Chapter 16 Flashcards
Electric charge
There is an electric field around a charged object:
-Electric charge in measured in (C) Coulombs and is either positive or negative
Opposite charges attract and…
like charges repel
If a charged object is placed in an electric field then it…
will experience a force.
Electron guns make charged particles move more rapidly by…
sending charged particles down a potential energy ‘hill’, the higher the hill the more kinetic energy is gained. The potential energy hill is created by putting a p.d across two electrodes, positive charges then accelerate towards the negative electrode and visa versa. When going from one electrode to another, a charge loses potential energy and gains kinetic energy.
Loss of electrical potential energy=…
gain of kinetic energy= mgh
The equation for the speed of a particle
v= √(2qV)/m
where qV = 1/2mv^2
Equation to calculate the force of attraction or repulsion between two point charges…
F= (K.Q1.Q2)/(R^2)
Where k = 1/ 4πε
The force on Q1 is always…
equal and opposite force on Q2
The further apart the chares are…
The weaker the force between them
Electric field strength
E= force per unit positive charge, the force that a charge of +1C would experience when placed in the electric field.
all particles with the same charge gain the same kinetic energy when they move though the same potential difference, but…
Some may move faster than others depending on their size
e.g. a electron moves much faster then a proton because its mass is much smaller
Equation for electric field strength
E= F/q
Where E = electric field strength (NC^-1), F= force (N) and q= charge (C)
Gravitational field strength
g= F/m
where g= gravitational field strength (NKg^-1), F= force (N) and m = mass (kg)
Radial field
-In a radial field, E depends on the distance r from the point charge Q.
Equation for E in a radial field
E = kQ/ r^2
E is inversely proportional to r^2
The further you go away from Q…
The field strength decreases and the field lines get further apart.
Electrical potential energy
-The work that would need to be done to move a small chare, q, from infinity to a distance r away from a point charge Q.
Equation for energy in an electric field
E= KQq/ r
Repulsive force field
Graph: E decreases as r increases
-In a repulsive force field ( positive charge, lines away from Q)- you have to do work against the repulsion to bring q closer to Q. The charge q gains potential energy as r decreases.
Attractive force field
Graph: E increases negatively as r increases positively
- In an attractive field, the charge q gains potential energy as r increases.
- Also gradient of a tangent gives he electric force at that point.
Electric potential..
V= electric potential energy per unit positive charge
Equation for electric potential energy
V= E(electric)/ Q and substituting for E gives V = KQ/r
V is measured in…
Volts or joules per coulomb
As with E, V is..
Positive when the force is repulsive and negative when the force if attractive (same graphs)
-Also the gradient f a tangent gives the field strength
Field strength is the same everywhere in…
A uniform field
Uniform field can be created using…
Two parallel plates to the opposite poles of a battery
Between these plates
The field strength is the same at all point
Equation for field strength in a uniform field
E= v/d where v= the p.d and d= the distance between the plates
E can be measured in …
Vm^-1
The field lines are
parallel to each other
The surfaces are
Equipotential and are parallel to the plates, and at 90 degrees to the field lines.
Principle of linear acceleration
-The negative electron will accelerate to the positive electrode
Switching P.D to keep accelerating electrons
- The first group of negative electrons will accelerate towards the positive electrode.
- The p.d is then alternated so once they pass the positive electrode, it is now negative, so the electron accelerate to the positive electrode in-front of it.
- The alternating p.d switches back and forth so that the electrons are accelerated as they pass between successive electrodes.
If the electron is accelerating, gaining potential energy, the change in the potential Δ V and the change in potential energy qΔV must both be…
Negative!
Change in kinetic energy = -change in potential energy
The change in the kinetic energy is produced by a force, F….
change in energy = FΔx = -qΔV
Force on the particle =
F= - qΔV/ Δx
Force = - potential energy gradient
Therefore Force on particle can be arranged to…
E= F/ Q => - ΔV/ Δx
electric field strength = -potential gradient
To find the uniform electric field between a pair of conducting charged plates…
- Put a voltmeter across to measure the p.d (V)
- Measure the distance between plates (d)
- The field is uniform, so the equipotentials are equally spaced and the gradient is the same right across the gap.
=» E = V / d
Units = Vm ^-1
Similar units…
NC^-1 = Vm^-1
Nkg^-1 = ms^-2
Field lines are always … to eqipotential surfaces
Perpendicular
Drawing field lines
- Always start and end on charges
- Cannot cross each other
- Always perpendicular to equipotentials
- Point from higher potential (more positive) to lower potential
- Close together in strong fields, far apart in weak fields
Millikans experiement
- Used an atomiser to fire a mist of ionised oil drops that are charged by friction s they leave the atomiser
- Some fell through the hole and were viewed in the microscope
- He then applied p.d across the plates to produce a uniform field that exerts a force on the charged drops. By adjusting the p.d he varied the strength of the field.
- He applied enough p.d until the drops were stationary so when Upwards force = Downwards force
- The voltage at which the oil was stationary was measured, the charge was then calculated
When the oil drop is held stationary
F= qE and W= mg
=> qE = mg
=> E= V/d
=> V= mgd/q
=> V = mgd/ ne
(If the charges are multiples n of electron charge e, the nq= ne)
Conclusion to Millikans experiment
- Charge exists in discrete ‘packet’ size 1.6 x 10 ^-19 C.
- He found that the charges were all multiples of 1.6 x 10^-19, thus showing that each drop was made up of smaller charges with a charge of 1.6 x 10^-19 (electrons)
Charged particle are affected by magnetic fields and so a current-carrying wire…
Experiences a force in a magnetic field
Equation for the force exerted on a wire in a magnetic field perpendicular to the field
F = BIL
A charged particle moves a distance l in t…
Velocity= L/ t
and I = q/t
so t = L/ v
I= qv/ L goes in F = BIL so …
F= qvB
By using flemmings left hand rule…
The force is always perpendicular to the magnetic field
Centripetal force
The centripetal force and the electromagnetic force are equivalent for a charged particle along a circular path
If F = mv^2 / r where f=qvB then…
qvB= mv^2 / r
So r = mv/ Bq
When a electron beams are deflected …
There will be a positive plate above or below the deflection path to make the electron no stay in a straight line
If the electron beam is staying stationary with a positive plate above and a negative plate below, when the electric field is turned on, what is happening?
The charge is must be negative as It wants to travel up as it is attracted to the positive plate, however the force of gravity is pulling I down so it remains stationary
The momentum of a particle is proportional to…
The radius of the curvature of the path
Therefore p = qrB
where p = momentum
Disadvantage to linear accelerators and advantage to circular accelerators
D- They have to be extremely long in order for the particle to reach a high energy
A- Particles with the same mass but different charges can be accelerated in opposite directions
The smaller the rings are in circular accelerators…
-the larger the centripetal acceleration a so it takes away energy from the particles
Charged spheres
-Small particles behave like small charged spheres
The electrical field of a sphere obeys the inverse square law
E = k q / r^2
The electric field strength near an isolated charged sphere obeys an inverse square law
E ∝ 1/ r^2
The field strength must also depend on the amount of charge therefore…
E ∝ q/ r^2
FIELD STRENGTH
E = -K Q / r^2
FORCE
Field strength x q
E = -K Q q/ r^2
ENERGY
Force integrated
We = K Q q/ r
POTENIAL
Energy ÷ q
Ve = K Q/ r
