7B. I take this magnetic force of a man to be my lover [COMPLETE] Flashcards
electric and magentic fields (magnetic field focus)
What is magnetic flux density?
The density of magnetic field lines.
What determines the strength of a magnetic field?
The magnetic flux density.
What is magnetic flux density’s symbol?
B.
What units is magnetic flux density measured in?
Tesla (T)
What’s the equation for magnetic flux density?
Magnetic flux density (T) = magnetic force on a current-carrying wire (N)/ (current (A) x length of the wire (m)). B=F/IL
What is the magnetic flux density sometimes known as?
The magnetic field strength
What is magnetic flux defined as?
The product of magnetic flux density and the cross-sectional area perpendicular to the direction of the magnetic flux density.
When is the magnetic flux a maximum?
When the magnetic field lines are perpendicular to the coil area.
When is the magnetic flux a minimum?
When the magnetic field lines are parallel to the coil area.
What is magnetic flux’s symbol?
Φ
What is the units magnetic flux measured in?
Webers (Wb)
What’s the equation for magnetic flux?
Magnetic flux (Wb) = Magnetic flux density (T) * cross-sectional area (m^2) Φ=BA
What’s the equation for magnetic flux when the magnetic field is not perpendicular to the cross-sectional area?
Φ=BAcos(θ), where θ is the angle between the magnetic field lines and the line perpendicular (normal) to the plane of the area [in degrees]
Why is magnetic flux a maximum when the field lines are perpendicular to the area?
Φ=BAcos(θ), θ=0, therefore cos(θ)=1 Hence Φ=BA*1=BA
Why is magnetic flux a minimum when the field lines are parallel to the area?
Φ=BAcos(θ), θ=90, therefore cos(θ)=0. Hence Φ=BA*0=0
How can e.m.f. be induced in a circuit?
When the magnetic flux linkage changes with respect to time.
What’s the equation for the induced e.m.f. in a circuit?
ε=N (d(BA))/dt, where N is no. of turns in a coil.
What is flux linkage defined as?
The product of the magnetic flux and the number of turns of the coil.
What’s the equation for flux linkage?
Flux linkage= ΦN=BAN, where N is no. of turns in a coil.
What units is flux linkage measured in?
Weber turns (Wb turns)
What’s the equation for the magnetic force on an isolated moving charged particle?
F=BQv
When is the magnetic force on the charged particle a maximum?
When F, B and v are mutually perpendicular. When the charged particle travels perpendicular to a magnetic field.
When is the magnetic force on the charged particle a minimum?
When the charged particle travels parallel to a magnetic field.
What is current?
Current is the rate of flow of POSITIVE charge.
What is the direction of the flow of electron beams?
The opposite direction to the current.
What is the equation of the magnetic force on a charged particle moving at an angle θ to the magnetic field lines?
F=BQv sinθ
What fingers represent which quantities in Fleming’s Left Hand Rule?
Thumb: Motion/Force; First/Index: Magnetic Field; Middle: Current.
What is the acronym for Fleming’s Left Hand Rule?
FBI: Force, B(m)agnetic field, I(c)urrent.
What is the symbol for the magnetic field going out of the page?
Dots, resembles an arrow coming towards you.
What is the symbol for the magnetic field going into the page?
Crosses, resembles an arrow moving away from you.
Where does the max magnetic force act on a moving charged particle?
Acts perpendicular to its velocity.
How do charged particles move in a magnetic field?
They move in a circular path.
If the charged particles move in a circular path, what could the magnetic force also be?
Centripetal force.
How can Fleming’s Left Rule be used to find velocity?
The middle, representing current, can be used to represent the direction of the velocity of a positive charge.
What does a current-carrying conductor produce?
It produces its own magnetic field.
When would a current-carrying conductor experience the maximum magnetic force?
When the current through it is perpendicular to the direction of magnetic flux lines.
When would a current-carrying conductor experience the minimum magnetic force?
