Download the field

Day 3.3
Force Fields
Instructions: The students learn about magnetic fields in grade 11. The new idea in grade 12 is that electrical
and gravitational forces can also be described through fields.
1) Sometimes you can exert forces without contact.
a) How can you pick up bits of paper without touching? Charged balloon etc.
b) How can you pick up paper clips without touching? Magnet.
c) How can you pick up a ball without touching? They won’t be able to do this. However, the ball can be
picked ‘down’ by letting go.
Instructions: We are used to forces acting at a distance and so we are usually not bothered by this.
How do electrical, magnetic and gravitational forces work at a distance? How does the Sun pull the Earth? We
need the concept of field to explain this.
2) Watch MinutePhysics: Real World Telekinesis. How does telekinesis work?
https://www.youtube.com/watch?v=NMgcX8UNIGY&list=UUUHW94eEFW7hkUMVaZz4eDg
Instructions: Just watch the first 2 minutes of the five minutes.
Forces are transmitted by fields. OK, so what are fields? This is NOT explained in the video and really seems to
just put a name to something we still don`t understand. However, we will take this as a provisional model and
see what we can do with it.
3) A magnetic field around a magnet can be made visible by 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 lines inside the magnet. The strength of the field can be indicated with the thickness of the line. The
lines do not have a direction. By convention we have them point out of a north pole and into a south pole.
b) Predict and then observe the field between two north poles.
c) Predict and then observe the field between a north and a south pole.
The field lines of two bar magnets lined up to attract are like the lines for a single bar magnet.
4) The Earth produces a gravitational field, g = F/m.
a) Sketch the gravitational field lines in the classroom. How can you make these 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/s2 rather than 9.8 N/kg. The lines point down and this can be
seen with many masses attached to strings or raindrops on a still day etc.
b) What are the units of the gravitational field?
They are N/kg from looking at the formula but students will be more familiar with m/s2. Challenge them
to show that they are the same. Hint: A Newton is a kg m/s2.
c) Calculate the field, g = Fg/m, near the surface of the Earth. (rE = 6.38 x 106 m, ME = 5.98 x 1024 kg)
g = Fg/m = G Mm/mr2 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 model this with
magnets and iron fillings?
You would need two magnets and one should be stronger than the other. Put similar poles together.
f) How does the field strength vary inside the Earth? 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 the formula
is not valid inside the Earth. From symmetry we know that the forces should cancel and therefore the
force per mass – the field – should be zero at the centre. 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/kg.
5) Electrical 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 gravitational 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. The electric field
around a negative charge is like the gravitational field around a planet and opposite to the field around a
positive charge.
b) What will the electric field near two opposite charges look like? How can you show this with a
balloon and a person with long hair? How can you model this with two magnets and filings?
This can be shown with the hair attracted to the balloon or with opposite poles.
c) What will the electric field near two similar charges look like? How can you show this with two
balloons? How can you model with two magnets and filings?
Two people with charged hair might show this. Two north poles will show this.
d) What will the field in between parallel charged plates look like? What gravitational field is this
similar to? This is like the constant field near the Earth s surface.
This can be demonstrated with a raised Plexiglas plate and small paper bits.
e) How is it the field of a hollow spherical conductor similar to and different from that of the Earth?
Hint: Look at p. 381 Figure 7 and p. 346 Figures 20-23.
This can be somewhat simulated by making a large circle of positive charges. It needs to be really
symmetric. The fields outside both spheres are radial and drop off as the inverse squared. Inside, the
fields differ. The gravitational field inside drops linearly to zero – but the electrical field is zero
throughout. The Earth’s field would also be zero inside if it were hollow.
f) What is the formula for the electrical field, E = Fe/q, around a point or spherical charge? Hint: What is
the formula for the gravitational field around a point or spherical mass?
E = kQ/r2.
6) 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 conductor redistribute themselves so that the
field inside the cage is zero. A microwave’s case acts as a Faraday cage to keep the EM radiation inside.
7) 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 positive 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
where 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.
8) Go to Phet http://phet.colorado.edu/en/simulation/electric-hockey Do level 1 with just 2 charges.
Textbook
7.3, 8.1
p. 347 # 8-10