Download 12.1 Electricity force, field and potential

12.1 Electricity force, field and potential
[edit] 12.1.1
Coulomb's Law : F = Q1Q2 / 4 x π x Eo x r2. Q is in coulombs, r in meters. The force is
dependent on the two charges and the square of the distance between them. Opposite
charges attract and like charges repel. Vector addition of these forces may be necessary in
some problems, but shouldn't present any major difficulties. The force on each of the two
charges in question is opposite in direction, but equal in magnitude.
[edit] 12.1.2
Electric field strength is equal to Force/charge. To find the field strength at a given point, put
a point charge of 1 coulomb at the point, and then calculate the force on it. From there
you can find the field strength and direction by vector addition.
[edit] 12.1.3
Electrostatic potential : The work done (in joules) to move a charge from an infinite
distance to a given point in the field, divided by the charge (in coulombs). In equation
form, V = W/q. The SI unit is volts (where a volt is a joule per coulomb). This value is a
scalar, so it doesn't matter what path you take between two points.
In a radial filed, V = 1/4 x π x Eo x q/r. Note, this equation only applies as long as r is outside
the conductor producing the field. If it's inside, then V = 0. In this case, q is the charge on
the point creating the field. Both of these equations are in the data book.
[edit] 12.1.4
1 electron volt is defined as the work done to move an electron through a potential
difference of 1 volt. This can be plugged into the above equation, to show that 1eV = 1.6
x 10-19J.
[edit] 12.1.5
The potential at a point in a field around a point charge or a uniformly charged sphere can
be found with the above equation (V = 1/4 x π x Eo x q/r).
For a collection of point charges, the fact that electric potential is a scalar greatly
simplifies the problem, because direction need not be considered. Simply add up all the
different potentials from each point charge to find the total.
Inside a hollow sphere, the value of V will be zero until you're outside the sphere, at
which point it jumps to a max, then falls away to zero at infinity.
[edit] 12.1.6
For parallel plates, the equipotential lines run parallel between the plates, and then
diverge out from either end.
For point charges, two opposite charges create circles of equipotential around them, but
squashed in on the side closes to the other point. Eventually there will be a line running
straight between them.
For like charges (maybe not necessary), the equipotential lines produce a sort of figure 8
with the center cut out, and each point charge in one of the 'holes' there are a bunch of
these figure 8 shapes radiating out.
Between parallel plates, equipotential lines can be related to electric field strengths,
because they evenly divide up the space between the plates, as does the field strength.
Also, equipotential lines will always cross filed lines at 90o, so that should help find
exactly where they go.
[edit] 12.2 Magnetic fields
[edit] 12.2.1
To find the force on a moving charge, or an electric current in a magnetic field :
Moving charge : Using the right-hand (palm) rule, find the direction of the force
(remember we're talking about conventional current, so it goes forward for positive
charges, backward for negative). Find the acute angle between the direction of the
magnetic field at that point, and the direction in which the particle is traveling. Substitute
into F = qVBsinØ to find the magnitude of the force (Ø is the angle, B is the field
strength, V is the velocity and q is the charge).
Current carrying wire : Find the direction of current flow in the wire, and thus the
direction of the force. Find the angle between the field lines and current flow. Substitute
into F = IlBsinØ to find the magnitude.
Note that when Ø = 0o (i.e. when the motion is parallel to the field) there will be no force.
Both of these equations are in the data book.
[edit] 12.2.2
Galvanometer : A permanent magnet is set up around a loop of wire. This wire is
allowed to rotate on a axis, but has a spring attached to always pull it pack to parallel
when there is no force. When a current is passed through the loop, this causes a force on
the loop, and it rotates. This pivoting moves a marker attached to the axis, and so shows
the current which is flowing through, since the grater the current, the further it will turn
before the force is equalized by the spring.
Loud speaker : A loud speaker consists of a metal coil attached to the stiffened
cardboard of the speaker, with a permanent magnet surrounding the coil. When there is
current running through the coil, this causes forces back and forth (as appropriate) on the
coil, and thus it vibrates the cardboard. With the right alternating current, different
frequency vibrations can be produced, thus producing sound.
Electromagnetic relay : The object of a relay is to complete a circuit when a current is
passed through an electromagnet. This is done with a coil of wire which, when current is
passed through it, pulls a piece of metal near it towards it. Since this metal is on a pivot, it
forces a wire attached to it above up, and onto the other wire, thus completing the circuit.
