# Electric and Magnetic Fields

• Created by: dkoning00
• Created on: 02-04-18 11:38
Define Electric Field Strength at a point
Force per Coulomb at that point in the field, that would act on a small, positive charge
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Why are electric field strengths produced by negative charges always negative?
Because they attract positive charges to that point with a force
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State Coulombs Law
The force between two point charges is proportional to the product of their charges and inversely proportional the the square of their distance of separation
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What are Electric Field lines?
Arrowed lines showing the direction in which a free positive charge will move in a field
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Define a Uniform Electric Field
An E-Field between two charged parallel plates where electric field strength has the same value at all points in the field
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How is EFS of a Uniform Electric Field found?
E = V/d
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How are E-Field lines drawn for a Uniform E-Field?
Equidistant and parallel to each other, pointing from the positive plate to the negative
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The electric field created by a point charge acting in all directions
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How is EFS of a Radial field found?
E = kQ/r^2
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What is the effect on field strength on a charge when doubling the distance from the point source?
EFS falls to one quarter
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How are E-Field lines drawn for a Radial E-Field?
Directly away from or towards the point charge (depending on its sign) with equal angular separation
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Define Electric Potential
The amount of electric potential energy that a 1C charge would hold at a point in the field
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Define Capacitance
The charge required to raise the voltage across a capacitor by one volt
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How can it be shown that a discharge curve of a capacitor is an exponential decay?
Falls by equal fractions in equal time
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What is the time constant of a given capacitor?
The time taken for a discharging capacitor's p.d/Q/I to fall to 1/e of its original value
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How would the time constant be found from a Q-t graph?
Use natural logs to plot ln(Q) = (-1/RC)t + Qo, RC can be found form the gradient
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Define a Magnetic Field
A magnetic field is a region of space where a moving electrical charge feels a force
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What are the units of Magnetic Field Strength (B)?
Tesla
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What is Magnetic Flux and its units?
Product of B and perpendicular Area, Weber (Wb)
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When a magnetic field links N coils of wire, the flux linkage = N(phi) for N turns
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What are Magnetic Field Lines around a source of magnetic flux?
Lines showing the direction in which a free North pole would move in the field
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How do you calculate the force on a current carrying wire in a magnetic field?
F = BILsinθ
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Why will a charged particle moving in a magnetic field travel with circular motion?
Because magnetic force always acts perpendicularly current, a moving particle produces a current flow so there is a centripetal force
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In Flemming's Left-Hand Rule, what does each digit dennote?
Thumb - Magnetic Force, Index finger - B field N-->S, Middle finger - Current
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The emf induced in a conductor is proportional to the rate of change of flux linkage
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State Lenz's Law
Any current driven by an induced emf opposes the change in flux linkage that caused it
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Give the resulting equation of the two laws
ε = -d(NΦ)/dt
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Describe an experiment to study the force acting on a current-carrying wire in a B field
A wire clamped in a magnetic field on a top pan balance will experience changing force with changing current as can be seen on the balance reading
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Describe an experiment to show that changing flux linkage induces an emf in a wire
Connect a coil of wire to a sensitive voltmeter and move a magnet in and out of the coil
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How does electromagnetic damping work?
A conductor moving through a magnetic field, it has induced across it an emf in the oppostire direction to motion, thus creating current. The movement of electrons creates an opposing magnetic field and therefore force which opposes motion
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Why can electromagnetic damping never bring an object to rest
For there to be current produced in the conductor, there must be a rate of change of flux linkage meaning the object must be moving
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What are Root Mean Square p.d. and current vlaues?
The effective average value of p.d. or current for an AC supply
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How is Vrms Found?
Vmax/root 2
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How does the Right-hand Grip Rule work?
Thumb shows current flow, fingers grip around wire to show shape and direction of B field
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How do you show current into and out of a page
Into page - CIrcle with a cross in it, Out of page - cirlce with a dot in it
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## Other cards in this set

### Card 2

#### Front

Why are electric field strengths produced by negative charges always negative?

#### Back

Because they attract positive charges to that point with a force

### Card 3

#### Front

State Coulombs Law

### Card 4

#### Front

What are Electric Field lines?

### Card 5

#### Front

Define a Uniform Electric Field