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click on the picture to play click on the picture to play electrical hokeyelectrical hokey
Let's take a single electric charge, Let's take a single electric charge, QQ, and put it somewhere. The , and put it somewhere. The space around it is different from the space without charge. We have space around it is different from the space without charge. We have created a situation in which we created a situation in which we couldcould have an electric force. All we have an electric force. All we have to do is bring in a second charge, have to do is bring in a second charge, qq, to , to feelfeel the force. Without the force. Without qq, there is no force ....but we still have the condition that we , there is no force ....but we still have the condition that we couldcould have a force. We say that the space around charge contains have a force. We say that the space around charge contains ELECTRIC FIELDELECTRIC FIELD. .
Electric Field Electric Field
How to measure/find the strength (magnitude and direction) How to measure/find the strength (magnitude and direction) of electric field at particular location of electric field at particular location PP due to charge due to charge QQ??
A test charge,A test charge, q q, placed at, placed at P P will experience an electric will experience an electric force,force, FF - either attractive or repulsive.- either attractive or repulsive.
qqQQ
rr FFPP
The magnitude of the electric field is The magnitude of the electric field is defined as the force per unit charge.defined as the force per unit charge. E =E =
FF
As As FF contains contains qq, , EE DOESN’T DOESN’T depend on depend on qq at all, only on at all, only on QQ..
Electric field at any point P in space is Electric field at any point P in space is alwaysalways in the direction of the force on a in the direction of the force on a positivepositive test test charge if it were placed at the pointcharge if it were placed at the point P. P.
Definition of electric field, E, at a point P distance r away from Q.Definition of electric field, E, at a point P distance r away from Q.
C
NE
qqQQ
rr FFPP
The other way around:The other way around:
If you know electric field E at a point where you place If you know electric field E at a point where you place a charge q, that charge will experience the force F: a charge q, that charge will experience the force F:
F = q E F = q E
EE
qqFFEE
qq FF
++Q=1.6x10Q=1.6x10-19 -19 CC
Electric field of a charged particle/point chargeElectric field of a charged particle/point charge
◊ ◊ magnitude magnitude
the same value on the sphere of radius r aroundthe same value on the sphere of radius r around
◊ ◊ direction – radially outward or inwarddirection – radially outward or inward
r = 1x10r = 1x10-10-10 m m
EE
(to the right)(to the right)
A charged particle Q creates an electric field.A charged particle Q creates an electric field.
E = 9E = 910109 9 = 2.9= 2.910101111 N/C N/C1.61.61010-19-19
(10(10-10-10))22
E E = k = kq q FF
rr22 Q Q E Field independent E Field independent
of test charge of test charge
q positive test chargeq positive test charge
example:example:
QuestionQuestion
Say the electric field from an isolated point charge has a certain value at a Say the electric field from an isolated point charge has a certain value at a distance of 1m. How will the electric field strength compare at a distance of 2 m distance of 1m. How will the electric field strength compare at a distance of 2 m from the charge? from the charge?
It will be ¼ as much – inverse square law for force It will be ¼ as much – inverse square law for force between two charges carries over to the electric field between two charges carries over to the electric field from a point charge.from a point charge.
Direction indicates direction in which a positive test charge would be pushed – direction of the force!!!.
We use “Electric Field Lines” to visualize el. field.We use “Electric Field Lines” to visualize el. field.
Convention / agreementConvention / agreement
Electric Field of a Point ChargeElectric Field of a Point Charge
+E
2.91011 N/C
32
1011
N/C 0.8
1011 N/C
2510 11 N/C
This is becoming a mess!!!
Electric Field Lines
1. Density gives strength# lines proportional to Qlines never cross!
2. Arrow gives directionStart on +, end on -
negative negative chargecharge
positive positive chargecharge
So always point away fromSo always point away from ++ charges, towards charges, towards – – charges…charges…
Denser lines - stronger field Denser lines - stronger field el. field decreases with distanceel. field decreases with distance
more lines revels stronger more lines revels stronger field due to greater chargefield due to greater charge
Electric field lines can never cross. If they crossed, that Electric field lines can never cross. If they crossed, that would mean that a charge placed at the intersection, would mean that a charge placed at the intersection, would be accelerated in TWO directions at once! This is would be accelerated in TWO directions at once! This is impossible! If two sources are creating electric fields in the impossible! If two sources are creating electric fields in the same place, we have to add the two vectors and get a same place, we have to add the two vectors and get a resultant vector representing the NET ELECTRIC FIELD.resultant vector representing the NET ELECTRIC FIELD.
