What are force fields in physics?

Force fields in physics refer to the area over which a force is exerted, such as gravitational or electric fields, without direct contact between objects.

How do gravitational and electric fields compare?

Both gravitational and electric fields operate by exerting forces over a distance. They are vector quantities describing forces between masses or charges.

What is the unit of a gravitational or electric field?

The unit for both gravitational and electric fields is Newton per kilogram (N/kg) for gravitational fields and Newton per Coulomb (N/C) for electric fields.

How is the force on a charge in an electric field calculated?

The force is calculated by multiplying the charge by the electric field strength (F = qE).

What happens inside a charged conductor regarding the electric field?

Inside a charged conductor, the electric field strength is zero because all field lines cancel each other out.

What is special about the field between a pair of positive charges?

The field between two positive charges results in zero directly at the midpoint between them due to equal repelling forces.

Why does hair stand on end when touching a Van de Graaff generator?

Hair strands stand on end because they become electrically charged and repel each other due to similar charges.

Why is touching a charged Van de Graaff generator generally safe?

It is safe because the charges involved carry very low energy, despite the high voltage, due to small charge amounts.

What is required to understand about electric fields when considering voltage?

When considering voltage, it's important to understand it as a ratio of energy per charge (Joules/Coulomb).

What determines whether combined electric fields cancel to zero between charged particles?

The signs (like or unlike) of the charged particles determine this; typically, opposite charges can mutually cancel out their fields at a midpoint.

Hewitt-Drew-it! PHYSICS 89. Electric Fields

00:09:38

https://www.youtube.com/watch?v=9nlflkR8tvs

Summary

TLDRThe video elaborates on the concept of force fields in both gravitational and electric contexts, illustrating how these forces operate over a distance between two bodies. It explains that a force field represents the region where a force can be felt by nearby objects, using gravitational and electric examples. The gravitational field around a planet is depicted with vector lines to show the direction and strength, which diminishes with distance (inverse square law). Similarly, an electric field around a charge is described, highlighting that the force on a charge is equal to the electric field strength times the charge. The video further details the vector nature of fields, clarifying how direction and magnitude play roles in field-pattern creation. It shows how sum of field vectors can lead to curly lines signifying field direction. In practical terms, the operation of a Van de Graaff generator is explained, illustrating how charge accumulates without increasing the inner field, which remains zero. The video concludes with an exploration of concepts like electric induction, field cancellation, and potential applications, prompting viewers to think critically about these forces.

Takeaways

🔋 Electric and gravitational forces operate over distances without contact.
📏 Force fields describe these non-contact forces from a conceptual standpoint.
⚡ Electric fields for charged particles are vector quantities like gravitational fields.
➡️ The magnitude of the electric field is the force experienced per unit charge.
🌀 Field lines for electric forces can be straight for single charges or curved between pairs.
🔍 Inside charged conductors, the electric field cancels and is zero.
🌩️ Van de Graaff generators demonstrate field concepts by creating high voltages safely.
🔅 Hair standing on end near Van de Graaff is due to repulsive electric forces between same-charged strands.
⚖️ Voltage should be understood as energy per charge unit, differentiating from direct current cause of shock.
🔁 The cancellation of fields between charges emphasizes symmetrical force interactions.

Timeline

00:00:00 - 00:09:38
The video continues to explore electric field patterns, showing that a composite of force vectors creates a curved field from positive to negative charges. It illustrates this with photographs of electric fields in different setups, such as terminals in oil and plates in a capacitor. The video highlights that no field results inside a charged conductor, covered in detail later in textbooks. Using a Vandegraff generator as an example, it illustrates how charges build up outside but remain zero inside, enabling continuous charge accumulation until electric discharge occurs. It discusses why people touching the charged generator aren't shocked despite high voltages due to energy per charge being minimal. Finally, it poses a conceptual question about the nature of charges when their electric fields cancel out midway between them, prompting viewers to consider if the charges are like or unlike.

