7. Electrical quantities are related to each other

Ohm´s law

Voltage (V), current (I) and resistance (R) in an electrical circuit are related to each other.
As the resistance to the passage of charge increases, more energy is required to move the electrons. Therefore, voltage and resistance must be directly proportional.
The greater the amount of charge we want to move per second (I), the greater the energy required, so voltage and current must also be directly proportional.

Ohm's Law describes this relationship:

V = I ⋅ times R

And it can also be expressed in these other ways, if what we want to calculate is I or R, instead of V:

I = V /dividedby R

R = V /divided by I

An easy way to memorize these equations, if you have trouble solving them, is with the triangle in the image. If you cover the magnitude you want to calculate with your hand, what you have left is the formula to calculate it.

Ohm´s law visual description. Interactive video (3 min)

Watch the following video (3 min) and answer the questions that will appear on the screen.

mp4

ohm´s law

File Attachments. Audio of the Ohm´s law video transcription

transcripcion audio ley de Ohm en ingles.odt (New Window)

Maths reinforcement: How to solve equations and apply it to Physics problems. Video (6 min) with exercises

Como resolver ecuaciones. Ana Dapena Mora, basado en CANVA

3·x = 12 x = Cloze (1):JXUwMDZj

3·R = 12 R = Cloze (2):JXUwMDZj

3·Q = 12 Q = Cloze (3):JXUwMDZj

When the unknown quantity is in the denominator, we will take an intermediate step. If 100 divided into x gives 4, 4 multiplied by x makes 100:

100 = 4 · x

And now we solve as before.

100/x = 4 x = Cloze (4):JXUwMDZhJXUwMDA3

100 / I = 4 I = Cloze (5):JXUwMDZhJXUwMDA3

100 / r ² = 4 r ² = Cloze (6):JXUwMDZhJXUwMDA3

If the unknown quantity is squared you must take its square root and, therefore, take it on the other side as well:

r ² = 25 r = Cloze (7):JXUwMDZk

Now do the same with letters that represent numbers instead of numbers:

V = I·R I = Cloze (8):JXUwMDBl Cloze (9):JXUwMDc3 Cloze (10):JXUwMDBh

v = e / t e = Cloze (11):JXUwMDJl Cloze (12):JXUwMGVm Cloze (13):JXUwMDJj

R = ρ · L / A L = R Cloze (14):JXUwMGVm A Cloze (15):JXUwMDc3 ρ

R = ρ · L / A A = ρ · Cloze (16):JXUwMDE0 / Cloze (17):JXUwMDBh

F = k · q · Q / r² r² = k · q · Q Cloze (18):JXUwMDc3 F r = √ (k · q · Q Cloze (19):JXUwMDc3 F)

Audio:

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Solved example

Calculate the resistance of a heater knowing that, if we connect it to a socket that supplies 220 V, the current intensity that flows through it is 11·10³ mA .

Data: V = 220 V, I = 11·10³ mA

Unknown: R

Unit change: I = 11 · 10³ mA ·1 A/10³ mA = 11 A

Equation: V = I·R

Clear equation and substitute data: R = V/I = 220/11 = 20 Ω

Audio:

Calculation exercises. Fill in the gaps

Fill in the blanks with the values of the quantities requested in the problem, calculated using the appropriate Ohm's law equation . Write the numerical value (with a point if it is decimal) and the unit in the answer, separated by a blank space.

Extension. Timeline: The stories of Coulomb, Ampère, Ohm and Volta

Do you know what the butterfly effect is? This concept, based on an ancient Chinese proverb, says that: "The flutter of a butterfly's wings can unleash a storm on the other side of the world." It means that small changes, often imperceptible, can transform the events that will affect our lives. Coulomb, Ampère, Ohm and Volta were scientists at a time when the world changed because science and reason took center stage. If it were not for the development of knowledge that took place at that time, thanks to them and many other intellectuals, and the value that was given to it, probably all the revolutions that have shaped the political and social reality that we live in today would not have taken place. In this timeline, you can see how our protagonists were related to the historical events that were occurring during their lives, imagine their reality, and understand how important science, knowledge, and the small contributions that we can all make throughout our lives are, no matter how insignificant they may seem to us.

The illustration , Anicer Charles Gabriel Lemonnier, Wikipedia , Public domain Industrial revolution . Albert Bridge, Geograph.ie , CC-BY- SA Freedom Guiding the People , Eugene Delacroix, Wikimedia Commons , Public domain Napoleon Crossing the Alps, Jacques-Louis David, Wikimedia.org , Public domain . Ampère , Ancient Fund of the Library of the University of Seville, Wikimedia Commons , CC-BY-2.0. Ohm , Unknown, Wikimedia Commons , Public domain Episode of the 1854 revolution at the Sun Gate , Lucas Velazquez, Wikimedia Commons , Public domain

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Maths reinforcement

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