Question?Question?
What is the direction of the electric field at point C?What is the direction of the electric field at point C?
1)1) Left Left
2)2) Right Right
3)3) ZeroZero
xx
yy
CC
Away from positive charge (right)Away from positive charge (right)
Towards negative charge (right)Towards negative charge (right)
Net E field is to right.Net E field is to right.
Question?Question?
What is the direction of the electric field at point A?What is the direction of the electric field at point A?
1)1) Up Up
2)2) Down Down
3)3) Left Left
4)4) Right Right
5)5) ZeroZero
xx
AA
Question?Question?
What is the direction of the electric field at point B?What is the direction of the electric field at point B?
1)1) Up Up
2)2) Down Down
3)3) Left Left
4)4) Right Right
5)5) ZeroZero
xx
yy
BB
Question?What is the direction of the electric field at point A, if the
two positive charges have equal magnitude?
1) Up
2) Down
3) Left
4) Right
5) Zero
x
A
- - -- - -- - -- - -
++++++++++++
Uniform electric fieldUniform electric field (the one that has constant magnitude and direction (the one that has constant magnitude and direction is generated between two oppositely charged parallel plates. Edge effect is generated between two oppositely charged parallel plates. Edge effect is minimazed when the length is long compared to their separation.is minimazed when the length is long compared to their separation.
Electric field Electric field of a capacitorof a capacitor
FOR MORE CLICK
RR
Electric field of a charged conducting sphere in Electric field of a charged conducting sphere in electrostatic equilibriumelectrostatic equilibrium
E = k E = k qqrr22
rr
Electric field outside a charged sphere (evenly distributed charge q over surface) at distance r from its center is the same as if the charge q is concentrated at the center of the sphere:
at the surfaceat the surfaceqqRR22
E = k E = k
What is electric field inside the sphere?What is electric field inside the sphere?
OK. Let’s start. In general if you have many charges you OK. Let’s start. In general if you have many charges you have to find el. field of each of them and then add them up have to find el. field of each of them and then add them up as vectors to get net el. field at certain point. So imagine a as vectors to get net el. field at certain point. So imagine a solid!!!!!!!!! Good luck.solid!!!!!!!!! Good luck.
Different approach:Different approach: Conductor is in electrostatic equilibrium Conductor is in electrostatic equilibrium when there is no when there is no netnet motion (flow ) of motion (flow ) of charge within a conductor or on its surface. charge within a conductor or on its surface.
• Conductor Conductor electrons free to move electrons free to move• charges in electric field feel the force F = Eqcharges in electric field feel the force F = Eq• only free electrons can move in the conductor so they will move until only free electrons can move in the conductor so they will move until
E = 0 inside a conductorE = 0 inside a conductor
if not then electrons would respond to its presence and be accelerated if not then electrons would respond to its presence and be accelerated within the conductor. And that is not conductor in electrostatic equilibrium.within the conductor. And that is not conductor in electrostatic equilibrium.
Q.E.DQ.E.D
If such conductor has excess charge, it resides If such conductor has excess charge, it resides entirely on the conductor’s outer surface running away entirely on the conductor’s outer surface running away from each other as far as possible. El. field is still zero from each other as far as possible. El. field is still zero everywhere inside the conductor.everywhere inside the conductor.
Therefore, electric field of charged conducting Therefore, electric field of charged conducting sphere in electrostatic equilibrium is:sphere in electrostatic equilibrium is:
RRqq
E = k E = k qqrr22
rr
at the surfaceat the surfaceqqRR22
E = k E = k
E=0E=0
El. field just outside a charged conductor is El. field just outside a charged conductor is perpendicular to the conductor’s surface.perpendicular to the conductor’s surface.
if notif not
Charge tends to accumulate where the curvature is the greatest Charge tends to accumulate where the curvature is the greatest (sharp points). (sharp points).
