Electromagnetism Flashcards
To understand Faradya's Law, Lenz's Law, eddy currents etc.
What is the Motor Effect?
A current-carrying conductor produces a magnetic field.
The Motor Effect is when the current-carrying conductor in an EXTERNAL magnetic field experiences a magnetic force due to the interaction between the external magnetic field and the current carrying conductor’s own magnetic field
Why do current carrying conductors produce a magnetic field?
A moving electrical charge creates a magnetic field. This is just assumed knowledge, the very basis of electromagnetism.
A current is the rate of flow of electricity. Therefore, a current describes the flow of an abundance of electrical charges, which thus create a stronger magnetic field.
What is the formula for magnetic force?
BIL sin(θ)
B = Magnetic Flux Density
I = Current
L = length of Conductor
What is meant by magnetic flux density?
Magnetic flux is the measurement of how compact or tight the number of magnetic field lines are when they pass through a given area
Flux density is a measurement of the density of magnetic field lines.
What even is density?
Density is mass per unit volume.
Measurement of how tightly a material is packed together
How ‘compact’ something is (closely tuck together)
What is a conductor in electromagnetism?
A conductor is an object or material that allows the flow of electrical current through it
What is Magnetic Flux?
Magnetic Flux is the number of magnetic field lines passing through a certain area
What is magnetic flux density?
Magnetic Flux Density is how tight/compresssed the magnetic field lines are when passing through a given area.
For example, a low magnetic flux density means that the magnetic field lines are more spread apart in the given area.
What is the symbol for Magnetic Flux Density in Formulas?
The magnetic flux density of a magnet is also called “B field” or “magnetic induction”. It is measured in tesla (SI unit)
What is the formula for magnetic flux?
Φ = BA cos(θ)
Where,
Phi = Mangetic Flux Density x Area x cos(θ)
What is the SI unti for magnetic flux?
(Hint, NOT MAGNETIC FLUX DENSITY)
Webers or Wb for short
Or Teslas per metre
Therefore, Wb or Tm^-1
What does the symbol B stand for in electromagnetism formulas? What is the si units?
Magnetic flux density, measured in T
Therefore, Magnetic flux density is measured in Teslas
In the formula for Magnetix Flux, what is the angle (θ) indicating?
Φ = BA cos(θ)
‘θ’ is the angle measured between the magnetic field lines and the normal of the conductor.
Therefore, construct a perpendicular (normal) to the conductor and measured the angle between this and the magnetic field lines)
What is the symbol for magnetic flux?
Phi or Φ
At what angle is the maximum magnetic flux experienced?
Maximum flux is experienced when the angle between the magnetic field lines and the normal to the plane is zero
(The plane is basically the surface of the conductor. Also this makes sense as cos 0 = 1)
Therefore, when the magnetic field lines are PERPENDICULAR to the plane, it experiences Maximum flux.
*Don’t get confused:
Max flux is when either:
- The magnetic field lines are perpendicular to the surface of the plane/conductor
OR
- When the angle between the magnetic field lines and the normal of the plane.conductor is 0 degrees
Therefore, it is the same thing - just a different way of expressing it. Both options mean the same, look at a visual diagram for support.
What will happen when the magnetic field lines are in line with the direction of the normal?
This means that the angle between the magnetic field lines and the normal is zero. (Since it is both pointing the same way) Therefore, this means that maximum flux is produced.
When is minimum magnetic flux produced?
Minimum magnetic flux is when the angle between the normal of the conductor/plane and the magnetic field lines is 90 degrees.
This makes sense as cos 90 = 0
What are transformers?
Transformers are a pair of coils wound on the same metal core. (Typically copper or iron)The core is often shaped as a square loop with primary and secondary coils wound on opposite sides. The construction of a transformer allows the magnetic flux generated by a current changing in one coil to induce a current in the neighboring coil.
Therefore, the function of a transformer is to transfer electrical energy from one circuit into another through electrical induction. It does this by changing the voltage of level of an alternating current. (AC)
Why are transformers used and why are they efficient/helpful?
Transformers are helpful because they distribute electricity efficiently over long distances and convert the voltage to fit one’s requirements. In summary, transformers help to efficiently manage and distribute electrical power across different circuits and voltage levels.
Why do we use iron cores for a transformer?
We use an iron core to increase the strength of the magnetic field, that is produced in the primary coil
Why are Transformers not 100% efficient? In an ideal scenario, they would be due to the Law of Conservation of Energy. But in real life, why not?
However, it is not 100% efficient, since transformers also lose energy through the form of electrical energy being converted into heat energy. This is due to the concepts of Eddy currents being restricted.
When the magnetic field in the core changes due to the alternating current passing through the windings, small circulating currents, known as eddy currents, are induced in the core material. These currents encounter resistance within the core material, leading to the conversion of electrical energy into heat. Eddy current losses can be minimized by using laminated cores, which reduce the paths available for these currents to flow.
What is the definition of conventional current? Why is it not really used in physics and what is it supposed to be?
Conventional current is defined as the flow of positive charges. (Protons)
Therefore conventional current shows that current flows from the positive charge to the negative charge in a circuit/power supply.
However, electrical flow is the opposite direction of conventional current. It goes from the negative plate to the positive plate.
What is alternating current? What is the difference between alternating current and direct current?
Alternating current is when the current flow switches back and forth. Therefore, it does not maintain a constant voltage and can range from positive to negative.
In DC (direct current) the electron current flows in one direction. Power sources with DC current often contain a constant voltage.
Transformers use electromagnetic induction to transfer electrical current from one coil to another
True. A transformer is a device that transfers electric energy from one alternating-current circuit to one or more other circuits, either increasing (stepping up) or reducing (stepping down) the voltage.
