Module 6: Chapter 23 - Magnetic Fields Flashcards
What is a magnetic field?
The region around a permanent magnet or a current carrying conductor in which another magnetic object will experience a force
In which direction are magnetic field lines drawn?
In the direction a “free” north pole would move
What is electromagnetism?
When a charged particle moves, it creates a magnetic field. Therefore, when a current passes through a wire a magnetic field is produced
How can you determine the magnetic field around a current carrying wire?
Right hand grip rule
Explain the right hand grip rule for a current carrying wire
The direction of a magnetic field around a current carrying wire can be calculated using the right hand grip rule. Imagine your thumb represents the direction of the current, your curved fingers will give the direction of the field lines
What represents a current coming out of the page?
A circle with a dot (like the tip an arrow head facing you)
What represents a current going into a page?
A circle with a cross in it (like the cross feathers of an arrow facing away from you)
What does the magnetic field of a single coil look like?
What is a solenoid?
A helical coil of wire in which a current travels through
What is the magnetic field around a solenoid?
What are 2 methods of producing a uniform magnetic field?
- Use 2 opposite magnetic poles, the magnetic field is uniform in the space between 2 opposite poles of magnets
- Use a current-carrying solenoid. The magnetic field is uniform at the centre of a solenoid carrying a current
Sketch the magnetic field pattern around these 2 current carrying wires
What is fleming’s left hand rule?
A rule used to determine the direction of the force on a current carrying conductor in an external magnetic field
used for the motor effect, NOT the generator effect
What is your index finger in flemings left hand rule?
Magnetic field
First finger Field
What is your middle finger in flemings left hand rule?
Electric Current
seCond finger Current
What is your thumb in flemings left hand rule?
Force
What is magnetic flux density?
the vector quantity measuring the strength of a magnetic field
What is the equation for the size of the force acting on a current-carrying wire within a magnetic field?
F = BIL sinθ
θ = angle between the magnetic field and current direction
What is the SI unit for Magnetic flux density?
Tesla (T)
What is one Tesla?
The magnetic flux density when a wire of length one metre, carrying a current of one ampere at a right angle to the field experiences a force of one newton
What is a magnetic field produced by?
Moving charges
What does a magnetic field act upon?
Moving charges
What is the equation for the magnetic field strength?
B = F/ILsinθ
Where θ is measured between the magnetic field and current direction
How can magnetic flux density be determined in a laboratory?
The magnetic field between the magnets is almost uniform. When a current is passed through the wire, the wire experiences a force and according to newtons third law of motion the magents experience an equal and opposite force. This force can be determined by finding the change in the mass reading and using F = mg. B can then be determined by B = F/(ILsin90)
Why does a current carrying wire experience a force when placed close to a magnet?
A current carrying wire generates its own magnetic field, the interaction of this field and the field of the magnet produces a force on the wire
What type of motion does a free charged particle moving perpendicular through a magnetic field experience?
Circular motion
Explain why a free charged particle moving perpendicular through a magnetic field experiences circular motion
The charged particles are always acted on by a force perpendicular to its motion, this gives it a circular motion with the radius depending on the force
What is the equation for the size of the force acting on a moving charge within a magnetic field?
F = BQv sinθ
Where θ is measured between the magnetic field and velocity direction
What is the radius of the circular motion of a charge moving perpendicular through a magnetic field
r = mv/Bq
Derive the equation for the radius of the circular motion of a charge moving perpendicular through a magnetic field
F = mv²/r
Bqv = mv²/r
r = mv²/Bqv
r = mv/Bq
What happens to the radius of the circular motion of a charge moving perpendicular through a magnetic field when you increase the velocity?
The radius will increase as r∝v
What happens to the radius of the circular motion of a charge moving perpendicular through a magnetic field when you increase the mass of the charge?
The radius will increase as r∝m
What happens to the radius of the circular motion of a charge moving perpendicular through a magnetic field when you increase the magnetic flux density of the field?
