The Electric Field Flashcards
What shape do electric charges move in
a) uniform electric fields?
b) uniform magnetic fields?
a) move in parabolas
b) move in circles (force acts perpendicular to velocity)
Electric field definition
An electric field is a region where an electric charge experienced an electric force
How do you draw a strong uniform magnetic field?
With field lines closer together
How do the forces vary on 3 electrons placed in a uniform electric field at different heights?
The force is equal on all 3, because it is a uniform field
In what direction are field lines drawn?
From positive to negative
What is a test charge?
A small charge that doesn’t affect the field it is in (a normal charge would alter the field)
In what direction/position are equipotential surfaces compared to field lines?
Equipotential surfaces are perpendicular to field lines
When are electric charges affected by magnetic fields?
When they are moving. They feel the force at right angles
Equation for
a) total force in a magnetic field?
b) force on a charge in a magnetic field?
a) F total = ILB
b) F charge = qvB
Why do electrons travel in circles in magnetic fields?
The electron experienced a force perpendicular to its velocity
Equation for centripetal motion force?
F(c.m.) = mv^2 / m
Why don’t positive ions in a lattice feel a force like electrons do?
Because they aren’t moving - v=0, so F=Bqv=0
How are charges distributed on a
a) isolated metal sphere?
b) plastic/polystyrene sphere?
a) evenly distributed
b) charges stay closest to where they are put on the sphere
What happens when a charge (of equal charge to sphere) is brought close to a NOT isolated metal sphere?
The equal charge repels the charges on the sphere to the back - distribution becomes uneven
In what type of sphere is there no
a) electric force
b) gravitational force
felt?
a) in a hollow charged sphere
b) in a hollow massive sphere
What is electric potential?
The work done in bringing a unit positive charge from infinity to that point
What do particle accelerators ‘do’?
Use EM fields to propel charges particles to very high speeds, producing very high energies
2 uses of particle accelerators?
Particle therapy in oncology
Computer chip manufacturing
What do the electric fields ‘do’ in particle accelerators?
The fields switch between +ve and -ve to create radio waves that accelerate particles in bunches
Why are circular accelerators used?
So protons can be accelerated round many times to build up speed
What kind of magnetic and electric field are used in a cyclotron?
- A constant magnetic field
- An electric field of constant frequency and high alternating voltage
What kind of magnetic and electric field are used in a synchrotron?
- A varying magnetic field
- A varying electric field
Why do particles move in a spiral in a cyclotron rather than a circle?
The frequency of the voltage is set so the particles’ speed isn’t constant, so they move in a spiral
What is emitted when electrons are pumped really fast into a synchrotron?
Waves of high frequency are emitted
Because E=hf, this means lots of energy is lost too
3 different equations for electric field strength
E = V/d E = F/q E = kQq/r^2
Equation for force involving current and flux density
F = ILB
Equation for potential
V = -kQ / r
