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Ideas to Implementation
1. Increased understanding of cathode rays
led to the development of television
1.4. Identify that charged plates produce an electric field/1.6. Discuss qualitatively the electric
field strength due to a point charge, positive and negative charges and oppositely charged parallel
plates/1.7. Describe quantitatively the electric field due to oppositely charged parallel plates
 An electric field is a region in which charged particles experiences a force
 Note: Electric field is a vector; so a magnitude and a direction is required (important in calculations)
 The direction of an electric field is the direction of force that a positive particle would experience within
the field
 The strength of the field is the force experienced by a unit charge.
 Mathematically: 𝐸 =
𝐹
𝑞
(where E = Electric field [Vm-1 or NC-1], F = Force [N] and q = charge [C])
 However, 𝐹 = 𝑞𝐸 and 𝑊 = 𝑞𝑉 = 𝐹𝑑 = 𝑞𝐸𝑑, ∴ 𝑞𝑉 = 𝑞𝐸𝑑 and dividing both sides by ‘q’ we get 𝑉 = 𝐸𝑑 or
𝑉
E=𝑑
 A uniform electric field can be established by supplying two oppositely charged parallel plates with
potential difference
 Electric fields around oppositely charged parallel plates:
 Electric field between point charges and like/opposite charges (basic year 11 knowledge):
1.3. Identify that moving charged particles in a magnetic field experience a force/ 1.5. Describe
quantitatively the force acting on a charge moving through a magnetic field 𝑭 = 𝒒𝒗𝑩 𝐬𝐢𝐧 𝜽
 Recall from motors and generators that charged particles moving at a constant velocity emanate a
magnetic field
 When moving charged particles are placed in a magnetic field, the particle’s magnetic field and the
external field interact, resulting in the particle experiencing a force
 Note: The direction of the force on a charged particle can be determined using the right hand grip rule.
Remember to reverse that direction that your thumb points in when dealing with an electron
 Now, the magnitude of the force acting on the charged particle can be determined using the formula:
𝑭 = 𝒒𝒗𝑩 𝐬𝐢𝐧 𝜽 WHERE F = Force [N], v = velocity of the particle [ms-1], B = magnetic field strength [T] and
𝜃 is the angle with which the charged particle enters the magnetic field. Note: DIRECTION IS ALWAYS
REQUIRED
2.3 Solve problems and analyse information using: 𝐹 = 𝑞𝑣𝐵 𝑠𝑖𝑛 𝜃 and 𝐹 = 𝑞𝐸
A few tricky examples:
1. Suppose a very strong magnetic field is directed into the page. An electron moving at a constant velocity
enters this magnetic field from the right
a) Describe the subsequent motion of this electron (1 mark)
It’s will undergo circular motion as indicated on the diagram
b) Explain why it moves in this manner (3 marks)
By applying the right hand palm rule, it can be determined
that the electron will experience a force upwards,
perpendicular to the direction of its motion, causing it to curve
up as indicated on the diagram. By applying the right hand
palm rule to its motion as it curves, the direction of the force it
experiences is always perpendicular to its linear velocity.
Thus, by definition, this is a centripetal force and therefore the
electron undergoes circular motion. Note: The magnetic field must be strong for this to occur. If the
magnetic field is weak, then the electron will not describe a full circle within the limited magnetic field.
Rather, it will curve up as an arc of a circle.
2. A top plate of a horizontal pair of parallel electric plates is positive. The plates produce a uniform electric
field of 9500 NC-1 between them. An electron is projected into the electric field horizontally to the right
a) Determine the force experienced by the electron due to the electric
field as it passes through the field
F = qE = (-1.602 x 10-19)(9500) = -1.5219 x 10-15 N downwards =
1.5219 x 10-15 N upwards
b) Describe the motion of the electron as it moves through the electric
field
As the electron moves through the electric field, its horizontal velocity is constant (assuming no friction)
and is unaffected by the electric field. However its initial vertical velocity is zero, but it experiences a
constant force upwards due to the electric field, and it therefore accelerates. By definition, the electron
will follow a parabolic path upwards, like an upside-down projectile.
c) Does gravity affect the motion of the electron? Discuss
Gravity will affect the motion of the electron and will decrease the acceleration it experiences upwards.