The radius will decrease as r∝1/B
What happens to the radius of the circular motion of a charge moving perpendicular through a magnetic field when you increase the charge of the particle?
The radius will decrease as r∝1/Q
What happens to the motion of a charge if it passes through a magnetic field not at a right angle?
(it has a component of velocity along the magnetic field)
It will spiral around the field lines as it will have a constant velocity in the direction of the field lines but will be spiralling as it also has a component of velocity perpendicular to the field lines
What is a velocity selector and how does it work?
A velocity selector is a device that uses both electric and magnetic fields to select charged particles of a specific velocity. It consists of 2 parallel plates connected to a power supply, they produce a uniform electric field between them and a uniform magnetic field is applied perpendicular to the electric field (and the direction of motion). The charged particles enter at different speeds, the electric and magnetic fields deflect them in opposite directions, only for particles with a specific v will the deflections cancel out so that they travel in straight line and emerge through the slit. For an undeflected particle electric force = magnetic force, EQ = BQv. This equation becomes v = E/B
What is the equation for the selected particle in a velocity selector
v = E/B
What are the 6 stages of a mass spectrometer?
- Vaporisation
- Ionisation
- Particle Accelerator
- Velocity Selector
- Deflection
- Detector
How are charged particles accelerated in a mass spectrometer?
They are accelerated through a potential difference between 2 parallel plates (one is an anode and one is a cathode)
Why is a velocity selector required in a mass spectrometer?
So that all particles enter the deflection chamber at the same speed. The deflection chamber uses a magnetic field to deflect the particles, therefore if they are all of the same velocity and charge, each particle will experience the same force (F = BQv sinθ). Therefore, the deflection depends solely on the mass of the particles, which is what is being determined.
What is Fleming’s right hand rule?
A rule used to determine the direction of the current induced when a conductor is moving through an external magnetic field
Used for the generator effect, NOT the motor effect
What does your index finger represent in Fleming’s right hand rule?
Magnetic Field
Same as flemings left hand rule
What does your thumb represent in Fleming’s right hand rule?
Direction of Movement of the wire
thuMb - Movement
What does your middle finger represent in Fleming’s right hand rule?
Current
Why is a current induced in the generator effect?
The conductor is experiencing a changing magnetic field and it “cutting” field lines
What is magnetic flux?
The product of the component of the magnetic flux density perpendicular to the area and the cross sectional area
What is the equation for Magnetic flux (Φ)?
Φ = BA cosθ
θ is measured to the normal of the surface of the area
Where is θ measured in the equation for magnetic flux?
To the normal of the surface
not to the surface!!!
What is the unit for Magnetic flux?
Weber (Wb)
What is one Weber?
The magnetic flux when a field of magnetic flux density of one tesla passes at right angles through a coil of area one metre squared
Find the Magnetic flux for a b and c where the area is 1 m² for each of them
a) 0.045 Wb
b) 0.0345 Wb
c) 0 Wb
What is Magnetic flux linkage?
The product of the number of turns in a coil and the magnetic flux
What are the units for magnetic flux linkage?
Weber Turns
What is the equation for magnetic flux linkage?
magnetic flux linkage = NΦ
What is Faradays law of electromagnetic induction?
The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux linkage
What is the equation for Faraday’s law of electromagnetic induction?
ε ∝ Δ(NΦ)/Δt
What is Lenz’s law?
The direction of any induced emf or current is always in a direction that opposes the change that is producing it
What happens when you combine Faraday’s law and Lenz’s law?
ε = -Δ(NΦ)/Δt
Draw a graph for the flux linkage and induced emf as a coil rotates in a magnetic field (alternating current generator)
When is the induced emf at a maximum?
- When the rate of flux linkage change is at a maximum
- When the flux linkage is 0
When is the induced emf 0?
- When the rate of flux linkage change is 0
- When the flux linkage is at a maximum
What is the circuit symbol for a transformer?
