previous question, it was brought to me that LEDs do not obey the Ohm's law. (See Calculate expected voltage around a resistor) Simply put: how is that? What makes them behave so differently? ...
1answer
11k views

### How to choose correct resistor with proper wattage and ohms for 6 leds connected in a serial circuit?

Let's suppose I've six LED lights with each 3.2 Volt and 20mA and all six of them are connected in one serial circuit. I know there is better setup but for this I'm using this circuit setup. I'm ...
4answers
663 views

### I do not understand what power actually is

I realize this is probably a super common question, but I have to put it in my own words. I try to understand ohm's law with the water analogy. Two tanks with water, one with a higher level than the ...
2answers
3k views

### How a DC motor works when connected to a power supply but no resistor?

I connected a 12V DC motor to a battery of 12V only. The motor rotated perfectly fine. But my question is that if you don't have a resistor attached to the circuit, as per theory the current is ...
3answers
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### Why is my circuit's current far lower than expected with 2.6v / 220Ohm / and LED?

I have a very simple circuit which I have measured the current to be only 3mA but should be in the range of 18mA. I have a simple circuit using 2 AA batteries (NiMH) at 1.3V each in series for a total ...
7answers
9k views

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

An association of 4 resistors of 30ohms each, how can one assemble them so you get a resistance equivalent of 18ohms?
4answers
12k views

### Does a generator produce voltage or current?

I know that this question might sound very stupid for +90% of you, but my brain doesn't really get the electricity generation. The thing that I don't understand is what does a generator produce, ...
3answers
2k views

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

Can anyone offer suggestions as to where I am going wrong please? In my circuit (diagram below) the system voltage is 12.03 V and the lamp resistance is 2.9 ohms (measured with power disconnected ...
6answers
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### How do resistors limit current for an LED? [closed]

If I have an LED with Vf = 2.1V and I = 20mA, Ohm's Law says I should have a resistor in series at a value of R = 145Ω, if the voltage supply is Vs = 5V. However, I believe can put a higher valued ...
4answers
508 views

### How can current-limiting supplies be possible?

Knowing Ohm's law, the current drawn is directly proportional to the resistance. In some power supplies, you can limit the current. Let's say the voltage is 12 volts, it supplies 1 amp, and the ...
2answers
1k views

### How to understand this circuit?

I'm studying about Ohm's Law, and the following problem perplexed me. The answer is $I_{t} = 11$mA and $U_{r} = 41$V. What I need to find is $U_{t}$ and $I_{t}$. The only thing I've found so ...
3answers
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### How is current lowered as Voltage is increased in AC power transmission

I was reading about power transmission on wikipedia and came across the explanation of the efficiency of AC power transmission that said power in the load = (I^2)* R while power transmitted= IV. The ...
1answer
8k views

### Joule–Lenz Law.. Forgotten history? [closed]

We are all, or should be, extremely familiar with this graphic. It is course the magic triangle used to represent Ohm's Law. We should all also be very familiar with the similar power triangle. ...
7answers
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### Batteries have zero voltage

According to Ohm's law V=IR. This means that if the current is zero there is no voltage. Does this mean also that an unplugged battery has zero voltage? Then why does it say otherwise in the package. ...
4answers
1k views

### How current works

I apologize for the very basic question but I seem to be lacking some fundamental knowledge. I am using an infra red LED in a circuit and it seems like it is drawing around 15mA at 5v. I was told ...
3answers
755 views

### How does a 'voltage drop' work?

It is my understanding that, in an electric circuit, voltage is divided amongst components in a ratio determined by their various resistances. That is, for example, if a 5Ω‎ and a10Ω‎ resistor were ...
3answers
812 views

### With parallel termination of a transmission line, why does the receiver see the full voltage?

With parallel termination of a transmission line, the line characteristic impedance $Z0$ is matched with the pull-down resistance $Rt$. But, in that case, shouldn't the receiver see half of the ...
1answer
801 views

### When does a resistor affect current and voltage?

I was playing around with a computer power supply and some resistors and noticed that raising the ohmage decreased the amperage but the voltage was unaffected. Does this mean that voltage is a ...
2answers
209 views