The fact that pointed objects create The fact that pointed objects create strong electric fields if charged is the strong electric fields if charged is the reason for the shape of lightning rod. reason for the shape of lightning rod. You’ll see. You’ll see.
Summary: Conductors in electrostatic equilibriumSummary: Conductors in electrostatic equilibrium
no net motion (flow) of charge within a conductor . Then:no net motion (flow) of charge within a conductor . Then:
El. field is zero everywhere inside the conductorEl. field is zero everywhere inside the conductor
If such conductor has excess charge, it resides entirely on the If such conductor has excess charge, it resides entirely on the conductor’s outer surface. El. field is still zero everywhere inside the conductor’s outer surface. El. field is still zero everywhere inside the conductor.conductor.
El. field just outside a charged conductor is perpendicular to the El. field just outside a charged conductor is perpendicular to the conductor’s surface.conductor’s surface.
Charge tends to accumulate where the radius of curvature is Charge tends to accumulate where the radius of curvature is smallest (sharp points). smallest (sharp points).
Electric field is zero inside conductor, regardles how strong Electric field is zero inside conductor, regardles how strong the electric field is outside.the electric field is outside.
Even more: electric field inside metal cavities is zero, Even more: electric field inside metal cavities is zero, regardles what’s going outside - so put electrical equipment in regardles what’s going outside - so put electrical equipment in metal boxes. Outside may be very strong fields and high metal boxes. Outside may be very strong fields and high charges, but the charges on the metal surface rearrange to give charges, but the charges on the metal surface rearrange to give 0 inside.0 inside.
This is why if lightning hits a car and you’re inside, you’re This is why if lightning hits a car and you’re inside, you’re safe, at least if it is not too strong ot make everything melt. safe, at least if it is not too strong ot make everything melt. Electrons from the lightening bolt mutually repel and spread over Electrons from the lightening bolt mutually repel and spread over the outer surface. Although the electric field they set up maybe the outer surface. Although the electric field they set up maybe great outside the car, the overall electric field inside the car great outside the car, the overall electric field inside the car practically cancels to zero.practically cancels to zero.
Radio signals cannot penetrate through a metal enclosureRadio signals cannot penetrate through a metal enclosure (mobile in an elevator)(mobile in an elevator)
The metal bars (rebar) that reinforce the concrete in walls The metal bars (rebar) that reinforce the concrete in walls can interfere alsocan interfere also
Electrical ShieldingElectrical Shielding
Electrostatic shieldingElectrostatic shielding
• The effect of the high voltage on The effect of the high voltage on the van de Graff generator stops on the van de Graff generator stops on the outside of the metal cage the outside of the metal cage Homer is SAFE!Homer is SAFE!
Lightning – great weapon of the GodsLightning – great weapon of the Gods
• causes 80 million dollars in damage each year causes 80 million dollars in damage each year in the USin the US
• kills ~ 85 people a year in the USkills ~ 85 people a year in the US• is all over in a thousandth of a secondis all over in a thousandth of a second• carries up to 200,000 Amperes (currant)carries up to 200,000 Amperes (currant)• temperature of a lightning bolt can reach temperature of a lightning bolt can reach
30,000°C – 5x hotter than the surface of the 30,000°C – 5x hotter than the surface of the sun sun
• causes the thunder!causes the thunder!
1. step:1. step: Positive and negative charges separate. Positive and negative charges separate. A cloud becomes positively charged at the top A cloud becomes positively charged at the top and negatively charged at the bottom. Reasons and negatively charged at the bottom. Reasons for this charge distribution are complex and for this charge distribution are complex and only partially understood.only partially understood.
