07 - electric and magnetic fields Flashcards
What is a force field?
A region in which a non-contact force is experienced by an corresponding interacting particle.
What is an electric field?
A region in which a charged particle will experience a non-contact force.
What is electric field strength?
The force per unit charge experienced by a charged particle when placed at that point in the field.
State the equation used to calculate the force a charge experiences in an electric field.
Force = Electric Field Strength x Charge
F = EQ
What law determines the magnitude of the electric force between two charges?
Coulomb’s Law
State Coulomb’s law in words.
The force between two charges is directly proportional to the product of their charges and inversely proportional to the square of their separation.
Give Coulomb’s law in equation form.
F = KQaQb /r^2
where k = coulombs constant
Describe the electric field pattern around a positive point charge.
A radial field, acting outwards.
Describe the electric field pattern around a negative point charge.
A radial field, acting inwards.
What equation is used to calculate the electric field between parallel plates?
E = V/d
What is electric potential?
The amount of energy required to move a positive test charge from infinity to a given point in an electric field.
How do you calculate electric potential for a radial field?
F = kQ / R
where k is coulombs constant
What are equipotentials?
Equipotentials are lines along which the electric potential remains the same and is perpendicular to the directions of the electric field
How do you calculate the work done in moving a charge along an equipotential?
No work is done when moving a charge along an equipotential since the electric potential doesn’t change.
What does a capacitor do?
Capacitors are used to store charge in a circuit.
What two factors determine how much charge can be stored by a capacitor?
- The potential difference across it
- The capacitance
What is capacitance?
Capacitance is a measure of how much charge can be stored by a capacitor per unit potential difference across it.
State the equation used to calculate the capacitance from the charge and potential difference.
Capacitance = Charge / Potential
Difference
C = Q/V
What is the unit of capacitance?
CV^-1 or farads (F)
What is represented by the area under a capacitor’s charge-potential graph?
The energy stored by a capacitor.
What is represented by the gradient of a capacitor’s charge-potential graph?
Capacitance
Give three equations to calculate the energy stored by a capacitor.
W =½QV
W = ½CV^2
W = ½Q^2/C
What value is given by the product of resistance and capacitance?
The time constant of the circuit.
What does the time constant of a circuit tell you?
The time the capacitor will take to reach 63% of its total charge, and the time taken for it to discharge to 37% of its full charge.
What is a magnetic field?
A magnetic field is a region in which a magnetic pole will experience a non-contact force.
In which direction do magnetic field lines point?
From North to South.
What is magnetic flux density?
Magnetic flux density is a measure of the strength of a magnetic field. It can be viewed as the number of magnetic field lines that pass through a given area.
What three factors determine the force exerted on a charge moving through a magnetic field?
- The magnetic flux density
- The charge of the particle
- The velocity of the particle perpendicular to the field
State the equation used to calculate the force exerted on a charge moving through a magnetic field.
F = Bqvsinθ
What is the relationship between the direction of a charge’s motion and the direction of the magnetic force it experiences?
The force is always perpendicular to the charge’s motion.
Describe and explain the path taken by a charge in a magnetic field.
The charge will move in a circular path.
This is because the magnetic force always acts perpendicular to the charge’s motion and so acts as a centripetal force.
What is produced by a current-carrying wire?
A magnetic field is produced in concentric circles around a wire when a current passes through it.
What occurs when a current-carrying wire is placed in a magnetic field?
The wire will experience a force due the permanent magnetic field interacting with the wire’s magnetic field.
What three factors affect the force experienced by a current-carrying wire placed in a magnetic field?
- The magnetic flux density of the field
- The current passing through the wire
- The length of the wire
State the equation used to calculate the force experienced by a current-carrying wire placed in a magnetic field
F = BILsinθ
What is Fleming’s left-hand rule used for?
To determine the direction of the force experienced by a current-carrying wire or moving charge in a magnetic field.
What does the thumb represent when using
Fleming’s left-hand rule?
The thumb represents the direction of the force.
What does the first finger represent when using Fleming’s left-hand rule?
The direction of the field.
What does the second finger represent when using Fleming’s left-hand rule for a moving charge?
The direction that a positive charge would move. This means that if it is a negative charge, you must point you second finger on the opposite direction to its motion.
What does the second finger represent when using Fleming’s left-hand rule for a current-carrying wire?
The direction of conventional current
flow.
