7.3 - Electromagnetic Effects Flashcards
What are magnetic fields similar to
Electric fields
What do electric fields affect and what do magnetic fields affect
Electric fields affect charges
A magnetic field affects poles
What is a magnetic field
A region of space that will cause a magnetic pole to feel a force is called a magnetic field.
What a magnetic field according to miss Flaherty
A magnetic field is a region of space in which a magnetic material feels a force.
Only 3 magnetic materials
What are the 3 magnetic materials
Cobalt, iron and nickel
What do field lines of a magnetic field show
Field lines show the direction in which a lone North Pole will be pushed
In reality, poles always exist in north and south pairs, but the convention is to take the field as acting from north to south.
What way do field lines go in a magnetic field
The convention is to take the field as acting from north to south, and this is the direction of arrows drawn onto magnetic field lines (also called lines of magnetic flux)
What are magnetic field lines also known as
Lines of magnetic flux
Like all field patterns, the closer the field lines are together the….
Stronger the field is.
What is the term referring to the strength of a magnetic field
Magnetic flux density, B, and SI unit for it is the Tesla (T)
What are the symbol and units for magnetic flux density
B is symbol
Tesla, T is units
What is the quantity of flux (symbol is a circle with a capital i in the middle)
The quantity of flux through any given area indicates the strength of the effect of the field there.
Measured in weber, Wb
How can you calculate quantity of flux
This can be determined for a particular region by multiplying the area enclosed by the region by the component of flux density perpendicular to the area.
Quantity of flux = B x sin(theta) x A
If the magnetic flux is in a uniform direction and lies perpendicular to the area in question, sin(90) = 1 so equation simplifies to
Phi = BA
How can we calculate magnetic flux density
In a situation where the flux is perpendicular to the area we are considering, a rearrangement of the equation shows why quantity B is also known as the magnetic flux density
B = quantity of flux(phi)/A
By sharing the flux over the area, B indicates how closely packed the magnetic field lines are - how dense they are. How dense field lines are indicates how strong the field is, so B is also known as the magnetic field strength
What is magnetic flux density the same as
Magnetic field strength
Tell me about flux linkage
The amount of magnetic flux interacting with a coil of wire is known as flux linkage
The interaction between magnetic fields and charged particles, or conductors, allows motors to operate, and electricity to be generated. In most practical applications, magnetic flux is made to interact with a coil of wire, as the effect on a single strand of wire is too small to be useful. If the single wire is coiled up, then the magnetic field can interact with each turn on the coil, and so any effect is multiplied N times, where N is the number of turns on the coil.
How do we calculate flux linkage
Simply it is the product of the number of turns of wire and the flux in that region.
Flux linkage = N x quantity of flux(Phi)
Remembering that phi = BA
Flux linkage also equals = BAN
Define poles
Poles are the magnetic equivalent of a charge on a particle, North Pole or South Pole
Define a magnetic field
It’s a region of space that will cause a magnetic pole to feel a force
Define magnetic flux
Is an alternative phrase referring to magnetic field lines.
Define magnetic flux density
Is the ratio of magnetic flux to the area it is passing through.
Define Tesla, T
The unit for magnetic flux density, or magnetic field strength
Define Weber, Wb
Is the unit of measurement of magnetic flux, symbol phi, (and magnetic flux linkage N x phi)
Define magnetic field strength
Is an alternative phrase for magnetic flux density
Define flux linkage
It’s the amount of magnetic flux interacting with a coil of wire.
What can magnetic fields affect
Magnetic fields can affect moving electric charges, as well as magnetic poles.
How can a magnetic field affect electric charges
If you were to place a wire in a magnetic field and pass a current through it, the wire will feel a force on it. This is called the motor effect
What is the motor effect
When you place a wire in a magnetic field and pass a current through it, the wire will feel a force on it.
When is the motor effect greatest
When the wire and the magnetic field are at right angles.
In this instance, the force will be at right angles to both current and field direction in the third dimension as shown by Flemings left hand rule.
What is Flemings left hand rule
Thumb = motion/force
First finger = magnetic field direction
Second finger = current
Flemings left hand rule gives the relative directions of the field, current and movement in the motor effect.