Thus, it will take a longer time to hit the plate and will have a greater horizontal displacement. However,
the force experienced by the electron = mg = (9.109 x 10-31)(-9.8) = -8.92682 x 10-30 N upwards = 8.92682
x 10-30 N downwards. This is very small compared to the force it experiences due to the electric field.
Hence the effect of gravity on the motion of the electron is negligible.
1.2.Explain that cathode rays allowed the manipulation of a stream of charged particles
 A cathode ray tube (CRT) or discharge tube is a sealed glass
tube from which most of the air is removed by a vacuum
pump.
 It contains two electrodes: the positive electrode is called
the anode and the negative electrode is called the cathode
 A high potential difference was applied across the two
electrodes, causing the appearance of cathode rays, which
are now known to be negatively charged particles called
electrons
 In 1896, Eugene Goldstein proved that the source of
radiation in a discharge tube was the cathode (hence
named them cathode ‘rays’)
 Due the obstruction of a few remaining gas molecules, it glows with a pale green light. It appears as on the
diagram above
 Structures built into or around the basic CRT allows the cathode rays (streams of charged particles) to be
manipulated. For example, placing another set of electrodes that sets up an electric field that deflects the
cathode rays. Likewise, a magnetic field may be set up through the use of electromagnet coils to also
deflect the cathode rays. Even solid objects such as paddle wheels and Maltese crosses can be places
inside. This is seen in the diagram above
1.1. Explain why the apparent inconsistent behaviour of cathode rays caused debate as to whether
they were charged particles or electromagnetic waves
 Today we know that cathode rays are beams of negatively charged electrons. However, a little over 100
years ago (when the atom was the smallest unit of matter known), conflicting experiments caused
scientists to vigorously debate whether they were indeed charged particles or electromagnetic waves
 The Magnetic field experiment: In 1865, H. Sprengel showed that cathode rays were deflected by a
magnet. This suggests that there were charged, and must therefore be charged particles (as waves do
not carry a charge)
 The Paddle wheel experiment: In 1875, William Crooke’s designed a cathode ray oscilloscope with a
paddle wheel in it. When the cathode rays tube was turned on, the paddle wheel moved and turned from
the cathode to anode (like the cathode rays themselves). This suggests that Cathode rays passed their
momentum onto the paddle wheel, and therefore must have mass (p = mv) and be charged particles
(waves do not have a mass)
 The Electric field experiment: In 1883, Hertz incorrectly showed that cathode rays could not be deflected
by an electric field (this mistake was due to the fact that his CRT had small amounts of gas in it which got
ionised upon contact with the cathode rays, neutralising the electric field) suggesting that they must be
electromagnetic waves
 The Maltese Cross experiment: An experiment was conducted where the anode was in the shape of a
Maltese cross. A sharp shadow was produced by the cross, suggesting that they travel in straight lines and
caused fluorescence, suggesting that they must be waves.
 The metal foil experiment: In 1892, Hertz directed cathode rays at a thin metal foil and observed that they
passed through the foil. The smallest unit of matter known at the time was the atom, and this was too big
to pass through the atoms of the metal. Hence, it was believed that they must be electromagnetic waves
(as these are small enough to permeate the metal foil)
 Corrected Electric field and charge/mass experiment: J.J. Thomson repeated Hertz’s experiment with
electric fields and cathode rays (but reduced the gas in the CRT) and found that they were deflected by
electric fields. He also successfully determined the charge/mass ratio of cathode rays, providing the final
proof that cathode rays are chargedparticle
 Scientific method: A theory is a scientist’s explanation of a principle. Since scientific explanations are
provisional, and scientific views at any time depend on the evidence available to support these views,
theories may change—we say therefore that science is ‘tentative’. Historically, this debate is an apt
example of the ‘tentative’ nature of science and the scientific method, i.e. observations from experiments
are interpreted and a hypothesis developed to explain what is thought to be happening. Opposing models
are then resolved through improved experimentation, allowing us to gain a greater understanding of the
nature of cathode rays.