Why is current inversely proportional to voltage in a transformer, when technically it should be proportional? (As shown in V = IR)
In a transformer, the voltage and current are inversely proportional due to the principle of conservation of energy. According to the transformer equation, V1 * I1 = V2 * I2, where V1 and I1 are the voltage and current on the primary side, and V2 and I2 are the voltage and current on the secondary side. When the voltage is stepped up on the secondary side, the current is stepped down proportionally to maintain the conservation of power. Similarly, when the voltage is stepped down, the current is stepped up. This relationship allows for efficient transfer of electrical energy between the primary and secondary coils of the transformer.
Therefore, since P = VI and power is the rate of change in work (as well as Law of Conservation of energy), if we increase the voltage, then the current should decrease, in order to balance out the power in the transformer.
In an ideal transformer, the input power should be equal to the output power. True or false?
True
What is the relationship between the number of turns in the coil, current and voltage in a transformer?
The number of turns in a coil is directly proportional to the voltage in a transformer.
Conversely, current is inversely proportional to the voltage, ONLY FOR TRANSFORMERS
The B-field is the magnetic field. In what direction does the b-field naturally go in? (Hint: Whenever we solve any electromagnetism questions, we use conventional current)
The B-field naturally tends to go from the positive to the negative. Therefore, the B-field moves from North to South
What does a change in magnetic flux produce?
A change in magnetic flux induces an EMF, an electromotive force (EMF is basically the potential difference/ voltage difference)
The change in magnetic flux inducing an EMF is because of Faraday’s Law.
When and how are eddy currents produced?
Eddy currents are produced due to changes in the magnetic field. They are created when a conductor moves through a magnetic field or when the magnetic field around a stationary conductor varies.
A change in magnetic flux density or change in direction of the magnetic field will produce an eddy current.
Induced voltage/emf produces an eddy current
What factors change the magnetic flux of a conductor?
Magnetic field strength, angle between conductor and magnetic field, area of conductor
Just think about formula for magnetic flux, which is BAcos theta
What is an eddy current? What causes it?
An eddy current is a loop of electrical current.
Eddy current is the induced current from a metal plate as a result of Faraday and Lenz’s Law.
Eddy currents are only induced in conductors of large surface area, usually metal plates, cylindrical pipes made of conducting material.
A magnet in a solenoid is stationary. Will it produce an induced emf? What will the ammeter say?
No, since there is no change in magnetic flux. The ammeter will read the same value as the original current, which is when there was no magnet going through the solenoid.
Make sure to revise the general shit flashcards, they important too
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Will there be an induced emf when a magnet is flowing inwards and outwards of a solenoid?
Yes as it produces a change in magnetic flux. Therefore, emf is induced and the ammeter readings will ‘deflect’ in one direction
What is deflection?
The change of direction
What is the difference between a galvanometer and an ammeter? Similarities?
They both measure current. However, an ammeter shows the magnitude of current whilst a galvanometer shows both magnitude and direction
What does a change in magnetic flux result in? (In relation to emf and current?)
A change in magnetic flux induced EMF, according to Faraday’s Law and this induced emf will cause current to flow. (note that this induced current will be in the opposite direction to the original current, since we have to consider Lenz’s Law)
Back emf is always opposite in direction to the emf produced by the DC motor
yes
As back emf increases, the current decreases. Why?
Current decreases as back emf increases. This is due to Ohm’s Law, which states V = IR and voltage is directly proportional to current. Basically, the net voltage is the emf - back emf. Therefore, as back emf increases, net voltage gets smaller and thus current also decreases.
What is Lorentz force?
Lorentz force is the force that is exerted by a magnetic field on a moving electrical charge.
What is the difference between
F = BIL sin theta and F = qvB sin theta?
f = BIL sin theta is the force on a current in a magnetic field. However, F = qvB is the magnetic force on a moving charge.
In F = qvB sin theta, what does the v stand for?
The ‘v’ stands for velocity, or otherwise the speed of the charge/object
In which direction do the magnetic field lines go?
Magnetic field lines travel from the north to south direction.
Think of electrons and protons. The electric field lines come out of a proton and go into an electron. The same thing applies here.
Also think about conventional current and how it is positive to negative.
Therefore, the B-field always goes from the north pole to the south pole, as it is positive to negative
What does the ideal ammeter represent? What about the ideal voltmeter?
The ideal ammeter has zero electrical resistance.
The ideal voltmeter has infinite electrical resistance.
What is the Lorentz force?
The Lorentz force is the force experienced by a charged particle moving through an electric field and a magnetic field.
What is the difference between F = qvB and F = BIL?
The force of magnetism = q(v*b) is defined for a point charge q moving in the magnetic field b with a velocity v. On the other hand, the force = ILB , it is the force of magnetism on a current carrying straight rod of length l, carrying a current I in magnetic field b.
The formula sheet says sin theta for all angle formulas. What is the intuition behind maximum and minimum?
Maximum is when the angle is perpendicular. Therefore, cos 90 = 0 so this would be a minimum. However cos 0 is a maximum. For sin theta, it is the other way around.
- A galvanometer has very low eletrical resistance. Therefore, ideal galvanometer should have zero resistance. (A galvanometer is an ammeter with both magnitude and direction of current
- A voltmeter has electrical resistance, which means that an ‘ideal’ voltmeter would have infinite resistance
- An ammeter has very low electrical resistance. Therefore, an ‘ideal’ ammeter would have zero resistance
true
Transformers only work in AC motors, since change in current cannot be induced in DC motors, as it flows through only one direction. The transformer needs an alternating currentto create a changing magnetic field, which can only happen if the AC source is grounded.
Yeah
The faster a motor spins, the lower the current is
Facts
Back emf is only for DC motors
Remember that
