Magnetic Fields Flashcards
Why are magnets called dipoles?
Magnets have a North Pole and a South Pole
What is a magnetic field?
A region where a magnet exerts a force on objects made from magnetic materials or on other magnets
How do you represent magnetic fields?
Using arrows or flux lines to indicate the strength and direction fo the field in the region surrounding the magnet
Describe the field lines for magnets
- Flux lines represent the direction of the force experienced by the both pole of. magnet at any point in a magnetic field (N->S)
- A magnetic field is strongest where its flux density is highest and this is shown as flux lines closest together
- A magnetic field with twice the strength is drawn with twice the number of flux lines per unit area in the same region
- The magnetic field of more than one magnet is the combined field of the individual magnets
- Flux lines never cross
- If there is more than one magnets, the magnetic fields cancel out in some places and there is a neutral point
Which direction do magnetic field lines go?
- North to South
- Closer the lines are together the stronger the field
How do you mark a neutral point?
With an x
What will a magnet freely suspend in a magnetic field do?
Align itself with the field
What is magnetic flux density?
- The strength or intensity of a magnetic field is its magnetic flux density B also known as B field
- Magnetic flux density is the number of magnetic flux lines that pass through an area of 1m^2
How is magnetic flux density measured?
Magnetic flux density is measured in Teslas (T)
What is Tesla?
Tesla is the flux density that causes a force of 1N on a 1m wire carrying a current of 1A at right angles to the flux
What do moving charges cause?
- Moving charges cause a magnetic field, which we describe using flux lines
- When current flows in a wire or any other long straight conductor, a magnetic field is induced around the wire
How is the magnetic flux around a current carrying wire shown?
- As concentric circles indicating the magnitude and direction of the flux
1. Moving away from the wire, flux lines are further apart because the field gets weaker
2. If you look at the wire with the conventional current flowing away from you the flux lines circle the wire in a clockwise direction
What does the x indicate?
Current flowing away from you
What does the dot indicate?
Current flowing towards you
How do you work out the direction of a magnetic field around a current-carrying wire?
- Use the right hand rule
1. Thumb points in direction of conventional current
2. Curled up fingers point in the direction of the field
What is the direction of the magnetic field if you loop the wire into a coil?
If you loop the wire into a coil, the filed is doughnut shaped while a coil with a length (a solenoid) form a field like a bar magnet
What is a solenoid?
A solenoid is a current carrying coil of wire that produces magnetic flux (this is also described as an electromagnet formed form a coil of wire)
What is the magnetic flux around a solenoid like?
The magnetic flux outside a solenoid is similar to the magnetic flux for a bar magnet, with the North Pole at one end of the coil and the South Pole at the other end, depending on the current direction
Why does the current-carrying wire in a magnetic field move?
Because a force acts on it, the magnetic field making the wire move is called the catapult field
What is the catapult field due to?
The catapult field is due to the combined effect of the current carrying wire’s flux and the static flux
Why will a wire carrying a current in a magnetic field experience a force?
- If you put a current-carrying wire into an external magnetic file (e.g. between two magnets) the field around the wire and the field from the magnets are added together
- This causes a resultant feel (lines closer together show where the magnetic field is stronger) and this bunched up lines cause a ‘pushing’ force on the wire
What is the direction of the force on a current carrying wire in a magnetic field always?
- The direction of the force is always perpendicular to both he current direction and the magnetic field (given by FLHR)
- If the current is parallel to the field lines the size of the force is 0N as there is no component of the magnetic field perpendicular to the current
What is Fleming’s Left-Hand Rule (FLHR)?
- Thumb point in direction of force
- First finger is in direction fo the external uniform magnetic field
- Second finger points in direction of conventional current
How do you measure a magnetic field?
- A magnetic filed runs through a coil of wire as well as outside it
- You can measure the flux density of the field using a Hall probe
How does a Hall probe measure the flux density of the field?
- The probe contains a slice of semiconducting material
- If a current flows in the semiconductor when it is perpendicular to the magnetic flux, a potential difference is generated across the sides of the semiconductor
- This potential difference is directly proportional to the flux density
What is the force on a wire proportional to?
- The force on a wire is proportional to the flux density
1. The force on a current-carrying wire at a right angle to an external magnetic field is proportional to the magnetic flux density B and magnetic flux density is sometimes called the strength of the magnetic field
What is magnetic flux density defined as?
-The force on one metre of wire carrying a current of one amp at right angles to the magnetic field
-Flux density is a vector quantity with both a direction and magnitude and is measured in teslas, T
1 Tesla = Wb/m^2
What happen when the current is at 90 degrees to the magnetic field?
The size of the force F is proportional to the current I, the length of the wire in the field, as well as the flux density B so F=BIl
How do you calculate the direction of the force on a current carrying wire?
- Magnetic flux density is a vector quantity
- When the directions of the magnetic flux, current in the conductor and the force are all at right angels to each other, Fleming’s Left Hand motor rule helps to see the 3D arrangement of these vectors
How do you calculate the size of the force?
- We can measure the flux density at any point by measuring the force exerted on a current carrying wire at that point
- The Tesla is defined as the flux density that causes a force of 1N on 1m of a wire carrying a current of 1A at right angles to the magnetic field
F=BIl
Describe uses of forces on a charged particle moving in a magnetic field
- Charged particles moving in a magnetic filed also experience a force
- Old style TV and computer monitors use electron guns to produce beams of rapidly moving electrons in evacuated tubes, and their direction is controlled using a magnetic field
Describer how forces act on charged particles in a magnetic field
- A force acts on a charged particle moving in a magnetic filed
- This is why a current-carrying wire experience a force in a magnetic field as electric current is the flow of negatively charged electrons
1. The force on a current-carrying wire in a a magnetic field that is perpendicular to the current is given by F=BIl
2. Electric current I is the flow of charged Q per unit time so I = Q/t
3. A charged particle which moves a distance l in=n time v has a velocity, v = l/t so l=vt
What is the equation for the force acting on a single charged particle moving through a magnetic field where its velocity is perpendicular to the magnetic filed?
