2.1.2 Forces on Charged Particles

Fundamental forces - order of strength

1) the strong nuclear force, 2) the electromagnetic force, 3) the weak nuclear force, and 4) gravity.

Explain the operation of a synchrotron accelerator using the terms; alternating electric fields, magnetic fields, fixed radius and opposite direction collisions

A synchrotron has a constant radius no matter what the energy of the particle is - the radius is fixed. The frequency of the supply and the magnetic field are both altered constantly in order to maintain the path of the particles as their mass and energy both increase. The deflection caused by the magnetic field makes the path curved, while the frequency of the electric field increases the energy. Particles of opposite charges can be made to circle in opposite directions then the magnets can be adjusted by reversing the polarity to make an opposite direction collision take place when the particles are at full velocity

An electron enters a region of space where there is a uniform magnetic field. As it enters the field the velocity of the electron is at right angles to the magnetic field lines. The energy of the electron does not change although it accelerates in the field. Use your knowledge of physics to explain this effect.

Acceleration is associated with a change in velocity of an object, in this case the particle. Since velocity is composed of both magnitude and direction a change in either speed or direction will cause an acceleration. In this case the magnetic field acting on the charge generates a force that changes the direction of the particle. Since the speed of the particle in unchanged its energy is therefore also unchanged.

Describe how fundamental particles react - gravitational and electromagnetic

Both forces have infinite range. Gravity is weakest of all fundamental forces

Advantages of a linear accelerator

Cheapest of all the accelerators Most efficient at supplying energy to the particles Requires a large space (linear accelerators are several km in length)

An electron enters a uniform magnetic field at an angle to the magnetic field lines as shown below. Explain the spiral shape of the electron path in the magnetic field

Component velocity parallel to magnetic field is unchanged as no unbalanced force. Component of velocity perpendicular to magnetic field causes circular motion.

State and explain evidence for the first neutrino

During beta decay, a neutron in the nucleus of the atom splits into a proton and a fast moving electron. It has kinetic energy. However when conservation of energy was taken into account it appeared there was some energy missing on the products side. It was concluded that another particle was emitted. Since it was undetected they assumed it could travel far without being absorbed and had no charge. This was the first evidence for the neutrino because when the neutrino's mass was taken into account energy is conserved.(In fact, in beta decay an anti-neutrino is emitted along with the electron as lepton number is conserved particle reactions)

Purpose of electric and magnetic fields

Electric fields are used to increase the speed of charged particles and change their direction whilst magnetic fields are only used to change the direction of charged particles

Explain the shape of the electron path in the magnetic field

Electron is entering the magnetic field at right angles which causes a constant spiral path due to the constant vertical acceleration with a constant horizontal velocity

Why are protons fired at metal targets and not electrons

Electrons have a smaller mass so would relativistic speeds too quickly to be of any use Low chance an electron could travel through the electron cloud surrounding the nucleus so it might not reach it Electrons smaller mass would not have much of an impact (ping pong ball off brick wall)

Explain why particles do not cause an aurora above the equator

Enter poles and travel in spiral paths OR never reach the atmosphere above the equator.

Explain how particle accelerators, such as the Large Hadron Collider at CERN, are able to: accelerate charged particles; 1 deflect charged particles.

(i) electric field (to accelerate) (1) 1 (ii) magnetic field (to deflect) (1)

Explain the operation of a cyclotron accelerator in terms of alternating electric fields, magnetic fields and increasing radius

Similarly to the linear accelerator, the particles are only accelerated in the gaps between the Ds. The Ds are like drift tubes, but in this case they are used to turn the particles around so they return to the gap. The alternating voltage ensures that the particles are always accelerated across the gap between the Ds at the right time. The external magnetic field is used to change the direction of the moving particles. The faster the particles go, the greater the distance they will travel in the D; the particles will move around an arc with a greater radius. Used in fixed target experiments ADD DIAGRAM

A point charge of 4C is placed near a -2C charge. Explain why the electric field strength is not 0 at any point between these two charges

The forces acting on a unit positive charge between the plates will be in the same direction

If 1 Joule of work is done moving 1 coulomb of charge between 2 points, what does that mean?

The p.d. between the points in 1 volt

Explain why charged particles do not cause an aurora above the equator

The particles are entering an area of the Earth's atmosphere where the net magnetic field is 0 (in between poles). This causes the charged particles to experience no force so they will not reach the equator and will not cause an aurora

Field lines represent

The path a positive charge would take

Properties of electric fields

The strength of the field decreases further away from the charge. Similar to gravitational field around a planet

Explain why two protons in the nucleus do not fly apart

The strong force acting over a short range overcomes the electrostatic repulsion

Explain why a cyclotron needs an alternating voltage

To provide an oscillating electric field which will reverse the polarity across the gap each half revolution. This ensures that the particles are repelled by one dee and attracted towards the other

Work W is done when

a charge Q is moved in an electric field

Explain why an alternating voltage is used in the cyclotron.

a.c. voltage used to change the direction of the force on protons/polarity of the dees/electric field across the gap. 1 (Electric field must change direction to accelerate the protons because) the direction the protons cross the gap changes keeps changing. 1

In an electric field

an electric charge experiences a force

Fields exist around charged particles and

between charged parallel plates

An electric field applied to a conductor causes the free

electric charges in it to move

A moving charge

produces a magnetic field

The potential difference between 2 points is a measure of

the work done in moving one coulomb of charge between the two points

Other type of beta decay

β⁺ - proton decays into a neutron, and a positron and a neutrino are emitted.

