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### Course: MCAT>Unit 8

Lesson 10: Electrostatics

# Electric field

We can think of the forces between charges as something that comes from a property of space. That property is called the electric field. Charges shape the space around them, forming an electric field that interacts with other charges. The tutorial covers Coulomb's Law, electric field lines, and the role of distance in field strength. Ideal for premed students delving into electrostatics. Created by Sal Khan.

## Want to join the conversation?

• At , how do two protons or positive charges repel each other? They do repel each other, but how?
• For answering this you must know Feynman's Diagram which shows possible ways of how and why does positive and positive charges repel
• Is it possible to have vector addition of gravitational and electric field to get a total field ?
• No, it isnot that simple. Physicists have been trying to unify the forces into a single description for over 100 years.
• In a way, does Coulomb's Law mean that every particle, no matter how far away they are from each other, has some effect/force on the other?? Just that its too small to really make a difference...
• Why positive electric fields of lines are radially outwards?
• The test charge (a unit charge that is used to test the effect of the source charge) is assumed to be positive. So, the source charge, which is the charge from which the electric fields are generated, pushes the test charge outwards. And so, the electric field lines are radially outwards.
Totally sure now. Hope that helped. :)
• I did not get what exactly an electric field is? Can someone give a simple definition?
• It has been awhile. But I just saw this video.
Basically Electric field is the surrounding of the charges particles. You can imagine the charge particle is center of circle, the electric field will be everything in the circle
• The Lightning Which We See In The Sky Is Said To Be A Result Of Static Electricity And Static Electricity Does Not Move Or Flow Then How Does Electricity From Clouds Reach The Ground
• Lightning is a fairly complex process. You can model a cloud as a half circle with a round uniform top and a flat bottom. There will be a charge buildup in the cloud as a result of the evaporation and condensation cycle. The top of the cloud will have a net positive charge and the bottom obtains a net negative charge. The net negative charge on the bottom of the cloud will cause Earth's surface beneath the cloud to accumulate a net positive charge (the negative charge at the bottom of the cloud exerts a force towards the earth that repels electrons on Earth's surface causing a local positive charge to accumulate).

When there is a difference in charge potential separated by a distance (the negative underside of the cloud located say 1 mile from Earth's surface with a local positive charge) a voltage difference occurs. As charge accumulates in the cloud the total difference in electrical potential (voltage) between the cloud and Earth also increases. This causes the electric field between the underside of the cloud and the surface of Earth to become stronger.

At a certain point the electric field will grow strong enough to cause the ionization of air to occur. As a molecule ionizes it loses electron(s) from its valence shell. These electrons will experience a force from the electric field created by the cloud and Earth that causes them to accelerate towards the positive charge on Earth surface.

The lightning 'strikes' quickly because the potential builds until it reaches the point at which the air around it can ionize but once it reaches this point the large difference in potential energies created by the cloud and Earth accelerates these electrons seemingly instantaneously. The electrons 'flow' in a sense because the air ionizes, lowering the effective resistance, which results in the distinctive lightning bolt (the air being ionized) that you see.

I could be wrong on some of the details but I'm fairly sure this is an accurate description of what causes lightning and what it really is.
• If the electric field lines generated by a positively charged particle are radially outwards and those of an electron are radially inwards,then this is visualised as though the positively charged particle's electric field force is pushing the negative charge further away from itself while being simultaneously pulled towards the negative charge whose electric field lines point inwardly towards itself.Right?What I'm mainly confused about is that if opposite charges attract each other,then the respective electric fields' force vectors of the positive and negative charges should ppint inwardly towards each charge;say A and B are two oppositely charged particles,hence both particles are pulling upon each other,shouldn't we represent the force vector of the force of A on B as pointing inwardly towards A and the force vector of B on A as pointing inwardly towards B.If that's true,then why did Sal at schematize the electric field lines issued from A (which is +ve) point outwardly on B(which is -ve) as though it is repelling it.
• No problemo.

Most important thing in your work is to understand what the arrows indicate.

The direction of the arrow indicates the directon of the force acting on 'a positive charge'.

Thats why they are 'outwards' for a proton and 'inwards' for an electron.

and thats why a proton sitting in an electrons electric field would feel a force attracting it towards the centre of the electron. because the arrows point in and the proton is positive...

and for an electron sitting in a protons electric field, if the arrows from the proton point outwards, then the force is inwards... this is because the arrow is the direction of force that would act on a positive charge.... electron is negative, so the force is in opposite direction...ie inwards.

ok??
• When u put distance instead of radius.. we actually measure the distance between the centres of the charges or the distance between their respective outer surfaces ?
• A charge is actually represented by a single point. Therefore distance is measured from thecentres of respective charges
• To explain the charge interaction between a charged and neutral object we use the concept of polarisation. So if I talk about a positively charged balloon and a wall, then the electrons are attracted towards the balloon and get crowded on the wall near the balloon. Why don't they enter the balloon?
• Because the balloon is made of rubber, and charge can't easily move onto or through insulating materials like that. Also, the charges are still attracted by the positive charges in the wall.
• I have a question in this video. About 6min 45s, you talked about the direction of the force. And I agree that the q particle will be move farther from the Q particle. But my question is, would the Q particle is also going to be move farther from the q particle?? and if it does, which particle is going to be exerted more force??
• Yes Q particle is also going to move away as q does.
I have tried to know how much force both charges exert on each other.
Lets say Q particle has 2 Coulomb charge and q has 1 Coulomb charge.You can calculate the electric field created by charges Q and q as E(Q)=F/q= k.Q/d2 and E(q)=F/Q= k.q/d2 respectively.In this way you get E(Q)=1.8*10^10 N/C
E(q)=9*10^9 N/C

We can calculate force exerted on charge q by charge Q as F=E(Q)*q= 1.8 x 10^10 N
And force exerted on charge Q by charge q as
F=E(q) * Q = 9*10^9 x 2= 1.8 x 10^10 N

So we got to know both charges experiences same Force due to their somehow different electric fields.
Let me know if it helps.My first ever answer on Khan Academy since I joined yesterday.