F=BQv
What is important to remember about F=BQv?
- You can use LHR to predict the direction of the force
- The sign of he charge is important as a sportively charred particle and a negatively charged particle will move in opposite direction in the same field
- This is because if a negative charge moves to the left, the conventional current flows to the right
How is work done with a charged particle?
When a charged particle moves at right angles to a uniform flux, the charged particles moves in a circle because the force is always perpendicular to the motion, provided no energy is lost (e.g. when the charge is in a vacuum)
Why does the kinetic energy of the charged particle not change?
- Work is force multiplet by the distance moved in the direction of the force
- Since the force is perpendicular to the motion, no work is done by the magnetic on the charge, so the kinetic energy of the charge does not change
How does circular motion relate to the charged particle?
- FLHR says that the force on a moving charge in a magnetic filed is always perpendicular to its direction of travel
- This condition is circular motion
- To use FLHR for charged particles, use second finger (normally current) as the direction of motion for a POSITIVE CHARGE
- If the particle carries a negative charge (e.g. an electron) point your second finger int he opposite direction to its motion
- The force due to the magnetic field (F=BQv) experienced by a particle travelling through a magnetic filed is independent of the particles mass, but the centripetal acceleration it experience will depend on the mass (NL2)
Describe circular motion and the charged particle
-Since the particle follows a circular path in a magnetic filed, we know it experiences a centripetal force
-For circular motion the centripetal force must equal the force exerted by the magnetic field:
F = mv^2 / r = BQv so mv/r = BQ
How is the radius affected for the charged particle?
- When B and Q are constant mv=BQr, the radius is proportional to the momentum of the particle
- Using r=mv/BQ
1. The radius increases (i.e. the particle is deflected less) if the mass or velocity of the particle increases
2. The radius decreases (i.e. the particle is deflected more) if the strength of the magnetic filed or the charge on the particle increases
What is a cyclotron used for?
- A cyclotron is used to force charged particles into a circular path that accelerated them at very high speeds
- Cyclotrons are often used with heavier particles like alpha particles and protons
- Experiments using particle accelerators investigate the structures of complex molecules like proteins as well as sub-nuclear structures
What is a cyclotron?
A particle accelerator that accelerates charged particles through a spiral path using a fixed magnetic filed and an alternating potential difference
Describe the structure of a cyclotron
- The cyclotron is formed from two semi-circular ‘dees’ separated by a small gap and connected to a high-frequency alternating potential difference
- A strong magnetic filed is applied perpendicular to the dees
- The perpendicular magnetic field forces charged particles to move in a circular path inside the dees
What happens the particles in the cyclotron?
- The particles experience a potential difference when they travel across the gaps and gain energy equal to QV (where Q is the particle’s charge and V is the potential energy in volts)
- Since the particle shave more kinetic energy then move faster and accelerate to the next dee
How is the ac voltage in the cyclotron controlled?
- The ac voltage is timed to change direction every time the particles reach the gap between the dees
- It must alternate to accelerate the particles each time they reach a gap
What would happen if the voltage did not alternate in a cyclotron?
The particles would follow a cycle of accelerate-decelerate-accelerate
How much time do the particles spend in the dee?
Particles spend the same time in each dee, but the radius of their path increases after each gap and they travel further in the same time
What does the centripetal force acting on the charged particle equal?
The magnetic force on the particle so mv^2/r = BQv or v=BQr/m
What happens to the particles as they accelerate through the cyclotron and why?
Because the radius is proportional to the speed of the charged particle, the particles spiral outwards as they accelerate through the cyclotron
How do you work out the time spent in each dee? (1)
t = Pir/v
because air is half the ccirumpference of the circle
What is another way to work out the time spent in each dee?
t=pir/ BQr/m
t=pim/BQ
-Which does not depend on the either the radius or the speed
What limits a particles speed in a cyclotron?
The effect of special relativity
How does special relativity limit a particles speed in a cyclotron?
- Particles get more massive as they travel close to the speed of light
- As particles move faster and they mass increases, the time spent in each dee increases, and the more massive particles get out of step with the alternating potential difference
What is a synchrotron?
A particle accelerator that accelerates charged particles through a circular path using a varying magnetic field
How does a synchrotron overcome the effects of special relativity?
- A synchrotron overcomes the problem as described above by increasing the magnetic field as the speed of the particles increases
- The radius of their path remains constant even though the particles travel faster
How is circular deflection used?
- Circular deflection is used in particle accelerators such as cyclotrons
What are the uses of cyclotrons?
- Cyclotrons have many sues for example in medicine
- Cyclotrons are used to produce radioactive tracer or high-energy beams of radiation for use in radiotherapy
What is the basic structure of a cyclotron?
A cyclotron is made up of two hollow semicircular electrons with a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes
How does a cyclotron work?
- Charged particles are fired unto one of the electrodes. The magnetic field makes them follow a (semi)circular path and then leave the electrode
- An applied potential difference between the electrodes accelerates the particles across the gap until they enter the next electrode
- Because the particle’s speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again
- The potential difference is reversed so the particle is accelerated again before entering the next electrode. This process repeats as the particle spirals outwards increasing in speed, before eventually exiting the cyclotron