In Flemings left hand rule current is….
Conventional
Positive to negative unless otherwise stated
The larger terminal of a cell is the….
Positive terminal
What happens if current is parallel to a magnetic field
There is NO force
How can we investigate “build your own motor”
The forced movement of a current carrying conductor within a magnetic field is the fundamental principle that causes motors to work.
From Flemings left hand rule, it is clear that if a coil of wire carrying a current were placed in a magnetic field it would feel a turning force, as the current travels in opposite directions on opposite sides of the coil. If it is free to move, then the coil (or motor) will spin continuously. Remember to use a low power DC supply.
How small are the smallest motors in the world
In 2003, a motor was produced which is less than 500 nanometres across!!! Since, one has been created of similar size but rotates faster and for longer than any previous nanomotor.
The entire set up of the motor would fit within one wavelength of red light.
Moving systems on a molecular scale have also been produced by other groups of scientists using chemical systems to generate the forces, but the motors described are the smallest to use electromagnetic forces as explained in this section.
What’s an electro magnet, what’s good about it
A metal core made into a magnet by being surrounded by a coil of wire carrying current.
Requires continuous current to keep a motor made with an electromagnet working. The strength of field can easily be changed by changing current.
Whereas, with a permanent magnet - it requires no power.
Define motor effect
It’s the phenomenon that a wire carrying a current, held within a magnetic field, will experience a force.
Define Flemings left hand rule
It’s a system for determining the direction of the force generated by the motor effect.
How can we investigate the magnetic force on a current carrying conductor
You can investigate how much force will be on a wire subjected to the motor effect.you can put two magnets on a top hand balance and put a current carrying wire through the field, depending on the current and field directions the mass read on the balance may decrease or increase. In this experiment it is easy to alter the current through the wire, the length that is within the magnetic field, and the angle it cuts across the field.
The strength of the force, F, on a wire has a current, I, through it whilst In a magnetic field, B, is given by the equation:
F = BIL x sin(theta)
Where L is the length of the wire within the field, and theta is the angle the current makes with the lines of the magnetic field. For simplicity, we will only consider uniform magnetic fields in which field lines are all parallel.
It is common to set up situations in which the angle theta is 90 so that sin theta is a maximum and equals one. This reduces the formula to F = BIL
Which is commonly remembered by students as Fred = BIL (what?🤨)
How can a motor be made faster/ more powerful as a consequence of the expression F = BIL
Increasing the current through the motor (I)
Increasing the number of turns of wire in the motor (L)
Increasing the magnetic field within the motor (B)
How is the magnetic field strength usually maximised
By making the coils core out of soft iron. Some motors use electromagnets to provide the field, and these could be strengthened by increasing the current through them.
Why does the motor effect happen
Because a charged particle moving at right angles to a magnetic field feels a force on it at right angles to its direction of motion and also at right angles to the field. If the charged particle is constrained - like an electron in a current in a wire - then the force will be transferred to the wire itself. If the particle is flying freely, its direction will change and it will travel a circular path whilst in the magnetic field.
Charged particles moving in a magnetic field follow a circular path as the motor effect provided a centripetal force.
The strength of the force on a charged particle moving across a magnetic field is given by the equation:
F = Bqv x sin(theta)
Where q is the charge on the particle, v, is it’s velocity, and theta is the angle between the velocity and the magnetic field lines. A simplified situation that is often considered is that of an electron (charge ‘e’) moving at right angles to the field (sin(90) = 1). This reduces the formula to F = Bev, which is commonly remembered by students as ‘Fred = Bev’
q may be referred to as e when talking about an electron
How can we investigate F = Bev
You can investigate how much force will be on an electron moving across a magnetic field using equipment like shown in the picture (page 69 idk how to describe it) the grid allows you to observe the path travelled by electrons, and there is a standard formula for calculating the magnetic field strength provided by a pair of parallel electromagnetic coils, often referred to as Helmholtz coils.
As the force on the charged particle is always at right angles to the direction of its _____ , it acts as a ______ force and the particle follows a _______ path
As the force on the charged particle is always at right angles to the direction of its velocity, it acts as a centripetal force, and the particle follows a circular path.
