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P3 Topic 4: Motion of particles
If an object is travelling in a circle it means it is constantly changing direction and that means they are
This means there must be a resultant force acting on it. This force acts towards the centre of the circle. This
force is called a centripetal force.
For example, in `hammer' throwing, a heavy mass is swung in a circle by its wire. To keep the hammer moving
it needs a centripetal force which is provided by the tension in the wire. When the hammer is released there
is no longer a centripetal force and it travels in a straight line at a tangent to the circular path it had been
Charged particles are used in cyclotrons and they are affected by a magnetic field.
The force on a moving charge in a magnetic field is always perpendicular to its direction of travel; therefore
the particle follows a curved track.
As the particle loses energy, the radius of the path decreases (so the spiral gets smaller).
The direction of the force depends on its charge. The paths of positive and negative particles travel in
The particles move in spirals because the particles lose energy and slow down when they interact with other
particles. The less energy, the more curved their path will be.
Cyclotrons use the above theory to accelerate particles; hence, it's called a particle accelerator. The charged
particles start at the centre of the cyclotron.
Two hollow, semicircular electrodes are used to accelerate particles across the gap between them. The
alternating potential difference between the two electrodes speeds up the particles.
A magnetic field is present to keep the particles moving in a circular motion. The particles spiral outward
because their energy increases.
If a cyclotron is used to bombard a high-energy proton with a stable element, the nucleus of this element is
changed to an unstable nucleus of a different element.
This is proton enrichment and is used to make unstable radioactive isotopes for medical purposes. The proton
has to be high energy to be absorbed into the nucleus. When it is, its atomic number changes, therefore
changing it into a different element.
Radioisotopes made by this method are usually positron emitters. Positron emitters are very useful in
hospitals as they are used in PET scanning.
It's important that radioactive isotopes used for PET scanning have a short half-life so that the patient's
exposure to radiation is minimised.
Particle accelerators are not only used for medical uses but also help scientists find out about the universe.
Huge particle accelerators, like the Large Hadron Collider in Geneva, are used to smash particles into each
other at very high speeds to see what happens i.e. what kind of radiation are emitted, new particles etc.
This gives clues about how the universe works so scientists can develop better explanations about the
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Collaboration of ideas and money are common in research such as this; it is done internationally. Not every
country can afford their own particle accelerator and sharing ideas is often useful for developing new ones.
Sometimes there are rival groups trying to find the same thing and want to be first to make a discovery.
The Large Hadron Collider at Geneva is an example of collaboration. This accelerator was built by CERN and is
the largest and most powerful built.…read more
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Some of the kinetic energy was transformed into heat and sound energy.
A bouncing ball as an example
When a ball is dropped, it will bounce. However, it will not bounce to the height you dropped it from. This is
because when it falls its gravitational potential energy is transferred into kinetic energy. When it hits the
ground, it loses some kinetic energy as heat or sound.…read more