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### Course: Electrical engineering>Unit 1

Lesson 1: Getting started

# Electric current

Current is the flow of charge. We measure current by counting the amount of charge passing through a boundary in one second. Created by Willy McAllister.

## Want to join the conversation?

• The video states that water does not conduct. Then why do you have to get out of a pool in a lightning storm? If it's not conductive why be cautious about the storm?
• Water free of any chemicals such as tiny conductors and impurities inside the body of water does not conduct electricity, but water that has tiny conductors and ions such as salt can conduct electricity very efficiently. So I think that what the video states is that pure water does not conduct electricity. Also, to answer your question about why one needs to get out of a pool during a thunderstorm is because pools are filled with lots of chemicals which many of them may conduct electricity.
• Hello!
This is a question I've had since I knew what batteries even were... If the electrons flow from the battery into the conductor, then back through the wiring into the battery, why do batteries loose their charge over time? Wouldn't the electrons flowing back into the battery "recharge" it to an extent? I think the answer has something to do with energy loss during the conversion of energy, but I want to know for sure.
• Dylan,

We must ask what caused the charges to separate in the first place.

The answer: chemical energy did work to make them separate. The amount of work is the very definition of voltage:

Voltage = energy required / amount of charge = Joules / Coulomb

As the chemical energy is depleted so follows the voltage.

Regards,

APD
• When it comes to electrical current, how does the electrical current get all over the world in a mater of seconds, to a point where everyone can use phones, computers(etc.)at the same time when billions of people use it?
• Hello Mathomon,

My hero Admiral Grace Hopper had a good way of explaining this. Ref:

Regards,

APD
• I have a question based on the conventional current direction and electron flow... why are they opposite in direction? if electron flow direction is the actual direction, the why did scientists knowing that flip it the other way around and make it confusing?>>>>
• The theory of charge and current has deep historical roots. Their definitions do not actually depend on the existence of the electron and proton (seems strange, but there is nothing in the theory of charge that predicts the existence of the electron.) The seemingly "backwards" arrow came about a long time before the discovery of the electron.

Scientists would "fix" this if it was actually a problem. The fix would be rather painful, since we would have to redraw every schematic and rewrite every textbook and republish every scholarly paper. And that would be to fix a problem that does not actually exist.

Yes, the arrow direction is confusing to beginners, but, I promise you if you move ahead and learn about Ohm's Law and solve a few resistor circuits you will get used to the arrow direction and your confusion will melt away.

I wrote this introduction to Charge that might help... https://spinningnumbers.org/a/charge.html, and here's another relevant article... https://spinningnumbers.org/a/conventional-vs-electron-current.html
• Could current be considered a form of flux?
• Yes definitely current in charges is analogous to flux in magnetic fields.Here voltage is called the driving force and in case of flux mmf (Magneto motive force) is called the driving force.
• What is the meaning of steady current?what are its sources?DC or AC which one is steady current?
• DC would be considered a "stable current" because the current isn't fluctuating. AC on the other hand is basically what it says, Alternating Current.
(1 vote)
• if copper is a good conductor because it has one valence electron then why not Na or K used as conductors.
• Sodium (Na) and potassium (K) in pure form are extremely reactive. If you put a drop of water on pure metallic sodium there is a violent reaction with flames. Not something you want to build circuits with.
• So I have been thinking ever since my mom got me a battery charger to literally charge the batterys that were dead when ever I played Wii. I was wondering how does the electrons from an outlet end up back in a battery then when its time to be used still powers what you are using the battery for? Is this the same way people use to charge there electronics or is it a different way when you plug in the charger
• Rechargeable batteries contain chemical reactions that can be forced to run both ways. Non-rechargeable battery chemistry does not have this feature.

In normal operation, electrons come out of the negative terminal and flow through your device (Wii, or phone, or toothbrush).

When it is time to recharge, you connect a recharging device to the battery. It takes AC current out of the wall, converts it to DC, and applies a voltage to the battery that's a little higher than it is used to. Typically 18% higher than the battery's rated voltage.

That extra-high voltage causes the chemistry in the battery to run backwards, so electrons go into the negative terminal.

The power circuit is designed so you can operate your device at the same time you are charging. If it takes 0.5 amp to charge the battery and 1 amp to run the device, then there is 1.5 amp being pulled out of the wall.
• How is current actually sent through a salt water solution? In the solution there are Na+ ions that are drawn towards the negative wire lead (attached to the negative battery terminal), and Cl- ions drawn towards the positive wire lead (attached to the positive battery terminal). If all the Na+ ions congregate to the negative end, and all the Cl- ions congregate to the positive end, then there are no ions in between and current flow stops. So how does this solution produce consistent current flow.
• Once an ion deposits its charge, its forced to the back; crowded out to the back where it picks up another charge. They don't congregate, they pick up, deposit and then move to the back.
• If the equation is [i=Σ(q)/sec], wouldn't the positive charges cancel out the negative charges in the equation?
• If the + and - charges are moving in opposite directions, no, they don't cancel each other out. They add.

Let's say the positive charges move left to right. Each + charge that moves counts as a +1 contribution to current. The amount of + charge on the right is going up.

The - charges moving right to left leave behind a decreasing amount of negative charge on the right side, which is exactly equivalent to saying you have an increasing amount of positive charge on the right. So each - charge that departs contributes +1 to the current.

Again with numbers: Imagine you have a neutral mixture of + and - charges everywhere in some kind of tube, say 100 protons and electrons on the left and 100 of each on the right. The net charge on both sides is 0, because there are equal + and -.

Now some magical force causes a proton to move from the left side to the right side. The left has a charge imbalance, with 99 protons and 100 electrons, or a net charge of -1. The right side has 101 protons and 100 electrons, a net charge of +1. That means there has been a current of +1 going from left to right.

Now the magical force causes an electron to go the other way, from right to left. On the left side you have 99 protons and 101 electrons, for a net charge of -2. Over on the right you have 101 protons and 99 electrons, so net charge +2. What current does that represent? The moving electron caused the right side to increase by +1, just like the first proton did, so the overall current now is +2.

And one final thought. Suppose you have a bucket of sand. It is filled with zillions of protons and electrons, an equal number of both. You pick up the bucket and walk 10 meters. Does that mean there is an electric current happening (charge moving)? NO it does not. Since the + and - charges are moving together in the same direction everything stays neutral at all times. The net charge everywhere is 0.