How could a particle be made to continuously orbit a central point
With The right combination of conditions, a moving charger particle could be held by a magnetic field, continuously orbiting a central point.
This is the principle by which artificially generated anti matter is contained to save it from annihilation, for future use or study.
What’s a mass spectrometer used for
This ain’t required by the spec but it says its useful
Scientists often need to identify unknown chemicals. This is particularly important, for example, in the field of forensic science, where a crime scene technician will take a sample of unknown material which they need to identify. A machine called a mass spectrometer can separate chemicals according to their charge/mass ratio, which allows unique identification of each substance within a sample.
A chemical to be Identified enters the machine and is ionised. This charge will then allow it to be accelerated in two different ways within the mass spectrometer. An electric field increases its speed. Then it feels a force when travelling through the field of the electromagnet, which changes its direction.
Check schematic diagram of mass spectrometer
Page 69
Tell me about the calculations involved in using a mass spectrometer
The force on a charged particle moving at right angles to a magnetic field is given by:
F = Bqv
Centripetal force is given by: F = mv^2/r
Thus for particles following the circular curve through the electromagnet to emerge on a path to the detector:
Bqv = mv^2/r
Which rearranged to
q/m = v/Br
The charge to mass ratio will identify the particles involved, so with B and r (radius of the curvature of the particle through the mass spectrometer) known from the calibration of the machine, all we need to know is how fast the particles were moving when they entered the electromagnet. They are accelerated to this speed by an electric field acting on their charge, and the kinetic energy gained comes from the potential difference, V, that they pass through according to:
1/2mv^2 = qV
v = square root of 2qV/m
Substituting this into our equation for the charge/mass ratio:
q/m = square root(2qV/m) divided by Br
If we square this and rearrange we get
q/m = 2V/ B^2r^2
Thus, by adjusting the accelerating voltage and the strength of the electromagnet (by altering the current through it) we can identify the various chemicals contained within a sample, as each is registered in the detector. The intensity of the current in the director can indicate the proportion of a given chemical within the sample.
What does the intensity of the current in the detector indicate with a mass spectrometer
The proportion of a given chemical within a sample.
The movement of a charged particle in a magnetic field causes it to
Feel a force.
Newton’s third law of motion reminds us that this force must have a counterpart that acts equally in the opposite direction. Moreover, this pair of electromagnetic forces is generated whenever their is relative motion between a charge and a magnetic field. Thus, a magnetic field moving past a stationary charge will create the same force.
What does the velocity term in the expression F = Bqv actually refer to
Actually refers to the relative (perpendicular) velocity between the magnetic field lines and q. (Also remember that if the movement is not at right angles, then we need to work out the component of it that is at right angles by including the sin theta term: F = Bqvsin theta
How can an emf be induced
If we move a magnet near a wire, the electrons in the wire will feel a force tending to make them move through the wire. This is an emf; if the wire is in a complete circuit, then the electrons will move, forming an electric current. We can use this principle to generate electricity. Reversing the direction of the magnetic field, or the direction of the relative motion, will reverse the direction of the force on the electrons, reversing the polarity of the emf. Faradays law states that the induced emf is proportional to the rate of change of flux linkage.
What does faradays law state
The induced emf is proportional to the rate of change of flux linkage.
How can we investigate faradays law
You can investigate faradays law using a magnet and a coil of wire connected to a voltage datalogger and moving the magnet relative to the wire to induce an emf.
Tell me about a bizarre result as a product of Lenzs law
If you drop a magnet down a copper tube, it falls more slowly than if you drop a similarly sized non magnetic piece of metal through it. As copper is not a magnetic material, the friction forces should be identical on the two falling objects. This somewhat bizarre result is a product of Lenzs law.
How does Lenzs law work using an example of a copper tube and a magnet being dropped down it
Imagine the copper tube Is like a series of coils of copper wire all stacked on top of each other. As the magnet falls through the tube, it will induce an emf in each copper circlet, which will cause a small current to flow around the tube. This circling current will then generate an electromagnetic field, which will interact with the falling magnet. The direction of this newly created magnetic field will then determine whether it slows up the falling magnet or repels it faster down the tube. If the latter were the case, then the magnet would end up with more kinetic energy than the gravitational potential energy it had at the start. This is impossible and so the law of conservation of energy dictates Lenzs law:
The direction of an induced emf is such as to oppose the change creating it.
