Electromagnetism: Fields Flashcards
What is an electric field?
The region that surrounds electrically charged particles where a force is exerted on other electrically charged particles.
What is electric field strength?
The electrical force acting on unit positive charge.
Sketch an electric field pattern in a single positive point charge.
Hint: Charge = outwards
Sketch an electric field pattern in a single negative point charge.
Hint: Charge = inwards
Sketch an electric field pattern in two oppositely charged parallel plates
Hint: Parallel, charge towards negative.
Sketch an electric field pattern in a positive and negative point charge.
Hint: side to side, energy towards negative.
Sketch an electric field pattern in two identical point charges.
Hint: side to side, charges curving away from each other
What is electrical potential at a point?
The work done in moving unit positive charge from infinity to that point.
What does any charge given to a conductor always reside on?
The outer surface of the conductor
What is a consequence of charge given to a conductor residing on the outer surface?
The electric field inside a conductor must be zero.
What is electrostatic shielding?
When devices that might be affected by an electric field are enclosed in a conductive material.
What is an example of where electrostatic shielding is used?
A Faraday Cage.
Sketch a graph of the electric field around a charged sphere or radius r
Hint: straight diagonal line at 0, then vertical line upwards, then long curve down
Sketch a graph of the electrostatic potential around a charged conducting sphere of radius r
Hint: Straight line across from top of y axis, then short curve downwards.
Summary of Millikan’s Oil Drop Experiment
- Atomiser used to produce spray of tiny oil droplets
- Pulled downwards through a small hole by gravity
- Observing terminal velocity to calculate mass
- Droplets fell between two plates where electric field was applied
- Varied voltage in order to balance downwards gravitational force with upwards electrical force.
What is the electronvolt (eV)?
A unit of energy, often used in high-energy particle physics.
What is 1eV equal to?
1.6 x 10^-19 J (energy of an electron)
What are ferromagnetic materials?
Ones which can become permanently magnetised.
What are some examples of ferromagnetic materials?
Iron, steel, cobalt, nickel
Magnetic field patterns in an isolated bar magnet
Travelling outwards from north, into south
Magnetic field patterns in two opposite poles adjacent.
Travelling outwards from north, into south
Magnetic field patterns in two like poles adjacent
Two south ends curving away from each other, north still travelling into south
What does the relative spacing of the field lines in a magnet indicate?
The field’s strength. Closer lines = stronger magnetic field
What does the direction of the magnetic field around a current carrying wire depend on?
The direction of current flow.
Sketch a magnetic field around a solenoid
Hint: shape of magnetic field same as that created around a bar magnet.
sketch a magnetic field around the earth
Hint: geographic north pole = magnetic south pole
What is magnetic induction, B?
The strength of a magnetic field at a point.
Right hand rule for a negative charge:
Thumb - direction of motion (force)
First finger - direction of magnetic field (north to south)
Second finger - direction of current flow (i.e electron flow)
Summarise charge moving perpendicular to a Magnetic field
- force of magnetic field on charged particle is at right angle to motion.
- Direction of motion is changed and leads to particle moving in circular path
- Force always acts towards centre of curvature and causes centripetal acceleration
Derive r = mv/qB
- F = mv^2/r, F = qvB
- qvB = mv^2/r
- qB = mv / r
- r = mv/qB
Summarise charge moving at an angle to a magnetic field
- Charge will follow helical path moving forward
- two components of the particle’s velocity; perpendicular to magnetic field (vsin0) and parallel to field (vcos0)
What is the job of the perpendicular component of the particle’s velocity
Creating the circular motion
What is the job of the parallel component of the particle’s velocity
Creating the pitch in the helilx.
What is the pitch of the helix in a magnetic field?
The distance between adjacent loops in the helix after one period.
What equation is used to find the pitch in a helix?
p = vcos0 T
What equation is used to find the frequency of the rotation in a helix?
f = qB / 2(pi)m
What equation is used to find the period of rotation in a helix?
T = 2(pi)m / qB
Summarise a bubble chamber
- Charged particle leaves track of tiny bubbles due to ionisation
- Tracks spiral inwards due to loss of energy and velocity
- particles with opposite charges (eg electron and positron) spiral in opposite directions.
What is the force mediating particle of a strong nuclear force?
Gluon
What is the force mediating particle of a weak nuclear force?
W(+-) & Z bosons
What is the force mediating particle of an electromagnetic force?
Photon
What is the force mediating particle of a gravitational force?
Graviton
What is the equation used to calculate an absolute uncertainty
(sqrt) (uncertainty X)^2 + (uncertainty Y)^2 + (uncertainty X=Z)^2
At what degrees dose maximum force occur in a current?
90 degrees (perpendicular to magnetic field)
At what degrees does zero force occur in a current?
0 degrees (parallel with magnetic field)
What is the equation used to find time taken in a semicircular path?
t = sv, s = (pi) r
t = (pi) r / v = (pi) / v, mv/ qB
:. t = (pi) m / qB