Mind Map

Video Q&A

What are force fields in physics?
Force fields in physics refer to the area over which a force is exerted, such as gravitational or electric fields, without direct contact between objects.
How do gravitational and electric fields compare?
Both gravitational and electric fields operate by exerting forces over a distance. They are vector quantities describing forces between masses or charges.
What is the unit of a gravitational or electric field?
The unit for both gravitational and electric fields is Newton per kilogram (N/kg) for gravitational fields and Newton per Coulomb (N/C) for electric fields.
How is the force on a charge in an electric field calculated?
The force is calculated by multiplying the charge by the electric field strength (F = qE).
What happens inside a charged conductor regarding the electric field?
Inside a charged conductor, the electric field strength is zero because all field lines cancel each other out.
What is special about the field between a pair of positive charges?
The field between two positive charges results in zero directly at the midpoint between them due to equal repelling forces.
Why does hair stand on end when touching a Van de Graaff generator?
Hair strands stand on end because they become electrically charged and repel each other due to similar charges.
Why is touching a charged Van de Graaff generator generally safe?
It is safe because the charges involved carry very low energy, despite the high voltage, due to small charge amounts.
What is required to understand about electric fields when considering voltage?
When considering voltage, it's important to understand it as a ratio of energy per charge (Joules/Coulomb).
What determines whether combined electric fields cancel to zero between charged particles?
The signs (like or unlike) of the charged particles determine this; typically, opposite charges can mutually cancel out their fields at a midpoint.