### Voltage distribution between components

I often see the same method for computing voltage distribution in electronic circuits implying components whose V-I curves are not trivial. For example, in a circuit implying a resistor and a LED ...
6answers
2k views

### What's the correct term to describe a conductor which doesn't always obey Ohm's law?

Non-ohmic comes to mind, but that gives an impression that the conductor never satisfies Ohm's law. Is there a better term which describes a conductor which might satisfy Ohm's law at certain voltages ...
7answers
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### Why does an infinitely decreasing resistance infinitely increase current?

So I have been mulling over Ohm's law for hours today. It makes sense to me that the current gets bigger as voltage rises, since the voltage is supplying more electrons and current is the number of ...
3answers
2k views

### LED current limiting resistor and Ohm's Law

Yesterday I was trying to figure out how many ohms of resistor should I be using for my LED, and found this post right here in the forum. The formula states that one should subtract the voltage drop ...
2answers
28k views

### How to calculate current and voltage draw of a single resistor circuit?

If I have a single 270 ohm resistor connected in a circuit to a 3.3V 50mA power supply, how can I measure the voltage and current for the resistor? I'm familiar with Ohm's law V=IR, where V is the ...
4answers
4k views

### Sharing a pull-up resistor

I have two IC chips. On one chip I have one 10K ohm pull-up resistor and on another chip I have three 10K ohm pull-up resistors. At first I was using four through hole pull-up resistors. Then I ...
4answers
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### Understanding “Current Draw” in terms of “Ohm's Law”

I'm having trouble understanding Current Draw as it relates to Ohm's Law (V = IR). I have a power supply rated at 5V and 500mA. I'm trying to power a string of LED lights. According to my ...
4answers
7k views

### How do current limiting resistors work between ICs?

In a simple battery+resistor circuit with a fixed voltage, the resistance of a series resistor determines the current flowing through it. But what happens if this resistor is placed between two ICs? (...
3answers
1k views

### Rule of thumb for LED Ballast Resistor

As well all know, using an LED without some sort of current control is a bad idea. One of the most basic types is a simple Ballast Resistor, paired with Ohm's Law. Based on Vin, Vf, and the desired ...
6answers
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### How does a transformer increase voltage while decreasing the current?

Ohm's law states V=I*R. That means when we increase voltage we must also increase the current(I.) But transformer increases the current while decreasing the voltage or decreasing the current while ...
5answers
11k views

### How do voltage, resistance and amperage work? (In DC circuits)

I have a (probably newb) question that I can't figure out. What I think I know is this: Voltage is electric potential (or rather a difference thereof) Amperage is the rate at which electrons move ...
2answers
1k views

### High voltage transmission, transformers and Ohm's law

I have difficulties with understanding high voltage transmission and transformers working process. Particularly with this expression: "the higher the transmission line voltage, the smaller the current"...
2answers
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### Unexpected resistance on breadboard

My first question on EE, please be kind. :-) I am trying to learn how the ADC works on this Cypress PSoC 4100 board. The circuit in the picture does nothing but power the board, connect it to the ...
5answers
1k views

### Why does voltage split when an additional resistor is added?

When you have one resistor in a series circuit, the amount of voltage taken up by that resistor is 100%. Adding another resistor (same Ohms) makes that voltage drop 50%. Why, since only current should ...
5answers
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### Ohm's Law confusion — can there be voltage without current?

I know there has been asked a similar question about this before but I still have some issues to understand the whole voltage and current thing. ( Current without Voltage and Voltage without Current? ...
4answers
379 views

### How does electrical power relate to Ohm's law?

I have some difficulty grasping these concepts. Let's say, for example, a power source of 10W operating at 5V is connected to a load of 0.5 ohms. According to Ohm's law, it is expected that a current ...
3answers
4k views

### Simple circuit with transistor using Raspberry Pi

I'm trying to use a Raspberry Pi to control a set of 4 LEDs. I'd like to control the LEDs in software by using a ...