A cathode is a negatively charged electrode.

An anode is a positively charged electrode.

What evidence from discharge tube experiments showed that cathode rays consist of negatively charged particles?

When a magnetic field was applied to the discharge tube, the path of the cathode rays was deflected , showing they were made of negatively charged particles .

Explain why a discharge tube must contain gas at low pressure for it to glow.

At low pressure, gas particles are widely spaced , giving accelerating electrons and ions room to gain enough kinetic energy to excite atoms on collision. These atoms then de-excite , emitting photons.

Explain what causes conduction across a discharge tube.

The electric field ionises the gas, producing electrons and positive ions . Electrons move towards the anode and positive ions move towards the cathode ; this movement of charge is the conduction current.

In a discharge tube, positive ions are attracted towards the .......... , while electrons are attracted towards the .......... .

In a discharge tube, positive ions are attracted towards the cathode , while electrons are attracted towards the anode .

True or False? Cathode rays are a form of electromagnetic radiation.

False. Cathode rays are negatively charged particles (electrons), not electromagnetic radiation, as shown by their deflection in a magnetic field.

Define thermionic emission .

Thermionic emission is the process by which heating a cathode (filament) gives its electrons enough kinetic energy to leave the metal's surface.

Why does heating the cathode make it easier to produce a cathode ray?

The electrons in the heated cathode gain more energy in their kinetic store , enough to leave the metal's surface and move towards the anode.

In an electron gun, what happens to electrons after they are accelerated towards the anode?

Most strike the anode, but some pass through a hole in its centre, forming a tight beam of electrons that continues towards the target.

Passing a current through a filament causes it to .......... , giving its electrons enough kinetic energy to escape the metal's surface.

Passing a current through a filament causes it to heat up , giving its electrons enough kinetic energy to escape the metal's surface.

True or False? It is the current itself, rather than the rise in temperature it causes, that gives electrons in the filament enough energy to be emitted.

False. The current heats the filament, and it is this higher temperature that gives the electrons enough kinetic energy to leave the metal's surface.

Describe J.J. Thomson's method for determining the specific charge of the electron using both electric and magnetic fields.

The electric and magnetic forces on the electron beam are balanced (using Helmholtz coils and charged plates) until the beam travels in a straight line , giving v = V /( Bd ). The electric field is then switched off, and the radius of the resulting circular path in the magnetic field alone is measured, allowing e / m e to be calculated.

In the specific-charge method using an electric field only, what two quantities must be measured to determine the electron beam's vertical acceleration a ?

The vertical displacement , y , and the width of the plates , w (with the horizontal speed v known).

In Thomson's method, the electric field was switched off and the electron beam formed a .......... path, allowing its radius to be measured.

In Thomson's method, the electric field was switched off and the electron beam formed a circular path, allowing its radius to be measured.

True or False? In Thomson's balanced-field method, the magnetic and electric forces act in the same direction on the electron beam.

False. The magnetic and electric forces act in opposite directions ; the potential difference is adjusted until they are equal and opposite, producing a straight, undeflected beam.

What did Thomson's deflection experiments show about the charge of the particles in cathode rays?

They must be negatively charged .

How did Thomson's measured specific charge for the electron compare to the previously known specific charge of the hydrogen ion?

It was around 1800 times larger than that of the hydrogen ion.

Why couldn't Thomson's large specific-charge result alone determine whether the electron had a small mass or a large charge?

Specific charge is a ratio , so a large value could be explained by either a smaller mass or a larger charge ; further experiments were needed to separate the two.

Thomson's specific charge for the electron was around .......... times larger than that of the hydrogen ion.

Thomson's specific charge for the electron was around 1800 times larger than that of the hydrogen ion.

True or False? Thomson's specific charge measurement alone was enough to determine the exact charge of the electron.

False. Specific charge is a ratio of charge to mass, so it could not by itself reveal the electron's charge; separate experiments (such as Millikan's) were needed.

Why was oil used rather than water in Millikan's oil drop experiment?

Oil does not evaporate quickly, so the mass of each drop stayed constant throughout the experiment.

How were the oil droplets given an electric charge in Millikan's experiment?

They were ionised by X-rays as they left the spray nozzle.

What determines whether an ionised oil drop becomes positively or negatively charged?

It becomes positively charged if it loses electrons, and negatively charged if it gains electrons.

What was the aim of Millikan's oil drop experiment?

