# 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.

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### If V=IR Why are voltage and current interchangeable through a constant resistance [duplicate]

I hear people say things like "I only put 5 amps through the circuit but I put a bunch of volts". I don't understand how this is possible if V=IR. Lets say you have a circuit with 5 ohms of ...
5answers
6k views

### Can I use V=IR in the analysis of AC circuits?

My textbooks usually just assert this relation so I don't really know when it doesn't apply. EDIT: Reflecting on this question, I wanted to clarify that I wanted to know if Ohm's Law is valid for ...
2answers
234 views

### A-Level Physics: What is the current needed to maintain the wire at a steady temperature of 387∘C?

I am stuck on answering an A Level physics question: When a current of 5.00A flows through a tungsten wire in a stream of air at 27.0∘C, the wire attains a steady temperature of 57.0∘C. It may be ...
2answers
2k views

### 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 ...
2answers
107 views

### why my power calculation for this resistor is wrong?

i encountered a very simple question in a test and i easily calculated the answer but somehow i am making a mistake because i can not find my answer between the available answers, what am i doing ...
3answers
758 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 ...
2answers
279 views

### How do voltage changers work? Can you change any voltage to any voltage?

I often hear about changing voltage. Like someone would say "I bumped 12V to 38V" or like "I converted these 100V to 10V". Can you change any voltage to any voltage? What's the exact mechanism for ...
1answer
376 views

### Verification on Ohm's Law?

I'm building some Heated Motorcycle Grips and I think my calculations are off. Can anyone help me verify that for 10 feet of 20 gauge Nichrome 60 wire, I have the correct Ohms, Watts, and Amps for the ...
2answers
202 views

### Basic circuit analysis question [duplicate]

Why current is negative in this situation? Because EMF and current directions are the opposites? simulate this circuit – Schematic created using CircuitLab Can someone give me a good ...
1answer
209 views

### arduino current

I want to start learning EE with Arduino so as a project I decided to build an Ohmmeter and measure a (specific) resistance that should be about 100 KΩ ±50KΩ. In order to probe for that resistance I ...
1answer
746 views

### Amp Limitation on 120v Circuit

If an engineer needs to limit how many amps come out of a wall's circuit, how is this accomplished? I know that there is nothing preventing more than 15 amps from coming through my wall other than the ...
1answer
174 views

### resistors misconception - is current-limiting effect circuit-wide?

Resistors are used to drop current Amp and Voltage, following Ohm's law. I can't understand some of their applications: we use them cause we sometimes want to drop the voltage a little in a specific ...
1answer
264 views

### What is the efficiency of electric power converted to heat?

I'm trying to understand how much heat is generated when a given amount of electric power is drawn. And from what I understand, it's pretty much 1 to 1 isn't it? Example: If I have a 1 HP motor ...
1answer
348 views

### Molybdenum Heater Wire

I have a 12 foot length of 14-gauge molybdenum wire that I have measured a resistance of 1.2 ohms. I would like to use it as a small heater coil. How can I calculate the safe number of volts that I ...
3answers
2k views

### Determining Resistor for Heating Element

I am attempting to run a Molybdenum heat coil. The best I can tell it is 0.3-0.4 ohms per rack and I would like to run say two racks at 230V at 3.9KW (so it doesn't trip my 230V 20 amp breaker). Does ...
7answers
36k views

### Power's relationship to Resistance

$$\frac{V^2}{R} = P = I^2 \cdot R$$ According to this equation, Power is inversely proportional to resistance on the left side and directly proportional to resistance on the right side. How can ...
2answers
97 views

### Why can Ohm's law be used in a linear circuit with multiple equivalent resistors without being given current?

I'm exploring voltage dividers and am puzzled by the conclusion made by the YouTuber in this video: "If you apply Ohm's law, you find that 1 V spread across two equal-value resistors gives you 0.5 V ...
2answers
4k views

### Calculating sum resistance of multiple series and parallel resistors

! I know how to find resistance, current, and voltage through series/parallel circuits. The problem I'm having is figuring which part of this circuit to simplify first. What is the correct order of ...
6answers
6k views

### 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
68 views

### Equation for splitting a charge between n wires

If I had a 10 amps of current that was split down two wires, one with 1 ohms of resistance and another with 2 ohms of resistance, what equation would I use to determine how much charge went down each ...
1answer
478 views

### Simulation measured voltage not matching calculations

Ok, so I have the following circuit: click here for simulation The source voltage is 5V. The voltage measured between A and B is 2.44V, which differs from what I get by calculating that same ...
2answers
572 views

### If adding resistors in parallel decreases total resistance, why doesn't the voltage in the circuit change?

I'm sure I'm missing something basic, but here we go: If adding resistors in parallel decreases total resistance, why doesn't the voltage in the circuit change? Does the current increase to match ...
1answer
816 views

### Choosing the correct thermistor

I need help choosing a thermistor. I’m not sure what I need to look for. Let’s say I have this circuit measuring temperature: I have a range of temperatures from 0 to 10 degrees that I need to ...
1answer
912 views

### Multimeter giving wrong current readings

I just got my first multimeter (TekPower TP8268) and I've been try to test it out, but I have been getting some strange readings with the current measurement. I have an AC adaptor to DC 12v and 5A. ...
2answers
198 views

### Device with independent voltage and current

I have had some philosophical reflections about the voltage/current relationship. Assume we are somehow able to have a determined fixed current AND voltage on a device. This device could theoretically ...
3answers
9k views