++++++
2. step:2. step: As the negative charges collect at As the negative charges collect at the bottom of the cloud it forces the the bottom of the cloud it forces the negative charges in the ground to be negative charges in the ground to be forced away from the surface. forced away from the surface. This leaves the ground positive. This leaves the ground positive. If attractive force is large enough electrons If attractive force is large enough electrons will leap from negative region downward will leap from negative region downward from the cloud to the ground.from the cloud to the ground.
development of a lightning boltdevelopment of a lightning bolt
++++++
++ ++ +++ ++ +
3. step: As the electrons approach the earth, 3. step: As the electrons approach the earth, el. field just above the surface becomes so el. field just above the surface becomes so intense that it ionizes atoms and molecules in intense that it ionizes atoms and molecules in the air. An intense flash of light is produced.the air. An intense flash of light is produced.
4. step: When first breakdown occurs, negative 4. step: When first breakdown occurs, negative charge in the column farther above the earth charge in the column farther above the earth can rush down through the region of ionized can rush down through the region of ionized air. air.
http://regentsprep.org/Regents/physics/phys03/alightnin/default.htm
Now that flow of electrons originated farther Now that flow of electrons originated farther above the earth causes a flash of light. That above the earth causes a flash of light. That way flash appears to move up.way flash appears to move up.
The rapidly moving electrons excite the air along the path so The rapidly moving electrons excite the air along the path so much that it emits light. It also heats the air so intensely that it much that it emits light. It also heats the air so intensely that it rapidly expands creating thunder.rapidly expands creating thunder.
One thing to notice is that the positive charges that make up both the One thing to notice is that the positive charges that make up both the cloud and the ground do not move. Even the positive streamer cloud and the ground do not move. Even the positive streamer launched by the ground (in the animation) is really only made up of launched by the ground (in the animation) is really only made up of positively charged air particles because the electrons left the particle. positively charged air particles because the electrons left the particle.
Electrons in atoms orbit the nucleus well defined orbits Electrons in atoms orbit the nucleus well defined orbits called stationary states. By absorbing energy atom can called stationary states. By absorbing energy atom can be raised to higher energy level called excited state. be raised to higher energy level called excited state. When excited electron jumps back to a lower level it When excited electron jumps back to a lower level it emits photon.emits photon.
At ordinary temperatures and when not in strong electric filed most At ordinary temperatures and when not in strong electric filed most atom are in the ground state. But if the temperature is high and/or atom are in the ground state. But if the temperature is high and/or electric field very strong the air will turn into plasma. The light is electric field very strong the air will turn into plasma. The light is generated when the stripped electrons in the plasma recombine generated when the stripped electrons in the plasma recombine with the ionized ions. The probability of recombination process is with the ionized ions. The probability of recombination process is pretty high, so the time for the recombination to occur is short. The pretty high, so the time for the recombination to occur is short. The streaks of light from the lightening fade away almost immediately. streaks of light from the lightening fade away almost immediately. When a meteor shower enters the very high regions of the When a meteor shower enters the very high regions of the atmosphere, streaks of light last for as long as 2 – 3 seconds atmosphere, streaks of light last for as long as 2 – 3 seconds before fading. Because the probability of freed electrons finding an before fading. Because the probability of freed electrons finding an ionized atom with which to recombine is very low (pressure and ionized atom with which to recombine is very low (pressure and density are low - atoms are relatively far apart).density are low - atoms are relatively far apart).
Lightening RodsLightening Rods
Place rod above a building, and connect it to Place rod above a building, and connect it to ground. Then the point of the rod picks up ground. Then the point of the rod picks up electrons from the air (“leakage”), so prevents electrons from the air (“leakage”), so prevents large build up of + on the building, hence large build up of + on the building, hence decreasing chance of a lightning strike. decreasing chance of a lightning strike.
But even if there is a lightning strike (if leakage not But even if there is a lightning strike (if leakage not enough), the electricity goes through rod to enough), the electricity goes through rod to ground, rather than through building. ground, rather than through building.
+ + + + +
++
++ +
++++
++ +
+
+
While enjoying the Sequoia National Park from a lookout platform, this woman found her hair rising from her head. Amused, her brother took her photograph. Why should
she run for cover?
Five minutes after they left, lightning struck the platform, killing one person and injuring seven.