Describe how fundamental particles react - Strong Nuclear

If all the protons in the nucleus are positively charged then the electrostatic repulsion should make them fly apart. Strong nuclear force is the strongest of the 4 fundamental forces but is also extremely short range in action. Only experienced by quarks and therefore the baryons and mesons that are made up from them.

Facts about cyclotrons

In a cyclotron the charged particles are accelerated by alternating electric fields. The particles travel in a spiral of increasing radius as a result of a constant magnetic field, which is perpendicular to the spiral. The radius of the spiral increases as the energy of the particles increases. The diameter of the cyclotron is limited by the physical size of the magnet. The resultant energy of the particles is limited by the diameter of the particles and by relativistic effects. This type of accelerator is used in fixed-target experiments.

Facts about linear accelerators

In a linear accelerator bunches of charged particles are accelerated by a series of alternating electric fields. The final energy of the particles is limited by the length of the accelerator. This type of accelerator is used in fixed-target experiments .

Facts about synchrotron

In s synchrotron bunches of charged particles travel in a circular path of fixed radius as a result of C shaped magnets whose strength can be varied. The particles are accelerated by alternating magnetic fields. As the energy of the particles increases the strength of the magnetic field is increased to maintain the radius of the path of the particles. In synchrotron accelerators the particles can have, in theory, an unlimited series of accelerations as the particles can transit indefinitely around the ring. There will be a limit caused by relativistic effects. In this type of accelerator particles with the same mass and opposite charge can circulate in opposite directions at the same time before colliding. This increases the energy of impact. This type of accelerator is used in colliding beam experiments.

Describe how fundamental particles react - Weak Nuclear

Involved in β decay. Called the weak nuclear force to distinguish it from the strong, but it isn't the weakest of all the fundamental forces. Extremely short range force

Explain why electron beams might follow curved paths/ straight paths

It enters the plate with a horizontal velocity and this will remain constant throughout as there is no unbalanced force horizontally. the electric field applies a vertical unbalanced force upwards resulting in an acceleration upwards. The combination causes the path to be curved./ no unbalanced force at all so continues in straight line at constant speed.

North pole is

Negative

Describe/ explain the paths which an alpha particle, a beta particle and a neutron would follow if each particle, with the same velocity enters an electric field

Neutron - straight through because has no charge so does not experience any force in a magnetic field, constant horizontal velocity, no acceleration in any direction Electron - Attracted to the positive plate as it is negatively charged so experiences a force and accelerates towards the positive Proton - Attracted to the negative plate as it is positively charged so experiences a force and accelerates towards the negative. It travels further than the beta before it is accelerated because it has a larger mass so from F=ma the same force cannot provide the same acceleration as the with the beta

Advantages of cyclotrons

Occupies smaller space

Explain the operation of a linear accelerator in terms of alternating electric fields and fixed targets

Particles such as electrons are accelerated between the gaps between the drift tubes. By connecting the drift tubes to an alternating signal, a potential difference can be set up between the tubes. This potential difference attracts and accelerates the particles across the gap to the next tube, increasing the velocity. The alternating signal then reverses the potential difference and this attracts the particles to the next drift tube, accelerating them again. The process repeats, increasing the velocity of the particles each time. Inside the drift tubes, the particles move with a constant velocity. The drift tubes get longer as you move along the accelerator to account for the increasing velocity of the particles. As the frequency of the alternating supply is constant, the particles must spend the same amount of time in each drift tube. The beam will collide directly with a stationary target, such as a metal block. Much of the beam energy is simply transferred to the block instead of creating new particles (fixed target) producing x-rays beams, neutrons, etc ADD DIAGRAM

South pole is

Positive

Explain why protons are accelerated in electric fields

Protons have a positive charge and are attracted towards the negative plate/experience a force when in an electric field.

How to approach a reflector question

Remember since the wave has to bounce back, for constructive interference p.d= 1/2 wavelength and for destructive interference p.d.= 1/4 wavelength

State and explain evidence for sub-nuclear particles

Rutherford got his students to fire alpha particles at a thin layer of gold foil. He noticed most particles passed straight through with hardly any deflection but the ones that were deflected changed course significantly. Since they could pass through so easily there must have been lots of empty space in the atom. When they were deflected the positive Alpha particles must have hit a large positive mass to change their course so much. This was evidence for the atom having positive protons and neutrons in the nucleus that give an atom their mass with orbiting electrons.