When the current through it is parallel to the direction of magnetic flux lines.
What is the equation for the force on a conductor carrying current in a magnetic field?
F=BIL sinθ, where θ is the angle between the conductor and external flux lines.
What does the equation for the force on a conductor carrying current in a magnetic field tell us?
The magnetic force is proportional to current, magnetic flux density, length of the conductor, sin θ.
When can you use Fleming’s Left Hand Rule?
When determining force/magnetic field/current on a charged particle or a current carrying conductor.
When there is a change in magnetic flux (or linkage), what energy is transformed?
Mechanical energy is transformed into electrical energy.
What is electromagnetic induction defined as?
The process in which an e.m.f is induced in a closed circuit due to changes in magnetic flux.
When does electromagnetic induction occur?
When a conductor cuts through a magnetic field. When the magnetic flux (linkage) through a coil changes.
Where is electromagnetic induction used?
Electrical generators, transformers.
How do electrical generators work?
They convert mechanical energy to electrical energy.
Where are transformers used?
Electrical power transmission.
How can you demonstrate the induction of e.m.f?
Connect a coil to a sensitive voltmeter. Move a bar magnet in and out of the coil.
What are the observations when the bar magnet is not moving on the voltmeter?
The voltmeter shows a zero reading.
Why does the voltmeter show a zero reading when the bar magnet is not moving?
The rate of change of flux is zero, so no e.m.f induced.
Why does the voltmeter show a reading when the bar magnet is moving?
The bar magnet’s magnetic field lines cut through the coil, generating a change in magnetic flux. This induces an e.m.f within the coil and is shown momentarily on the voltmeter.
What happens when the bar magnet is moved in the opposite direction it just moved in?
An e.m.f is induced in the opposite direction.
Why does an e.m.f induced in the opposite direction occur when the bar magnet moves in the opposite direction?
As the bar magnet moves in the opposite direction, the direction of the current changes. Hence the voltmeter shows a reading with an opposite sign momentarily.
What does increasing the speed of the bar magnet do?
It induces an e.m.f of a greater magnitude.
Why does the induced e.m.f increase when the speed of the bar magnet increases?
The rate of change of flux increases.
What is the direction of the electric current, and the induced e.m.f in the conductor?
The opposite to the change that produces it.
How can the induced e.m.f be increased?
By moving the bar magnet faster through the coil. Adding more turns in the coil. Increasing the strength of the bar magnet.
What happens when a coil rotates in a uniform magnetic field?
The magnetic flux will vary. Therefore, the flux linkage will vary. Hence the induced e.m.f will vary.
When will there be a maximum e.m.f?
When the coil cuts through the most field lines.
What is the varying e.m.f induced also called?
An alternating current.
When the normal of the plane of coils is parallel to the magnetic field line, is the flux linkage a minimum or a maximum?
A maximum.
When the normal of the plane of coils is parallel to the magnetic field line, is the change in flux linkage a minimum or a maximum as the coil rotates?
A minimum.
When the normal of the plane of coils is perpendicular to the magnetic field line, is the flux linkage a minimum or a maximum?
A minimum.
When the normal of the plane of coils is perpendicular to the magnetic field line, is the change of flux linkage a minimum or a maximum as the coil rotates?
A maximum.
What does increasing the coil’s frequency of rotation do?
Increases the frequency of the alternating voltage. Increases the amplitude of the alternating voltage.
What happens in a transformer?
An e.m.f is induced in a coil when there is a change of current in another coil linked with this coil.
What does a transformer do?
It changes high alternating voltages at low current to low alternating voltages at high current, and vice versa.
What is a transformer made of?
A primary coil, a secondary coil, and a soft laminated iron core.
Why is the soft laminated iron core necessary?
It creates flux linkage between the primary and secondary coils.
Why soft iron is used for the core?
It can be easily magnetised and demagnetised.
Why does the iron core have to be laminated?