To determine the charge , Q , of individual oil drops, and hence the value of the elementary (fundamental) charge .

A falling oil drop can be held stationary by increasing the upward .......... force until it is equal to the drop's .......... .

A falling oil drop can be held stationary by increasing the upward electric force until it is equal to the drop's weight .

Define terminal velocity .

Terminal velocity is the constant speed reached when the resultant force on an object is zero, for example when a falling oil droplet's weight is balanced by the viscous drag force.

State the three conditions required for Stokes' law to apply to an object moving through a fluid.

The object must be: Small Spherical Moving at low speed

What pattern did Millikan notice in the charge values he measured across many different oil droplets?

Every measured charge was an integer multiple of 1.60 × 10 -19 C.

What did the quantisation of oil-droplet charge values suggest about electric charge?

That 1.60 × 10 -19 C is the elementary (fundamental) charge , the charge carried by a single electron.

At terminal velocity, a falling oil droplet's .......... is balanced by the viscous .......... force.

At terminal velocity, a falling oil droplet's weight is balanced by the viscous drag force.

True or False? Stokes' law can be applied to any object moving through a fluid, regardless of its size, shape or speed.

False. Stokes' law only applies to an object that is small , spherical , and moving at low speed .

A LevelPhysicsAQAFlashcardsTurning Points in PhysicsThe Discovery of the Electron

The Discovery of the Electron (AQA A Level Physics): Flashcards

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Cards in this collection (41)