What is the relationship between field lines and how strong the elctric field is
the closer the field lines, the stronger the field
what is a current flowing through a coil called
a solinoid
what happens to electric field strength for radial fields
there is a weaker field the further you get away from the centre, following the inverse square law
what is the acceleration in an electric field formula
a = EQ/m
electric potential formula
electric potential = electric potential energy / charge
V = E / Q
What is the difference between electric potential and electric potential energy
The electric potential at any point in space is defined as the electric potential energy per unit charge at that point. Electric potential is the amount of work done to move a positive test charge from infinity to a point in space. Electric potential energy is the energy required to move a charge in an electric field.
explain why current will slow down in a capacitor over time
in a capacitor, if one side has a very high charge, and the other side has no charge, the current will be really large at first as they all go across to the other side. the current then slows down exponentially as they both become equally charged
what is magnetic flux density and what is its formula
the number of lines of force passing through a unit area of material; linked to the strength of the magnetic field. measuered in teslas. indicated by how close the field linesare. B = Φ / A
Φ = flux in webers
B = flux density
what is magnetic flux
Φ = a measure o the numberof field lines passing through a region of space. meausred in Tm^2
what is flux linkage
the linking of the magnetic field with the conductors of a coil when the magnetic field passes through the loops of the coil, expressed as a value in webers.
relationship between number of coils and magnetic field strength
more coils of wire = more magnetic field strength in th middle and round the outside
What is needed to induce an emf
You need relatuve motion between a wire and a magnetic field
For example pushing a magnet in and out of a coil of wire
Explain the motor effect
If a current carrying wire (which has a magnetic field) is placed in a region of magnetic field, it will experience a magnetic force as the 2 magnetic fields interact with each other. The magnitude of the force is maximised when the current is perpendicular to the magnetic field, hence F = BILsin θ
Explain why an alternating current is required for the motor effect in a loudspeaker
When a current flows through the coil in the speaker, the coil experiences a force. This force causes the coil and the paper cone to move. The current then must be reversed so that the coil and paper cone move in the opposite direction.
Why does the motor effect cause an alternating voltage
Because the magnet is put in and out of the magnetic field. Magnetic field strength is greatest when perpendicular to direction of magnetic field. As the magnetic field strength inside the coil is increased as the magnet enters, the induced voltage in the coil is directed one way. Emf’s are reversed by reversing poles.
What factors effect the magnitude of the emf in the motor effect
Strength of the magnetic field
Rate of change of magnetic field
Angle between the magnetic field and conductor (strongest at 90 degrees)
Area magnetic field is moving through
Number of turns on the coil
What is the relationship between emf and rate of change of magnetic flux
Proportional. As rate of change of magnetic flux increases, induced emf increases
Explain Lenz’s law
As a magnet moves through a metal wire, a current is induced. This has its own magnetic field which interferes with the magnets magnetic field. The direction will be in the opposite direction to the magnets direction ( the change in flux that created it). The emf induced always opposes the change of motion that induced it.
the induced current in the coil will produce a magnetic field that opposes the increase in flux
Explain faradays law
Emf induced is equal to the rate of flux linkage.
Explain how a p.d is induced over the secondary coil in a transformer
An AC supply is used to supply the primary coil
This creates an oscillating current in the primary coil
The current creates a magnetic field across the primary coil
The AC creates a moving magnetic field
An iron core links the moving coils to the secondary coils
The moving magnetic field over the second set coil induces an EMF and alternating current in the secondary coil, with a different value of EMF depending on the factors like how many coils.
What are the 2 magnetic field strength equations
F = BEV
F = BIL
Magnetic flux linkage formula
N Φ = NBAcos θ
B = flux density
N = number of turns in wire (ignore first N)
A = area
Explain how a capacitor discharges
• Electrons/charge transferred from negatively charged plate to positively charge plate through the resistor (1)
• Hence the charge on capacitor decreases (exponentially) (1)
• Until the charge on the capacitor equals 0/negligible (1)
Explain faradays law
It states that the induced emf in a conducting circuit is proportional to the rate of change of magnetic flux linkage Φ within the circuit
Explain lenz’s law
The current induced in a circuit due to a change in a magnetic field is directed to oppose the change in flux and to exert a mechanical force which opposes the motion.
Explain why particles travel in a circular path in a magnetic field
As there is a constant force that acts perpendicularly to the direction of motion causing an acceleration towards the centre of the circle
Explain how charge builds up on a capacitor
When charging a capacitor, the power supply ‘pushes’ electrons to one of the metal plates
It therefore does work on the electrons and electrical energy becomes stored on the plates
The power supply ‘pulls’ electrons off of the other metal plate, attracting them to the positive terminal
This leaves one side positively charged, while the other side becomes negatively charged
Gradually, this stored charge builds up
Adding more electrons to the negative plate at first is relatively easy since there is little repulsion
As the charge of the negative plate increases, i.e., becomes more negatively charged, the force of repulsion between the electrons on the plate and the new electrons being pushed onto it increases
This means a greater amount of work must be done to increase the charge on the negative plate or in other words:
The potential difference across the capacitor increases as the amount of charge increases
When is the flux linkage at its maximum
since cos (0°) = 1, the flux linkage is a maximum when the angle θ is zero. This means the flux and coil face are perpendicular (i.e. the normal line to the coil face and the flux lines are parallel).
When will a current carrying wire experience the greatest magnetic force
A current-carrying conductor (eg. a wire) will experience the maximum magnetic force if the current through it is perpendicular to the direction of the magnetic flux lines
What is resonance
Resonance is where the amplitude of oscillations of a system drastically increase due to gaining an increased amount of energy from the driving force.
