Day 3.
3
Force
Fields
Instructions
: The students learn about magnetic fields in grade 11
. T
he new id
ea
in grade 12 is that
electric
al and gravitational forces
can also be described through fields.
1)
A
magnetic
field around a magnet can be made visible b
y iron filings.
a)
Predict and then sketch
the magnetic field around a
bar magnet.
The filings show parts of the field lines.
Keep the diagrams simpler and clearer by drawing
continuous loops
including inside the magnet.
Strength of the field can be indicate
d with the thickness or darkness of the line.
The
North Pole
is defined to be the one where the field lines emerge from the magnet.
b)
Predict and then
observe
th
e field
between
two north poles.
c)
Predict and then
observe
the
field
between
a north a
nd a south pole.
The field lines of two bar magnets lined up to att
r
act are like the lines for a single bar magnet.
2)
The
Earth produces a
gravitational
field
, g = F/m
.
a)
S
ketch
the
gravitational
field
lines
in the classroom
.
How can you make
t
hese field lines
visible?
The students have already been using the field concept for gravity but maybe didn’t realize it, especially
if g is more strongly identified with 9.8 m/s
2
rather than 9.8 N/kg.
The lines point down and this can be
seen with many m
asses attached to strings or raindrops on a still day etc.
c)
Calculate
the
field
,
g = F
g
/m
,
near
the surface of the Earth. (
r
E
= 6.38 x 10
6
m, M
E
= 5.98 x 10
24
kg
)
Surprise! It is 9.8 N/kg.
d)
What do gravitational field lines
around
the
Earth
look like?
The lines all point to the center and get weaker further out.
e) What would the field lines
between
the Earth

Moon system look like?
How can you m
odel
this with
magnets
and iron fi
llings?
You would need two magnets and one should be stronger than the
other. Put the south poles together.
g)
How does the
field strength vary inside the E
arth?
Hint: What is the value at the centre?
This was hinted at in the first lesson. The formula says the force is infinite at the centre but it is not valid
inside the E
arth and from symmetry we know that the forces should cancel and therefore the force per
mass
–
the field
–
should be zero. It can be shown with integral calculus that the field drops off linearly
with r. Halfway to the centre, the field is 4.9 N/m.
3)
Ele
ctrical
charges produce a field
,
E
= F/Q
.
http://phet.colorado.edu/en/simulation/charges

and

fields
a
)
What
will
the electric field around a charged sphere look like?
What gravitation
al field is this
like
?
Instructions
: Have students predict by drawing on a white board before observing.
The electric field line direction is defined to be the direction on a positive charge. Therefore the
gravitational field is like the electric field ar
ound a negative charge.
b
)
What
will
the electric field near two opposite charges look like? How can you show this with a
balloon
a person with long hair
? How can you model
this
with two magnets and filings?
T
wo people with charged hair might show thi
s. T
wo north poles
will
south
c
)
What
will
the electric field near two same charges look like? How can you show this with two
balloons? How can you model with two magnets and filings?
This can be shown with the hair attracted to the balloon or with op
posite poles.
d
)
What
will
the field
in
between
parallel
charged plate
s
look like? What gravitational field is this
similar to?
This is like the constant field near the Earth s surface
.
g)
How is it the field of a hollow spherical conductor similar to
and different from that of the Earth?
This
can
be somewhat simulated by making a large circle of positive char
g
es.
The fields outside both
spheres are radial and drop off as the inverse squared. Inside, the fields differ. The gravitational field
inside dro
ps linearly to zero
–
but the electrical field is zero throughout. The Earth’s field would also be
zero inside if it were hollow.
h
) What is the formula for the electrical field
,
E
= F
e
/q,
around a point or spherical charge? Hint: What is
the formula for
the gravitational field around
a point or spherical mass?
E
= kQ/r
2
.
4)
Watch
Electric field inside a conductor
.
http://www.youtube.com/watch?v=WqvImbn9GG4
Tesla Cage of Death
http://www.youtube.com/watch?NR=1&v=Zi4kXgDBFhw&feature=endscreen
What is a Faraday cage and how does it protect you?
Where are they used?
A Faraday cage is made of a conductor.
The charges on the c
onductor redistribute themselves so that the
field inside the cage is zero.
A microwave oven’s case acts as a Faraday cage to keep the EM radiation
inside.
5)
The earth has a net negative charge resulting in a field of 100 N/C. What is the charge on the Eart
h?
Q = 6.38
2
/9 x
10
5
C.
= 4
.5 x 10
5
C.
6)
How do el
ectr
ic, magnetic and gravitational
forces
act without contact?
Force fields around the objects interact.
Are these real or a mathematical construct? Magnetic fields
seem most real as they are so easily to ma
ke visible. Electrical fields can also be seen with rayon fibres
in oil. See images in the textbook, section 7.3. If they are real, what are they made of?
7)
Draw and c
ompare Earth’s gravitational
, electric
and magnetic fields.
The gravitational
and electric
al
field
lines both point
toward the center of the Earth. The magnetic field
looks like that of a bar magnet and the North Pole is a south magnetic pole.
8)
To measure a field, you need to use a
small
test object. Why?
If you use a large charge, its field ad
ds significantly to the pre

existing field and changes it.
9)
Stretchy fabric can be used to model fields. How would you model the field of;
a)
a planet or charged sphere?
Pull down on the fabric at one point for gravity or a negative charge. Pull up for a posit
ive charge.
Place
a mass on this surface and it will fall toward or away from the centre. You can set up orbits with a ball.
b)
the Moon

Earth system
.
Pull down hard in one place and not so hard in another. A ball will roll to one or the other depending on
wh
ere you place it. It is possible to get a figure eight path.
c)
a
magnetic or electric
dipole
Pull up in one place and down in another.
Textbook
7.3
, 8.1
p. 347 # 8

10
10)
Look at p. 381 Figure 7 and p. 346 Figures 20

23. What are field
lines like at the sur
face
s
of conductors
and inside conductor
s
?
Why?
Field lines at the surface of conductors will be parallel to the surface once it is in equilibrium. If this was
not the case the remaining field would cause the electrons to move.
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