The solenoid that generates the magnetic field has a diameter of 10 cm. The simulation below uses metal plates 10 long with a spacing $d=$ 2 cm. The force on an electron due to the electric field is $\vec. The electrons were accelerated in the $x$-direction, the electric field was in the $y$-direction and the magnetic field was in the $z$-direction. Some of the electrons pass through a small hole in the plate and form and electron beam that travels to a region where an electric field and a magnetic field was present. The electron charge-to-mass ratio was measured by accelerating the electrons through a voltage $V_x$ towards a positively charged plate. He determined that the negatively charged particles (electrons) were much lighter than the positively charged particles. Thomson performed experiments to show that atoms consisted of sub atomic particles that had positive and negative charges. Thomson's experiment to determine the charge-to-mass ratio of electrons The correct direction of the force could also be done by using the right hand rule, but with your left hand this takes into account the negative sign from the charge of the electron.J. This causes the direction of the force to point in the opposite direction. Keep in mind that the charge of an electron is negative.
The direction of the force from the external magnetic field can be determined by the right hand rule. The cross product between the velocity of the cathode ray and the external magnetic field results in a force that is perpendicular to both the velocity of the cathode ray and the external magnetic field. If only a magnetic field is applied across the cathode ray, the beam of electrons will feel a force whose direction is give by the curl of the velocity and the magnetic field: If only an electric field is applied across the cathode ray, the beam of electrons will feel a force exerted on them in the opposite direction of the applied electric field: Where e is the charge of an electron, is the velocity of the particle, is the electric field, and is the magnetic field. The Lorentz force is the combination of the force that an electric field and a magnetic field exerts on a point charge This chemistry and physics video tutorial provides a basic introduction into the cathode ray tube experiment. The path that the beam of electrons follow can be altered by the Lorentz force. The anode allows the cathode ray to pass through and continue on in a straight line until it hits the end of the Crookes tube. This beam of electrons traveling from the cathode to the anode is known as a cathode ray. 10If an electronm 9.11 of-31 kgmassx 10is acceleratedpotential through a differenceV, then its kinetic energy is given by mv 2eV (2) wheree 1.602-19 C x is10 the charge of the electron. These emitted electrons are then accelerated on a straight path towards the anode by the electric field. h/p h/mv (1) wherep mvis the momentumh isofPlancks theconstant, particlen mely and 6.626-34 xJ-s. The cathode contains a filament, such that when it’s temperature increases due to the high voltage, a high number of electrons are released from the surface of the cathode. A high voltage applied between the electrodes creates a voltage and an electric field, where the negative end is the cathode, and the positive end is the anode. On one end of the Crookes tube, there is a metal electrode. The Crookes tube used in this demo is a partially evacuated glass bulb that contains a low-pressure hydrogen gas. If a magnet is introduced near the Crookes tube, a force will be exerted on the cathode ray that is perpendicular to both the direction of the velocity of the electrons and the direction of the magnetic field. To bend the cathode ray with the magnet, simply bring the magnet near the cathode ray. If the electric field was reversed, the cathode ray would deflect in the opposite direction.įigure 5: The cathode ray is deflected by a magnetic field.
This electric field acts on the cathode ray, causing it to deflect towards the positive plate, due to the negative charge of the electron. This results in an electric field that points from the positive metal plate to the negative metal plate. In Figure 4, a potential difference is created between the two metal plates inside of the Crookes tube. To have the cathode ray bend the other way, turn the current to 0A on the power supply, and then simply swap the banana cables that are only on the Crookes tube. This will create the potential difference between the two metal plates inside of the Crookes tube and cause the cathode ray to bend. Turn on the power supply and set the current to 0.3A, and slowly increase the voltage of the power supply. Make sure that current and voltage of the power supply are set to zero prior to turning the power supply on. Figure 4: The deflection of the cathode ray due to a potential difference between the two metal plates inside of the Crookes tube.
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