When the current in the coil is released, the spring action disconnects the circuit.
[edit] 12.2.3
The magnetic field around a current carrying wire is defined by B = µo/2 x π x I/r. I is the
current through the wire, and r is the distance away from the wire where the field is being
measured.
For a solenoid at least 10 time as long as it is wide, the field inside is constant, and
defined by B = µo x NI/l, where N is the number of loops of wire, l is the length, I is the
current and µo is a constant. The field strength is equal anywhere inside the coil as
mentioned above.
Both these equations are given in the data book.
[edit] 12.2.4
The force between two parallel conductors is defined by F = µo/2π x I1 x I2 x l/r. Where r is
the distance between them, I1 and I2 are the currents in each and l is the length of both
wires. Force per meter can be found simply by substituting in 1. As can be seen, if the
currents are in opposite directions, the force will be negative, or away from the other
conductor.
The ampere is defined based on this (the current which produces a force of 2 x 10-7N
between two wires 1 meter apart), since it is easy to accurately vary the current in a wire.
The coulomb is then defined from this (charge in coulombs = current x time).
[edit] 12.3 Electromagnetic induction
[edit] 12.3.1
Magnetic flux is defined as Φ = BA (field strength x area). If the field is not
perpendicular to the area in question, however, Φ = BAcosØ, where Ø is the acute angle
between the field direction and the normal to the area. Flux linkage is basically the
change in NØ, where N is the number of turns of wire, and Ø is the flux.
[edit] 12.3.2
Neumann's equation is E = -N x ΔØ/Δt (the induced emf is equal to the number of loops x
the change in flux over time).
Lenz's law says that an induced emf will always produce a current who's magnetic field
opposes the original change in flux. Farraday's law says that the induced emf is
proportional to the flux cut divided by the time taken.
[edit] 12.3.3
emf = Blv (a wire of length l moving perpendicularly through a field of strength B at a
velocity v).
This can be derived from F = ΔØ/Δt as follows :
F=
B x ΔA/Δt =
B x l x v x Δt/Δt =
Blv.
Or from F=IlB as follows :
(charge Q)(Voltage) = Work done
V = Work Done / Q = Force x distance / Q = IlB x s / Q = Q/tlBs / Q
The Qs cancel out and we're left with V = lB s / t = Blv.
[edit] 12.3.4
(I assume what we're talking about here is a loop of wire rotating in a magnetic field.)
We take a coil with N turns of wire, and an area A rotating in a field of strength B at an
angular velocity of w. The flux linkage at any given point is BAN x sin wt , where t is the
time, as it rotates, starting from to when the entire coil is parallel to the field, and so
produces no emf.
We then play with some calculus (substitute it into Neumann's equation, N being already
accounted for, and do it like a calculus equation) to get emf = BANw cos wt. This
equation is not in the data book, so it might be worth remembering.
[edit] 12.3.5
An AC generator is, just like above, a coil being forced to rotate in a magnetic field. This
produces an alternating current because each half turn, the effective orientation of the coil
is reversed (the side that was going left is going right), and so the current is also reversed.
The two ends of the loop are connected to slip rings which are allowed to turn, and
brushes rubbing on them, and running the alternating current out to the rest of the circuit.
[edit] 12.3.6
Average power consumption = IrmsVrms and Irms = Io/√2 (same for Vrms). This equation is in
the data book, and can be derived as follows.
P = IoVo x sin2wt, thus Pav = 1/2 IoVo and so the above rms bits can be found. Vrms = Vo/√2.
[edit] 12.4 The Cathode ray oscilloscope (CRO)
[edit] 12.4.1
A cathode ray oscilloscope (CRO) is a tool for measuring variations in current from a
source. The CRO provides a continual graph of the current over time on the screen. It's
difficult to describe how exactly to use one, but hopefully you'll have to have tried it.
At the back of the CRO there is an electron 'gun' where a beam of electrons are produced
by a large potential difference between an anode and cathode (the 'ray' comes off the
cathode, making it a cathode ray). It then passes through two sets of perpendicular
deflectors. The horizontal one is controlled by the settings of the CRO, and makes the
light trace from left to right (and then jump back to the start). The vertical one is
indirectly controlled by the source current, and produces the up and down sine type
curves.
The difference between this and a TV tube is that a TV strikes every pixel with the
cathode ray once per frame (and may have multiple pixels with different colours) The
CRO, however, strikes in a sine curve through each pass. Other than that, they both work
on the same principle.
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