In this way the emf induced in the tube acts to try and slow down the magnet, as the movement of the magnet is the change creating this emf. The induced current around the copper tube acts as an electromagnet, and the polarity of this induced magnetic field is opposite to that of the falling magnet, so it slows up the magnets fall.
What’s Lenzs law in words
The direction of an induced emf is such as to oppose the change creating it.
How can the direction of an induced emf be found using Flemings right hand rule
This is very similar to the left hand rule for the motor effect. The reason that the opposite hand is needed is a product of Lenzs law, in that the induced emf must be in a direction which opposes the change creating it.
It looks like it’s Flemings left hand rule but just on the right hand.
What expression does putting Faradays and Lenzs law together give us
It gives us an expression for calculating an induced emf
E = -d(N x phi)/dt
Or E = -triangle(N x phi)/triangle t
Faradays law told us that the emf would be proportional to the rate of change of flux linkage, and the minus sign in the equation comes from Lenzs law, to indicate the opposing direction.
How can phi be calculated
Phi = B x A
= magnetic field strength x cross sectional area
Define Lenzs law
Lenzs law is that the direction of an induced emf is such as to oppose the change creating it
Define faradays law
Faradays law says that the induced emf is proportional to the rate of change of flux linkage.
Can an emf be induced constantly
We have seen how an emf can be induced by changing the magnetic flux linkage with a conductor. If this is extended to a coil which is made to rotate continuously in the magnetic field, the emf will be induced constantly, although it will keep changing direction as the coils position relative to the field keeps switching back and forth.
In a complete circuit, the emf will cause a current to flow, and as the polarity (direction) of the emf is constantly switching, the current will keep changing direction.
What does an AC generator look like
An AC generator: continuously rotating this coil within the magnetic field will induce an emf all the time, and so an alternating current will flow in the circuit all the time.
Has carbon brushes, slip rings
North to south magnets with wire rotating inbetween.
What pattern does alternating current follow
Alternating current follows a waveform, and the same cycle will repeat continuously.
Tell me about the relationship of frequency and time period for AC current
As with any waveform, we can measure the time period for one complete cycle, and the reciprocal of this will be the frequency. The frequency is measured in hertz (Hz) and is the number of cycles completed per second.
In the uk, what frequency is mains electricity
In the uk, mains electricity is AC and it has a frequency of 50Hz. This means that power station generators are carefully controlled, so that they rotate exactly 50 times per second.
What waveform does alternating current follow
A sinusoidal waveform, it’s value is constantly changing. If we start at zero, it rises to a peak value, drops back through zero to negative values, reaching a maximum negative value and then returns to zero, before repeating the cycle again and again. This idea is true whether we are considering the voltage or the current - they both follow the same wave shape.
What’s the maximum value
The maximum value, regardless of sign, is called the peak voltage and we can measure the peak current (I subscript 0), or the peak voltage (V subscript 0).
In the Uk, what’s the peak voltage of mains electricity
+/- 325 V. The peak current value will depend on the actual circuit in use, following ohms law.
How can we investigate AC with an oscilloscope
An oscilloscope is essentially a voltmeter that gives measurements over an extended period of time.
The voltage reading is shown on the screen by a small dot. The vertical position of the dot indicates the voltage value. As time goes by, the dot is automatically moved from left to right across the screen, so that if the voltage reading changes over time, we will be able to see the changing height of the dots trace on the screen. If the dot moves fast enough, it will blur into a continuous line trace.
The two most important controls on an oscilloscope are the time base (x axis, dot speed control) and the volts-per-division (y axis, voltage sensitivity control). Eg 10mV/div. The time base might be 5ms/div so the dot will move horizontally across the screen at a rate of 1 square in 5 milliseconds. The voltage sensitivity is set at 10 mV/div in this example, so the dot will move vertically 1 square for every 10 millivolts measured.