Define cathode.
A cathode is a negatively charged electrode.
Define anode.
An anode is a positively charged electrode.
What evidence from discharge tube experiments showed that cathode rays consist of negatively charged particles?
When a magnetic field was applied to the discharge tube, the path of the cathode rays was deflected, showing they were made of negatively charged particles.
Explain why a discharge tube must contain gas at low pressure for it to glow.
At low pressure, gas particles are widely spaced, giving accelerating electrons and ions room to gain enough kinetic energy to excite atoms on collision. These atoms then de-excite, emitting photons.
Explain what causes conduction across a discharge tube.
The electric field ionises the gas, producing electrons and positive ions. Electrons move towards the anode and positive ions move towards the cathode; this movement of charge is the conduction current.
In a discharge tube, positive ions are attracted towards the .........., while electrons are attracted towards the ...........
In a discharge tube, positive ions are attracted towards the cathode, while electrons are attracted towards the anode.
True or False?
Cathode rays are a form of electromagnetic radiation.
False.
Cathode rays are negatively charged particles (electrons), not electromagnetic radiation, as shown by their deflection in a magnetic field.
Define thermionic emission.
Thermionic emission is the process by which heating a cathode (filament) gives its electrons enough kinetic energy to leave the metal's surface.
Why does heating the cathode make it easier to produce a cathode ray?
The electrons in the heated cathode gain more energy in their kinetic store, enough to leave the metal's surface and move towards the anode.
In an electron gun, what happens to electrons after they are accelerated towards the anode?
Most strike the anode, but some pass through a hole in its centre, forming a tight beam of electrons that continues towards the target.
State the equation for the work done, W, by an electric field of potential difference V on a charge q.
$W = q V$
An electron starting from rest is accelerated through a potential difference V. Derive an expression for its speed v in terms of e, V and electron mass m_e.
All work done is transferred to kinetic energy:
$e V = \frac{1}{2} m_{e} v^{2}$
Rearranging gives:
$v = \sqrt{\frac{2 e V}{m_{e}}}$
Passing a current through a filament causes it to .........., giving its electrons enough kinetic energy to escape the metal's surface.
Passing a current through a filament causes it to heat up, giving its electrons enough kinetic energy to escape the metal's surface.
True or False?
It is the current itself, rather than the rise in temperature it causes, that gives electrons in the filament enough energy to be emitted.
False.
The current heats the filament, and it is this higher temperature that gives the electrons enough kinetic energy to leave the metal's surface.
Define specific charge.
Specific charge is the charge per unit mass of a particle, measured in C kg^-1. For an electron this is $\frac{e}{m_{e}}$ .
Give the equation for the specific charge of an electron determined using a magnetic field only, in terms of accelerating potential difference V_A, magnetic flux density B and beam radius r.
$\frac{e}{m_{e}} = \frac{2 V_{A}}{B^{2} r^{2}}$
Describe J.J. Thomson's method for determining the specific charge of the electron using both electric and magnetic fields.
The electric and magnetic forces on the electron beam are balanced (using Helmholtz coils and charged plates) until the beam travels in a straight line, giving v = V/(Bd). The electric field is then switched off, and the radius of the resulting circular path in the magnetic field alone is measured, allowing e/m_e to be calculated.
In the specific-charge method using an electric field only, what two quantities must be measured to determine the electron beam's vertical acceleration a?
The vertical displacement, y, and the width of the plates, w (with the horizontal speed v known).
Give the equation for specific charge determined using an electric field only, in terms of acceleration a, plate separation d and potential difference V.
$\frac{e}{m_{e}} = \frac{a d}{V}$
In Thomson's method, the electric field was switched off and the electron beam formed a .......... path, allowing its radius to be measured.
In Thomson's method, the electric field was switched off and the electron beam formed a circular path, allowing its radius to be measured.
True or False?
In Thomson's balanced-field method, the magnetic and electric forces act in the same direction on the electron beam.
False.
The magnetic and electric forces act in opposite directions; the potential difference is adjusted until they are equal and opposite, producing a straight, undeflected beam.
What did Thomson's deflection experiments show about the charge of the particles in cathode rays?
They must be negatively charged.
How did Thomson's measured specific charge for the electron compare to the previously known specific charge of the hydrogen ion?
It was around 1800 times larger than that of the hydrogen ion.
Define specific charge.
$\frac{p a r t i c l e c h a r g e}{p a r t i c l e m a s s}$
Why couldn't Thomson's large specific-charge result alone determine whether the electron had a small mass or a large charge?
Specific charge is a ratio, so a large value could be explained by either a smaller mass or a larger charge; further experiments were needed to separate the two.
Thomson's specific charge for the electron was around .......... times larger than that of the hydrogen ion.
Thomson's specific charge for the electron was around 1800 times larger than that of the hydrogen ion.
True or False?
Thomson's specific charge measurement alone was enough to determine the exact charge of the electron.
False.
Specific charge is a ratio of charge to mass, so it could not by itself reveal the electron's charge; separate experiments (such as Millikan's) were needed.
Why was oil used rather than water in Millikan's oil drop experiment?
Oil does not evaporate quickly, so the mass of each drop stayed constant throughout the experiment.
How were the oil droplets given an electric charge in Millikan's experiment?
They were ionised by X-rays as they left the spray nozzle.
What determines whether an ionised oil drop becomes positively or negatively charged?
It becomes positively charged if it loses electrons, and negatively charged if it gains electrons.
State the condition for an oil drop of mass m and charge magnitude Q to be held stationary between plates separated by distance d with potential difference V.
$\frac{Q V}{d} = m g$
What was the aim of Millikan's oil drop experiment?
To determine the charge, Q, of individual oil drops, and hence the value of the elementary (fundamental) charge.
A falling oil drop can be held stationary by increasing the upward .......... force until it is equal to the drop's ...........
A falling oil drop can be held stationary by increasing the upward electric force until it is equal to the drop's weight.
True or False?
Increasing the potential difference across the plates decreases the upward force on a charged oil drop.
False.
Since $F = \frac{Q V}{d}$ , increasing the potential difference V increases the upward electric force on the drop.
Define terminal velocity.
Terminal velocity is the constant speed reached when the resultant force on an object is zero, for example when a falling oil droplet's weight is balanced by the viscous drag force.
State the three conditions required for Stokes' law to apply to an object moving through a fluid.
The object must be:
- Small
- Spherical
- Moving at low speed
State Stokes' law for the viscous drag force F on a sphere of radius r moving at velocity v through a fluid of viscosity η.
$F = 6 π η r v$
What pattern did Millikan notice in the charge values he measured across many different oil droplets?
Every measured charge was an integer multiple of 1.60 × 10^-19 C.
What did the quantisation of oil-droplet charge values suggest about electric charge?
That 1.60 × 10^-19 C is the elementary (fundamental) charge, the charge carried by a single electron.
At terminal velocity, a falling oil droplet's .......... is balanced by the viscous .......... force.
At terminal velocity, a falling oil droplet's weight is balanced by the viscous drag force.
True or False?
Stokes' law can be applied to any object moving through a fluid, regardless of its size, shape or speed.
False.
Stokes' law only applies to an object that is small, spherical, and moving at low speed.

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