Le Fevre High School
SACE Stage 2 Physics
Electric Fields 3
1.
What is meant by a corona discharge and how is it used to charge the
photoconducitve drum in a photocopier or laser printer?
The corona discharge is a result of strong electric fields around sharp points or edges
producing the acceleration of positive and negative ions in the electric field. This
results in the ions colliding with atoms generating more ions. The result is a net
movement of positive and negative charges (ions) in opposite directions in the electric
field. (If the electric field is high enough, it may produce high accelerations. As these
ions then collide with other atoms, they may release light energy.)
In a photocopier the corona discharge is produced by a potential difference of
approximately 6000 V between a thin wire and an aluminium conducting drum coated
with photosensitive selenium. The positively charged ions produced in the corona
discharge are repelled away from a positively charged corona wire towards an
earthed drum. As the drum rotates it is charged uniformly by these positive ions.
Corona Wire
6000 V
Selenium photoconducting coating
over aluminium
2.
What would be the likely result on the copy produced by a photocopier if the
charging corona wire became oxidized in spots along its length (which
reduces the adjacent electric field)?
If the corona wire became oxidised, it would no longer charge uniformly along its length,
and consequently would not produce a uniform electric field along its length. This
would result in a non-uniform charging of the selenium coating on the drum and
consequently when the latent image is produced on this drum it would not reproduce
truly. (It would result in white streaks across the paper).
Le Fevre High School
3.
Most photocopiers have a control that lighten or darken the image, suggest
how this might be achieved?
An intensity control could be achieved by reducing the potential difference between the
corona wire and the drum. A lower potential difference would mean the charge
deposited on the drum is reduced and hence when the latent image is formed it will
attract less toner and consequently be lighter in appearance.
OR use the trimming blade to remove more toner thus less available toner is transferred
to the drum, reducing the intensity.
4.
What is the difference between a photocopier and a laser printer in their
principles of operation?
The only difference is that the image is projected onto the drum in a different manner.
Photocopier: a strip lamp the full width of the original illuminates the original a strip at
a time. The reflected light is then directed via mirrors and focussing lenses onto the
drum. Mirrors enable the photocopier to be compact in the vertical direction. Lenses
enable the image to be enlarged or reduced. Additional mirrors reduce shadows by
illuminating the original in two directions.
Laser Printer: finely focussed laser spot scans the copy a line at a time. The laser is
switched on and off as it scans the line to control whether the image is dark or light.
5.
Step 1:
Step 2:
Step 3:
Step 4:
Step 5:
Explain using the aid of diagrams, the steps taken to produce a page from a
photocopier.
charging the drum via corona discharge
reflecting and focussing the light from the original onto the drum to create
a latent image
Applying the toner onto the latent image on the drum.
Fixing the image onto the paper and removing the latent image from the
drum.
Removing the charge from the paper (separating corona) and heat fusing
the toner onto the page.
mirror
mirror
Charging
corona
Erase
lamp
drum
Trimming
blade
Magnetic
roller
Cleaning
blade
paper
Fusing
rollers
Separation
corona
Transfer
corona
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6.
A corona wire discharge is typically occurs with electric fields exceeding
about 3 x 106 NC-1 under normal atmospheric conditions. (charge on an
electron = -1.6 x 10-16C)
a. What is the magnitude of the electric force on a singly charged ion in such
a field?
E = 3 x 106 N C-1.
Single ion q = +e = 1.6 x 10-19 C
F = Eq
= (3 x 106).( 1.6 x 10-19)
= 4.8 x 10-13 N
b. What is the magnitude of the acceleration if the ion is molecular oxygen
with a mass of 5.31 x 10-26 kg?
a=
F 4.8 x 10 13

m 5.31 x 10  26
= 9 x 1012 m s-2
c. Comment on your value in part b.
Value is reasonable given the large electric field produced by the 6000 volt
potential difference between the corona wire and the drum. This indicates the
time for ionisation is small and consequently the charging of the drum will be
rapid. Also in air the value would only be sustained between the collisions.
7.
A laser printer is rated at 600dpi (dots per inch). How many dots are needed
to cover an A4 sheet of paper, which measures 21.0cm x 29.7cm allowing for
an unused margin of 0.5cm around all sides of the paper.
A = (21 – 2(0.5)) x (29.7 – 2(0.5))
= 20 x 28.7
= 574 cm2
= 574  (2.54 x 2.54)
= 88.97 square inch.
One square inch = 600 x 600
= 360000 dots
Hence number of dots
= 88.97 x 360000
= 32029264
= 3.20 x 107