What and why are root-mean-square values given
As the voltage and current values for AC are constantly changing, what does it mean if we ask, ‘what is the voltage of this AC?’ If you look at the sinusoidal trace, you might say that on average it was zero. Alternatively you might choose to state the peak value. Scientists need to have an agreed system or the resulting confusion could lead to electrocution. LOL. Also, we need to be able to compare alternating current with direct current. DC has the voltage and current values constant and always in the same direction. Unless peak values are stated, the voltage and current values of alternating currents are the root mean square values.
What do root mean square values allow us to do
I (subscript rms) and V (subscript rms) allow direct comparison with DC as they will deliver the same amount of energy as a DC with the same value.
How do we calculate RMS values
V (subscript rms) = V(subscript 0)/square root 2
I(subscript rms) = I(subscript 0)/square root 2
Eg the rms voltage of the AC mains electricity is 230V and this is the normally stated voltage value for the uk mains electricity.
Root mean square values are a type of what
Averaging technique. For an AC waveform, we look over a whole cycle and for each value find the square of it. Then we look at all these squared answers (which will all be positive) and find the mean average of them. Finally take the square root of that average, and we have the ‘root of the mean of the squares’ - ‘root mean square’. You will see this comes up again for the average speed of molecules in a gas sample. Omg why
What’s the principle of operation of a transformer
We have seen the induction of emf was a result of the relative motion between a conductor, or coil, and a permanent magnet. The magnetic field which interacts with the coil could just as easily be produced electrically by another coil. This is the principle of operation of a transformer, and the coil producing the initial magnetic field is referred to as the primary coil.
Tell me how transformers operate /how to use an electromagnet to induce an emf in a coil
we have a pair of coils linked together by a soft iron core. Iron is extremely good at carrying magnetism and, in diagram shown (page 74), virtually all the magnetic field generated by the primary coil on the left would interact with the secondary coil on the right. When the primary switch is first closed, the primary suddenly produces a magnetic field that was not previously there. This means that a changing magnetic field is now within the secondary coil. This sudden change of flux linkage will generate an emf in the secondary. However, once the electromagnetic field is stable there will no longer be any change in flux linkage over time, and so there will be no further induced emf in the secondary. The voltmeter needle will kick and then return to zero.
If the primary circuit is switched off, the magnetic field it produces will suddenly disappear, and a brief emf will be induced in the opposite direction to the switch on voltage. The voltmeter needle will kick in the opposite direction and then return to zero again.
Define peak value
Peak value is the maximum value, regardless of sign, of a sinusoidal graph, such as that for alternating current.
Define peak current
Peak current is the maximum value, regardless of sign, of an alternating current.
Define peak voltage
Peak voltage is the maximum voltage value, regardless of sign, of an alternating current.
Define primary coil
A primary coil is the first coil in a transformer, in which the supply current passes.
Define secondary coil
A secondary coil is the second coil in a transformer, in which the output current passes.
Transformers ain’t required by spec
But it do be useful
What’s better than using an electromagnet to induce emf in a coil
This circuit and situation described before this Is not really of much practical use, although it can be responsible for current surges in circuitry, which can cause damage. The same principle is more usefully applied in the transformer, in which an alternating current is supplied to the primary.
Tell me about how transformers work
An alternating current is supplied to the primary. As this current constantly varies, the electromagnetic field produced by the primary coil alternates, which creates a varying induced emf in the secondary. This emf will vary at the same rate as the AC supplied to the primary. It will also change constantly depending on the varying rate of change of flux linkage. This comes from the strength of the magnetic field (which in turn depends on the number of turns on the primary coil) and the number of turns on the secondary coil.
The magnetic field is almost totally contained in the iron core
A current in the primary coil produces a magnetic field, like a solenoid.
What is the ratio of voltages between the primary and secondary coil equal to
It’s identical to the ratio of the number of turns on these coils:
V(primary)/V(secondary) = N(primary)/N(secondary)
This gives us the essential job of a transformer which is to change voltage. More turns on the secondary means a step up transformer in which the output voltage is higher than the input voltage and vice versa for a step down transformer.