What is the structure of a transformer?
Explain how a transformer works
- When there is an alternating current in the primary coil, it causes the iron core to be rapidly magnetised and demagnetised. This rapid change in fux means that there is an equally fast change in flux in the secondary coil which induces an emf in the coil which is equal and opposite in the direction to the change in flux causing it
What happens if you increase the frequency of the ac current in the primary coil of a transformer?
You increase the rate at which the iron core is magnetised and demagnetised. This increase in the rate of the change of flux causes a larger emf to be induced in the secondary coil
What is the turn ratio equation (for emf)?
n₂ / n₁ = v₂ / v₁
For an ideal transformer
What is a step up transformer?
A transformer with more turns on the secondary coil than on the primary coil. It has a higher output voltage than input voltage.
Steps up the output pd
What is a step down transformer?
A transformer with fewer turns on the secondary coil than on the primary coil. It has a lower output voltage than input voltage.
Steps down the output pd
What is a 100% efficient transformer?
A transformer where the output power from the secondary coil is equal to the output power from the primary coil
Why might a transformer not be 100% efficient?
- Due to heating of primary and secondary coils
- Due to eddy current set up in the core of a transformer
What are eddy currents (in terms of transformers)?
Currents induced in the core of a transformer
Why are the coils in transformers laminated?
To ensure eddy currents aren’t induced in the core of the transformer
How can you reveal the magentic field lines around a bar magnet?
- Using iron filings
- Using a plotting compass
What does the magnetic field around a bar magnet look like?
Describe the magnetic field around the earth:
The magnetic field around the earth is the same as that of a bar magnet. However, the magnetic north pole is located at the geographic south pole and the magnetic south pole is located at the geographic north pole
Describe how the earth generates its magnetic field
The outer core of the earth contains molten iron and nickel. The movement of the molten iron and nickel (due to convection currents and the coriolis effect) generates an electric current and therefore induces a magnetic field around the earth.
Explain why the direction of the current (and emf) is reversed when the direction of the magnet is reversed
When the north pole of the magnet is pushed towards the coil, an emf is induced in coil, causing a current to flow and generating a magnetif field around the coil. This magnetic field is generated so that the end of the coil closest to the magnet is a north pole, meaning that work must be done in order to push the magnet towards the coil. The work done is equal to the electrical energy produced in the coil. Therefore when the direction of the motion of the magnet is reversed, the current and emf are reversed so that the coil now attracts the magnet and work is done to pull the magnet away from the coil. This is due to Lenz’s law: The direction of the induced emf or current is always as to oppose the cahnge producing it
Which end of the solenoid is the north pole?
The end where the current flows anticlockwise
Which end of the solenoid is the south pole?
The end where the current flows clockwise
Why does a transformer not work with a direct current?
There is no changing direction of the current and therefore the magnetic field will not reverse direction. As a result there is no changing magnetic flux and therefore no induction of emf
What is the equation for an ideal transformer?
V₁I₁ = V₂I₂
What is the turn ratio equation (for current)?
n₂ / n₁ = I₁ / I₂
for an ideal transformer
Why are transformers used in the national grid?
To step up the voltage and transfer the power from the power station at a high voltage. By doing this you minimise current and therefore the loss of power due to heating in the cables. The voltage is then stepped back down (and current up) when it reaches the consumer
What is the equation for power loss due to heating in cables
- Pₗ = I²R
- Pₗ = P₀²R / V²
Power loss is proportional to current squared
Power loss is inversly proportional to voltage squared
How does a bar magnet generate a magnetic field?
Magnetic materials are composed of tiny regions called magnetic domains, where the magnetic moments of electrons in individual atoms are aligned. In an unmagnetized state, these domains are randomly oriented, resulting in a net magnetic effect of zero. When a magnetic material is magnetized (by being exposed to an external magnetic field), these domains become aligned in a specific direction, creating a magnetic field
