Electric and Magnetic fields Flashcards
which way do magnetic field lines point?
North to south
definition of a field
a field is a region of space in which an object experiences a force. Action-at-a-distance forces are sometimes referred to as field forces
Gravitational field definition
a gravitational field is a region of space in which a mass will experience a force
Magnetic field definition
a magnetic field is a region of space in which a magnetic material will experience a force
electrical field definition
is a region of space in which charged particles will experience a force from another charged object (only requires one charged particle)
How to show the magnitude of a force
close together = strong
far apart = weak
parallel and equally spaced = uniform
Direction of lines on an electric field
The direction a positive test charge goes to positive to negative
law of field lines
they can not cross
How to draw a radial field
The line has to be at 90 degrees to the point its coming out of
How to draw a uniform field
. lines leave surface at right angles
. direction of field is that a positive test charge would travel
. parallel and equidistance
what is the edge effect
where the lines curve at the edges of the paralell plates
electric field strength equation (E)
E= Force/Q(charge)
electric field strength definition
E is defined as the force per unit positive test charge (q)
it is a vector
whats Es units
NC^-1
what does force produce
an acceleration (newtons second law).
uniform electric field strength equation
E = V(pd)/ seperation of plates(d)
NC^-1 = Vm^-1
acceleration equation relating E and accelertation
a = EQ/m
can use suvat
Electric potential energy equation
E (potential energy) = V(pd) x Q
definition of an electric field
an electric field is a region around a charge where another charge would feel force
what is a line of equipotential
a line along which the electric potential is the same, a charge can move along the line without work being done on by it due to the field
what does this mean FA= EBxQA
the force charge A feels FA is equal to the charge of A (QA) multiplied by the electric field it is in caused by charge B (EB)
Write Fa=EbQa as newtons 3rd law
if Fa = EbQa then the force on B must be: Fb = EaQb but because the forces on A and B are equal.
we can say Fa = Fb = F
so F = EbQa =EaQb
that means that the force between the charged must depend on both charges so
F= CQaQb
where c is some constant
what is coulombs law
F = kQaQb
————
r^2
r is the separation in m
k is coulombs constant
radial electric field strength equation
E = KQ
—–
r^2
Electric potential at any point in a radial electric field
V = kQ
—–
r
radial electric field strength using Area
E = Q / 4xpixE0xr^2
coulombs law using area
F = QaQb
————–
4xpixE0xr^2
what are capacitors
a capacitor is a component that stores charge. it can discharge in the form of an electric current at a time convenient to the user.
How to charge a capacitor
- a capacitor is connected across a pd
- the negative terminal pushes electrons onto one plate (top). This repels the electrons from the opposite plate (bottom), leaving it positively charged.
-When the pd across the capacitor equals the supply pd the current stops flowing and the capacitor is fully charged
how do you make a capacitor store more charge
increase the area of the conducting plates
what is the capacitance equation and defintion
-the capacitance, C, of a capacitor is defined as the amount of charge, Q, stored per Volt, V.
-it is scalar
C=Q/V
it has units of Farads F = CV^-1
Capacitors in series
its the opposite rules for connecting resistors
1/ C total = 1/C1 + 1/C2 etc.
capacitors in parrallel
C total = C1 + C2
Both ways to work out time constant
t = RC (resistance x Capacitance) or current, pd, charge drops 37 Percent of the y axis
what is the equation for capacitor discharge and what quantities can it be applied to?
Pd/I/Q = Pd0/I0/Q0 e ^ -t/RC
pd ,current and charge
what is the motor effect
-if a current carrying wire is placed in a region of magnetic field, it will experience a magnetic force
-the magnitude of force is maximised when the current is perpindicular to the magnetic field
what is the force on the wire propotional to
- the current on the wire
- the length of the conductor in the field L
- The sine of the angle that the conductor makes with the field (theta)
- The strength of the field (magnetic flux density) B
the equation for force on current in magnetic field
F= BxIxL sin Theta
B = magnetic flux
what is flemings left hand rule
thumb is force
index finger is magnetic field
current is the middle finger
what do dots and crosses mean
dots = out of the page
crosses = into the page
what is electromagnwtic induction
Electromagnetic induction is the process by which an electromotive force (EMF) is generated in a conductor when it experiences a change in magnetic flux linkage. It occurs due to Faraday’s Law of electromagnetic induction, which states that the induced EMF is proportional to the rate of change of magnetic flux linkage.
what is faradays and lenzs law
Faraday’s Law: The magnitude of the induced EMF (
𝐸
E) is equal to the rate of change of magnetic flux linkage (Φ) through a circuit:
𝐸= −𝑑Φ/𝑑𝑡
The negative sign indicates that the direction of the induced EMF opposes the change in flux, as explained by Lenz’s Law.
Lenz’s Law: The direction of the induced current (and EMF) is such that it opposes the change in magnetic flux that caused it.
This is a consequence of the law of conservation of energy, ensuring that the induced EMF resists the motion or change creating it.
how is emf induced
motion of a magnetic field relative to a coil of wire
defnition of flux linkage
total number of field lines through a surface/plane x number of turns on a coil. it is also expressed as the linking of a magnetic field to a conductor
how does the number of turns on a solenoid affect the strength of em created
more turns = stronger
Describe the structure of a transformer and how it operates
A transformer consists of two coils, the primary and secondary, wound around a soft iron core. An alternating current in the primary coil creates a changing magnetic field, which induces an alternating voltage in the secondary coil via electromagnetic induction. The core ensures efficient magnetic flux linkage between the coils.
What is the ideal transformer equation, and what does it assume?
The ideal transformer equation is:
𝑉𝑝/𝑉𝑠=𝑁𝑝/𝑁𝑠
This assumes 100% efficiency, meaning no energy losses due to resistance, eddy currents, or hysteresis in the core.
What are eddy currents, and how are they minimized in transformers?
Eddy currents are circular currents induced in the iron core due to the changing magnetic flux. They waste energy as heat. To minimize them, the core is laminated, dividing it into thin insulated layers to reduce current paths.
Why are transformers used in the national grid, and how do they improve efficiency?
Transformers are used to step up voltage for transmission and step it down for distribution. High voltage reduces current for a given power, minimizing energy losses due to the resistance of transmission lines
P=IR^2
What are the main sources of energy loss in a transformer, and how are they addressed?
A:
Resistance in coils: Minimized by using low-resistance materials like copper.
Eddy currents: Reduced by laminating the core.
Hysteresis losses: Reduced by using soft iron, which requires less energy for magnetization cycles.
Magnetic flux leakage: Minimized by designing the core to ensure close coupling of the coils.