### What can the ohms of a DC motor tell me about itself?

I have a lot of motors and all type of size but mainly they are all small to mid size adult hand size or so , I took out my multimeter and switch to ohms settings and grabbed the ground and power ...
1answer
952 views

### USB data serial termination resistors. Need them for STM32F2 micro? If so, what wattage?

I'm a little confused. The STM32F2 datasheet says only this, vaguely stated in the features: HNP/SNP/IP inside (no need for any external resistor) Is that referring to the D+/D- termination ...
2answers
160 views

### Determine supply voltage for led from current limiting resistor

Everyone knows how to calculate the resistance of a current limiting resistor for a LED but is it possible to go the other way? (aka determine the supply voltage from the resistor's value) I ask ...
2answers
387 views

### How does one determine the voltage of a node?

This sounds like a very straightforward question, and probably has an obvious answer. I've read a section in a physics book on electricity, I've read a textbook on DC and this simple concept seems to ...
1answer
479 views

### How do I calculate voltage on a multi-powered DC circuit where all ends go to ground?

I'm writing software that will calculate the correct voltage on each component, given the state that the DC system is currently in (ex. what power sources are on, what relays are open, fuses popped, ...
3answers
6k views

### Pull-down resistor ; why is it crucial

These 3 diagrams represent 3 test cases. Dia 1: When I perform an analogRead of analog port 0, then I get a high value obviously. According to the manual, the <...
1answer
165 views

### Is Ohm's law required to calculate Watts?

A buddy and I were having a conversation. One of us is convinced that Ohm's Law and Watts are the same thing. After much research I can not find anything directly pointing Ohm's Law to defining Watts, ...
3answers
2k views

### Measuring the inductance of a coil?

I have a hand-wound coil I'm using in an LC Tank circuit. Using my multi-meter, I found out that the coil is drawing .6mA at 9v. Using Ohm's Law... $$R = V/I$$ $$R = 9/.0006$$ $$R = 15kΩ$$ Is there ...
2answers
17k views

### Voltage drop through a resistor using Ohm's Law?

I'm having a very hard time comprehending basic Ohm's Law. I was recently watching a video on the operation and use of Operational Amplifiers, and was completely lost because I couldn't understand ...
3answers
3k views

### Powering a device by USB?

I would like to power a device via the 5V USB port on a computer, because I will also be communicating with the computer via the USB data cables. I know that USB ports provide 5V, but from what I've ...
5answers
14k views

### 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? ...
1answer
59 views

### finding I(current)

I am learning about step response in RC and RL circuits and one of the problems i managed to redraw as the image above; however, when I am solving for i the answer is -5mA. I am just confused on why ...
2answers
850 views

### brief explanation of Ohm's law

How can I calculate ideal values of current for both short and open circuits by using Ohm's law. Further more usually said that it is not advisable to connect DC batteries of different voltages in ...
1answer
86 views

### Is this calculation for an optocoupler's resistor correct?

I'm trying to calculate what resistor to use for this optocoupler. Per the datasheet, it drops 1.3v at 50mA does this mean I should try choose a resistor that gives me 50mA? My supply is 3.3V so my ...
2answers
732 views

### How do I work out what resistor to use?

I am trying to work out what resistor to use in a circuit. The circuit is a series circuit. The power to the circuit is 5.5V, with a switch, which connects to a GPIO pin on the RasberryPi. I believe ...
3answers
720 views

### How can this circuit read a value for amps at 0 volts

I made a simple circuit using this: It can tell you the voltage and current along a wire if you mouse over it. The wire down the bottom reads 4.55mA and 0V (the little yellow dots actually move to ...
1answer
433 views

### Parallel circuit and Ohms law

Theoretical questions with ideal components.(R2 is 10 Ohm) 1) parallel circuit. R1 and ammeter, If I have a parallel circuit of 2 resistances, both zero Ohm, the current would split to 2 and each ...
3answers
593 views

### Are $\frac{1}{4}$watt 4x$\Omega$ Resistor and $\frac{1}{2}$watt x$\Omega$ Resistor equivalent?

We know Ohm`s law: $$\Omega = \frac{V}{I}$$ W=\frac{A}{t} \space\space\space\space\space\space V = \frac{A}{q} \space\space\space\space\space\space I = \frac{q}{t} \space\space\space =>\space\...
6answers
36k 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 ...
1answer
3k views

### why ohmic losses increase with resistance?

Electrical newbie here. I'm trying to get understanding how is resistance involved with power dissipation (ohmic losses, eg. heating). Primarily I'm looking at Electromagnet coil, it says the losses ...
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
3k views

### Does this RGB LED really need 220 ohm resistors per color?

I have an arduino kit (5v) and instead of just following their simple instructions I want to know why they are using certain resistors for their RGB LED's. The RGB LED that comes with this is ...
1answer
176 views

### How much resistance will I need to drop the voltage?

9 Volts is applied to circuit. There is a LED which runs on 3 Volts. I want to drop 6 Volts. how much resistance ($R_1$) would I need for that?
2answers
371 views

### Use battery voltage rating or fully charged voltage?

I am using a 3.7V (18650) battery, but its fully charged voltage is 3.85V. When calculating a resister for an LED circuit, do I use the battery's voltage rating (3.7V) or the fully charged voltage (3....
3answers
140 views

### Which current should I use to determine the Watt?

I am making a heated blanket and I have some troubles to determine the heat that will be generated. Resistance: Let say, I use this heated cable with a resistance of 0.3 Ohms/m*. If I am using 10m ...