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00:00:01
[Music]
00:00:04
electric forces like gravitational
00:00:06
forces most often act between bodies
00:00:09
that are not in contact these forces act
00:00:11
over a distance no contact necessary a
00:00:15
conceptual way to describe forces acting
00:00:17
at a distance involves the concept of a
00:00:19
force field consider this planet in
00:00:22
space these Orange Lines represent
00:00:25
gravitational field lines about the
00:00:27
planet any mass in this gravitational
00:00:30
force field experiences a
00:00:32
force is the force acting on a satellite
00:00:34
of mass m in the
00:00:36
field in previous lessons we've
00:00:38
described the gravitational force in
00:00:40
such a satellite with Newton's law of
00:00:43
gravity an equivalent way is multiplying
00:00:46
the mass of the Satellite by the
00:00:47
gravitational field of the planet where
00:00:50
the satellite is
00:00:52
located then the force F on the
00:00:54
satellite is its mass times the
00:00:57
gravitational field G
00:01:00
this is bold-faced g to distinguish it
00:01:03
from the italic G we often use for
00:01:05
acceleration due to gravity near Earth's
00:01:07
surface both G's have units Newtons per
00:01:11
kilogram the field is stronger up close
00:01:14
and weakens with the inverse square of
00:01:16
distance the field plays an intermediate
00:01:19
role between objects likewise in
00:01:22
electricity here's a proton surrounded
00:01:24
by its electric
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field the electric field is denoted by
00:01:28
the symbol e
00:01:31
and here's an electron circling the
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proton and what of the force on the
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electron much akin to gravity the force
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equals its electric charge multiplied by
00:01:40
the proton's electric field
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QE the magnitude of the electric field
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is simply the force per unit charge if a
00:01:49
charge Q experiences a force F at some
00:01:51
point in space then the electric field
00:01:53
at that point is f / Q when the force is
00:01:58
expressed in Newtons and the charge kums
00:02:00
denoted by the capital letter C the
00:02:03
electric field has the unit Newton per
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Kum n / C both the fields for gravity
00:02:10
and electricity are vector quantities
00:02:12
having direction as well as magnitude
00:02:14
and can be represented by Vector
00:02:17
arrows the direction of Earth's
00:02:19
gravitational field is toward Earth
00:02:21
inward the direction of force on a mass
00:02:23
in the field the direction of the
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electric field about the proton is the
00:02:28
direction of force on a POS positive
00:02:30
charge in the field outward since our
00:02:33
charge is negative here that of an
00:02:35
electron the force on it is opposite to
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the direction of the
00:02:40
field here's a positive charge with a
00:02:43
few of its field lines they Point
00:02:45
radially outward but what of the field
00:02:48
lines about a pair of equal and opposite
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charges we get this pattern a pattern of
00:02:54
curved lines it's interesting to realize
00:02:57
that the curved pattern is the composite
00:03:00
of pairs of straight lines outward from
00:03:02
the positive charge and inward toward
00:03:04
the negative charge for example consider
00:03:08
the field at this point where I place a
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Green
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Dot a positive test charge here would
00:03:14
feel a repelling force from the positive
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charge and an attractive Force toward
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the negative
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charge at this point here's the force
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Vector due to the positive
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charge here's the force Vector showing
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attraction to the negative charge
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we see it smaller in accord with the
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inverse Square
00:03:34
law we draw a parallelogram for these
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two vectors and this gives us the
00:03:39
resultant Force at this location right
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here where the Green Dot
00:03:45
is not its direction coincides with the
00:03:48
direction of the composite field at this
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point let's do the same for this point a
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force in a direction away from the
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positive
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charge and a force of attraction toward
00:04:00
the negative
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charge and using the parallelogram rule
00:04:06
we get the resultant vector and again it
00:04:09
aligns with a composite field at that
00:04:12
point we do the same for this
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point and the
00:04:18
parallelogram and see
00:04:20
that alignment with the field at this
00:04:23
point and for this point same
00:04:28
story I I could repeat this process with
00:04:31
all the points in the electric field and
00:04:33
you know what I'd get the curved pattern
00:04:36
you see a composite of resultant Force
00:04:39
vectors over an infinite number of
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points a curved field that begins at the
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positive charge and ends at the negative
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charge is this yum or
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what let's take a look at some
00:04:52
photographs of electric field
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patterns here's a photograph of such
00:04:59
field Lin about a positive and negative
00:05:01
terminal in a bath of oil the field is
00:05:05
shown by the orientation of bits of
00:05:07
thread suspended in the oil and not
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surprisingly the field formed by the
00:05:12
pair of equal and opposite charges is
00:05:14
like the field I just
00:05:16
sketched we can use the same Vector
00:05:19
technique to show the field between a
00:05:21
pair of positive terminals shown here
00:05:24
what's the resultant field halfway
00:05:26
between the equal charges did you say
00:05:29
zero if so
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yum here's a photo of field lines
00:05:34
between oppositely charged plates called
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a capacitor notice the field is uniform
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between the plates except near the edges
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why the field isn't uniform between the
00:05:45
edges is something we won't get into in
00:05:47
this
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lesson but here's detail for a
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capacitor both plates have equal and
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opposite
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charges and chyo physics instructor Mona
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Asser models a demonstration
00:06:03
capacitor back to our field patterns
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very interesting is the field pattern
00:06:08
for a plate and cylinder that have
00:06:10
opposite
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charges notice the absence of a field
00:06:15
inside the charged cylinder all field
00:06:18
lines cancel out why this is true is
00:06:22
even more interesting covered of course
00:06:24
in your textbook for now we just want to
00:06:27
say that inside any charged body the
00:06:29
field strength is zero all field vectors
00:06:33
cancel again covered in conceptual
00:06:37
physics the fact that no field results
00:06:39
inside a Charged conductor is nicely
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employed in the vandag graph
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generator let's look at the operation of
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a vandag graph
00:06:50
generator we see that a voltage source
00:06:53
leaks charge off metal points in this
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case
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electrons an insulating belt carries the
00:06:59
electron R up inside an electrically
00:07:01
insulated tube that supports the hollow
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doome
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above electrons leak off onto the metal
00:07:08
Point collectors at the top of their
00:07:10
path and repel one another and gather on
00:07:12
the outside of the metal
00:07:15
sphere no matter how much net charge
00:07:18
accumulates on the outside of the Dome
00:07:20
field strength inside is zero that means
00:07:24
charge can be added if introduced inside
00:07:26
the Dome without repelling away whereas
00:07:29
any charge that approaches the outside
00:07:31
of the charge Dome is repelled away
00:07:34
inside not the case the result is that
00:07:38
charge can keep building up of the
00:07:39
dome's outer surface until the buildup
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voltages produce Electric discharge
00:07:44
through the air then we have localized
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lightning
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bolts here we see lri Patterson touching
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a Charged vandag graph
00:07:53
generator note how her hair stands out
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that's because it becomes electrically
00:07:58
charged and strand of her hair are light
00:08:00
enough to show the electric repulsion
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between the
00:08:04
strands the Dome L's touching is at
00:08:07
thousands of volts so why isn't she
00:08:09
shocked as should be by handling a
00:08:11
faulty 110 volt light
00:08:13
socket the answer has to do with
00:08:21
energy voltage is energy per
00:08:25
charge Jews per Kum only a tiny fraction
00:08:29
of a kulum makes up the charge on the
00:08:31
generated Dome so a small amount of
00:08:35
energy per tiny amount of charge can
00:08:37
equal a huge
00:08:39
voltage so any energy transfer to luri
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is
00:08:43
minuscule but that's not the case with a
00:08:46
faulty light socket the flow of charge
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there is in kums not tiny fractions of
00:08:52
kums the flow of energy can be
00:08:55
fatal when thinking voltage think rati
00:08:59
IO of energy per
00:09:01
charge are you getting this I hope so
00:09:05
let me leave you with a concluding
00:09:08
question consider a pair of equally
00:09:10
charged particles separated by a certain
00:09:14
distance if exactly midway between these
00:09:17
particles their combined electric Fields
00:09:20
cancel to zero what can you say about
00:09:23
the signs of charge of the
00:09:25
particles are they like charges or
00:09:28
unlike charges
00:09:30
can you support your answer until next
00:09:33
time good
00:09:37
energy