1.7. Outline Thomson’s experiment to measure the charge: mass ratio of an electron
 Thomson set up his experiment as shown in the simplified diagram: (refer to book)
 As can be seen, he arranged the electric field such that the cathode rays would deflect upwards
 He then set up a magnetic field (using electromagnet coils) such that the magnetic field would produce a
downward force on the electron
 He then altered the strength of the electric field such that the downward force caused by the magnetic
field would be completely cancelled by the upward force provided by the magnetic field
𝐸
 He then equated the Forces: 𝐹𝐸 = 𝐹𝐵 , ∴ 𝐸𝑞 = 𝑞𝑣𝐵, ∴ 𝑣 = 𝐵
 He knew both the value for ‘E’ and ‘B’, allowing him to calculate the velocity of the cathode ray
 Now, he completely switched off the electric field. Since the force acting on the electron now (due to a
known magnetic field) will be perpendicular to its linear velocity, the force due to the magnetic field
becomes a centripetal force =
𝑚 𝑣2
.
𝑟
He then measured the displacement of the cathode ray beam (the
radius of circular motion)
 He then equated the centripetal force and the magnetic force: 𝐹𝐵 = 𝐹𝐶 , ∴ 𝑞𝑣𝐵 =
𝑚 𝑣2
,
𝑟
∴
𝑞
𝑚
𝑣
= 𝑟𝐵
 All the values on the right hand side are known. Despite the cathode material or gases, he always attained
a value of 1.76 x 1011 C/kg. This value was 1800 times less that the value for a hydrogen ion. Since the
charge of mass could not be determine, there were two possible conclusion he could make:
1. The charge of the cathode ray was a 1800 times greater than the charge of a hydrogen ion
2. The mass of the cathode ray was 1800 less than that of a hydrogen ion
 He concluded the latter. This undoubtedly contributed to the discovery of the electron and the development
of models of atoms: as his results paved the way for his conclusion that cathode rays were a new class of
negatively charged particles (later named electrons)
2.1. Perform an investigation and gather first-hand information to observe the occurrence of
different striation patterns for different pressures in discharge tubes
Aim: To observe the occurrence of different striation patterns for different pressures in discharge tubes
Equipment: Induction coil, one set of discharge tubes with varying pressure, two plug-clip leads, two
plug-plug leads and a power pack
Safety:
 As the cathode rays hit the glass or metal in the discharge tube, X-rays are generated. Prolonged exposure
to these X-rays may cause lethal diseases such as cancer. To prevent this, all students observed the
striation patterns from five metres away and the teacher operating the discharge tubes was wearing a
lead apron.
 Touching the induction coil while it is operating may cause electric shock and death. To avoid this, stay a
safe distance from the induction coil
 Take care when handling the discharge tubes, as the low pressure can cause them to easily IMPLODE
Method:
1. Connect the power pack to the induction coil and set it at 6V. Adjust the points on the induction coil so
that a strong steady spark is produced
2. Attach the negative terminal of the induction coil to the cathode of the discharge tube and marked with
the highest pressure (40mmHg) and attach the positive terminal to the anode. Switch on the power pack
3. Sketch a diagram of the pattern observed in this tube and describe it carefully
4. Repeat steps 1-3 for each of the other discharge tubes – tubes to be used should be 10mmHg, 6mmHg,
3mmHg, 0.14mmHg and 0.03mmHg
5. Repeat the experiment ten times
Results:
Pressure
(mmHg)
40
10
6
3
0.14
0.03
Striation pattern observed















Flashes of purple at anode and cathode
Black in the middle
Bright purple at anode and cathode
Pink stream of light in the body
Purple light at anode and cathode
One dark gap near cathode
Pink-purple body
Pink anode
Purple flashes at cathode
Pink-orange body with striations
Pink-purple glow at anode and cathode
Purple body
Less striations
Glass fluorescing
Tube is dark except for the purple fluorescence behind the anode
on the glass
1.1.9. Outline the role of the following in the cathode ray tube of conventional TV displays and
oscilloscopes
 Electrodes in the electrode gun: The electron gun produces a narrow beam of electrons. It consists of a
heating filament, a cathode, a grid and two open-cylinder anodes (the second one is more positively
charged than the first). (Note: There are three electrode guns in a colour TV). The filament heats the