To reduce eddy currents and improve efficiency.
What occurs in the primary coils of a transformer?
An alternating current, which produces an alternating voltage, is applied to the coil.
What occurs in the iron core of a transformer?
The alternating current and voltage create an alternating magnetic field. Therefore, a changing magnetic flux linkage.
What occurs in the secondary coils of a transformer?
The change in magnetic flux linkage from the iron core is also in the secondary coils. Due to Faraday’s Law, an e.m.f is induced.
What comes out of the secondary coils?
An alternating output voltage.
What is the similarity between the input voltage and the output voltage of a transformer?
They both have the same frequency.
What is the equation for a transformer?
V_s/V_p =N_s/N_p, where V is voltage, N is the number of turns in the coil, p is primary, and s is secondary.
What is Lenz’s Law used for?
To predict the direction of an induced e.m.f in a coil or wire.
What is Lenz’s Law?
The induced e.m.f is set up in a direction to produce effects that oppose the change causing it.
What equipment is needed to verify Lenz’s Law?
A bar magnet, a coil of wire, and a sensitive ammeter.
What happens in the experiment to verify Lenz’s Law?
A known pole of a bar magnet is pushed into the coil. An e.m.f is induced. A current is driven in the series circuit.
What does Lenz’s Law dictate?
The direction of the e.m.f induced and current must be set up to oppose the incoming magnet. If a north pole approaches the coil face, the e.m.f must be set up to create an induced north pole. This is because the two north poles will repel each other.
Why is a north pole induced not a south pole as the north pole of a bar magnet approaches the coil face?
Due to the principle of conservation of energy. The work done on the magnet from us must be equal to the electrical energy produced in the coil. If it was a south pole, the bar magnet would get attracted by the coils and energy would be seemingly created from nothing.
What is the right-hand rule for solenoids?
The thumb is where the north pole of the magnetic field is, and the curled fingers are the direction of the current.
What happens when we pull the north pole of the bar magnet away from the solenoid?
There will be an opposite reading on the voltmeter. A south pole is induced at the coil face.
What is Faraday’s Law?
The magnitude of the induced e.m.f is directly proportional to the rate of change of magnetic flux linkage.
What is the equation of Faraday’s Law?
ε=N (Δ(BA))/Δt.
What is the equation of Faraday’s Law as Δt → 0?
ε=N (d(BA))/dt.
What is the equation when both Lenz’s Law and Faraday’s Law are combined?
ε=-N (d(BA))/dt.
What does the negative sign mean in the equation of the Lenz’s + Faraday’s?
It represents Lenz’s Law. It shows the e.m.f is set up in an opposite direction to oppose the changing flux linkage.
What graph is the equation “ε=-N (d(BA))/dt” related to?
It represents the gradient of the graph of magnetic flux (linkage) against time.
What is the definition of an alternating current?
A current which periodically varies between a positive and negative values.
When does the direction of an alternating current switch?
It switches every half cycle.
What is the shape of the graph of current/voltage against time?
A sine curve or sinusoidal.
What is the motion of the electrons in a wire carrying an a.c. current?
They move back and forth with simple harmonic motion.
What is the peak current (I0) or peak voltage (V0) defined as?
The maximum value of the alternating current or voltage.
How do you find the peak current/voltage from the graph?
The amplitude of the graph.
What is the peak-to-peak current/voltage?
The distance between a positive and a consecutive negative peak.
What is the equation for peak current/voltage?
Peak-to-peak current/voltage ÷ 2
What are root-mean-square values (rms) used for?
To compare alternating current/voltage to its equivalent direct current/voltage.
What is the rms value of an alternating current mean?
The equivalent direct current that produces the same power.
What is the rms value of an alternating voltage mean?
The equivalent direct voltage that produces the same power.
What is the equation for the rms current?
I_rms=I_0/√2
What is the equation for the rms voltage?
V_rms=V_0/√2