cathode, causing thermionic emission. The positively charged anodes help to focus and accelerate the
electrons to the required speeds, so that they have enough energy on reaching the screen to cause
fluorescence. An electrode called the grid between the cathode and anode helps control the brightness of
the spot, by controlling the number of electrons emitted by the gun. The more positive the grid is, the
more the electrons get attracted and hit the screen, the greater the brightness of the spot
 The deflection plates or coils: Are either electrically charged (used in CROs), or electromagnets (used in
TV and computer screens) can be used to deflect the cathode ray beam to the required spot on the screen
o
For CROs: Two sets of parallel deflecting plates are charged to produce an electric field between
them. The horizontally placed parallel plates (Y plates) produce a vertical electric field, deflecting
the beam up and down. The vertically placed parallel plates (X plates) produce a horizontal
electric field, deflecting the beam right and left. The beam is always deflected toward the positive
plate, and the greater the voltage between the two plates, the greater the magnitude of the
deflection
o
For TVs: Electric current passing through the coils around the cathode-ray tube produces
magnetic field that control the movement of the electron beam. Magnetic coils allow for a
wider angle beam than what would be possible with electric fields from plates, which is why
they were used in CRT TVs
 The fluorescent screen: The inner surface of the glass screen is coated with layers of a fluorescent
material like Zinc Sulfide. When electrons strike the screen, the coating fluoresces, and a spot of light is
seen on the screen.
1.2.2. Perform an investigation to demonstrate and
identify properties of cathode rays using discharge
tubes: containing a Maltese Cross; containing
electric plates; with a fluorescent display screen;
containing a glass wheel; analyse the information
gathered to determine the sign of the charge on the
cathode rays
Aim: To determine some of the properties of the rays
which come from the cathode of a discharge tube
Apparatus: Two power packs, two plug-plug leads, one
pair of magnets, induction coil, four plug-clip leads and
discharge tubes (ones with a Maltese Cross, containing
electric plate, a fluorescent display screen and a glass
wheel)
Method:
1. Connect the terminals of the induction coils and set it at
6V. Adjust the points on the induction coil so that a
strong, steady spark is produced
2. Connect the terminals of the induction coil to the
discharge tube containing the Maltese Cross when the
cross is down and when it is up
3. Replace the Maltese cross tube with the tube containing
electric plates, and connect the terminals of the plate to
its high DC voltage supply. Observe the effects of the
electric field on the cathode rays
4. Connect the tube with the fluorescent screen display to
the induction coil and record the effect of placing a set
of bar magnets near the cathode rays
5. Finally, connect the tube containing the glass wheel on
tracks to the induction coil and observe the effects that
the cathode rays have on the when the tubs is horizontal
Results:
 Maltese cross: a clearly define shadow of the cross appears on the back of the tube opposite the cathode.
This showed that the cathode rays travelled in straight lines from the cathode to anode, and are blocked
by solid objects
 Electric plates: pairs of electric plates caused the cathode rays to be deflected toward the positive plate.
This showed that they ate negatively charged and consist of particles (since electromagnetic waves are
not charged)
 Fluorescent Screen: a thin straight green beam was formed on the fluorescent screen, and this was
deflected when a bar magnet was brought close. The deflection showed that the cathode rays were
charged, and the direction of deflection indicated a negative charge. The thin beam demonstrated they
travel in straight lines. Additionally, the fact that they cause fluorescence showed that they have energy
 Paddle Wheel: the lightweight glass paddle wheel, able to rotate freely, is placed in the path of the
cathode rays so that the rays strike one edge of the wheel at a tangent. The cathode rays caused the wheel
to spin and move away from the cathode. This demonstrates that the cathode rays must have momentum,
and therefore mass, and they are emitted from the cathode