June 2011 G485-Fields, Particles and Frontiers of Physics Watch

ChoYunEL
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#321
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(Original post by sulexk)
Right hand rule or left hand rule.

Let us unravel this problem with an example. Suppose an aircraft is flying east to west, its wings are an electrical conductor cutting across the earth's magnetic flux. In the northen hemisphere, which wingtip will become positively charged?
I'm going with the right wing becomes positively charged..

Edit - which happens to be incorrect... :s
Thumb is the motion the plane is moving, therefore the conventional current is moving towards the left wing, assuming the field is going towards the ground.
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sulexk
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(Original post by ChoYunEL)
I'm going with the right wing becomes positively charged..

Edit - which happens to be incorrect... :s
Thumb is the motion the plane is moving, therefore the conventional current is moving towards the left wing, assuming the field is going towards the ground.

Initially I was extremely confused!

I thought the right wing would become positive, but then I realised I was thinking about electron flow. If you just think about positive flow, it seems to work!

I think it helps to think about the positive charge only I guess?

But if does state electron direction in the question then we can use that information.
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sulexk
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(Original post by ChoYunEL)
I'm going with the right wing becomes positively charged..

Edit - which happens to be incorrect... :s
Thumb is the motion the plane is moving, therefore the conventional current is moving towards the left wing, assuming the field is going towards the ground.

It also works with a coil rotating in a magnetic field. Imagine that rectangular coil rotating clockwise in a magnetic field. If you use the idea of positive charge moving you can work out which terminal shall beocme positively charged.
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Ralphus J
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(Original post by Ralphus J)
Do we need to know anything about the actual ultrasound transducer? Such as dampening and what not, its not on the specification list

Whilst im on the topic of ultrasound can someone help me please with (i) explain qualitatively how the doppler effect can be used to determine the speed of blood.

ASAP would be really great
Bumpedy bump bump?
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sulexk
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It makes sense, as we are dealing with conventional current when dealing with flemings right and left hand rules.
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ChoYunEL
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(Original post by sulexk)
It makes sense, as we are dealing with conventional current when dealing with flemings right and left hand rules.
I used right hand rule since it is induced conventional current and therefore, current is flowing towards the left wing.
Which means it's negatively charged?

Edit - Oh wait! Nevermind ¬_¬ bad moment there...
All the electrons will be flowing towards the right wing therefore the right wing is negatively charged... Doh
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Oh my Ms. Coffey
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So we use RHR for conventional and LHR for normal current?
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sulexk
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(Original post by Ralphus J)
Bumpedy bump bump?
The Doppler effect can be used to measure the speed at which blood flows through veins or arteries. In this technique a beam of ultrasound is directed towards flowing blood. When the beam is reflected off the moving blood its frequency is altered. The difference between the emitted and reflected frequencies can be used to calculate the speed of blood flow.


Hope this helps!!
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sulexk
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(Original post by Oh my Ms. Coffey)
So we use RHR for conventional and LHR for normal current?
Both use conventional current

But if the question gives direction of electron flow then in order to determine the effect on conventional current we have to reverse the motion or force.
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ChoYunEL
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If you're trying to find out which way a generator is rotating...
You already have a current, you already have a field, therefore the left hand's thumb points in the direction the motor/turbine is moving.
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sulexk
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(Original post by ChoYunEL)
If you're trying to find out which way a generator is rotating...
You already have a current, you already have a field, therefore the left hand's thumb points in the direction the motor/turbine is moving.
But actually for the generator, you are producing the current, so you have motion and magnetic field, so would you use the right hand rule instead?
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Ralphus J
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(Original post by sulexk)
The Doppler effect can be used to measure the speed at which blood flows through veins or arteries. In this technique a beam of ultrasound is directed towards flowing blood. When the beam is reflected off the moving blood its frequency is altered. The difference between the emitted and reflected frequencies can be used to calculate the speed of blood flow.


Hope this helps!!
Thanks is this all we really need to know, my book goes into a ridicuous amount of detail?:eek:
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ChoYunEL
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(Original post by sulexk)
But actually for the generator, you are producing the current, so you have motion and magnetic field, so would you use the right hand rule instead?
Question 3 ai) of the OCR 2824 Jan06 paper, it asks for the direction of flow of liquid.
In the Physics OCR A2 text book, page 122.
I used left hand rule for that particular question.

There is no induction and flow is already there.
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M_I
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Just to make sure I've got this right:

You can have three outcomes of a star

1. Very low mass stars will stop after hydrogen shell burning and will fade away.

2. A low mass to medium mass star will become a white dwarf and fade away. Also do these stars undergo shell helium burning or not?

3. High mass stars will end up as either a Neutron star or a Black Hole.

Is that correct?
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ChoYunEL
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(Original post by M_I)
Just to make sure I've got this right:

You can have three outcomes of a star

1. Very low mass stars will stop after hydrogen shell burning and will fade away.

2. A low mass to medium mass star will become a white dwarf and fade away.

3. High mass stars will end up as either a Neutron star or a Black Hole.

Is that correct?

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Oh my Ms. Coffey
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(Original post by M_I)
Just to make sure I've got this right:

You can have three outcomes of a star

1. Very low mass stars will stop after hydrogen shell burning and will fade away.

2. A low mass to medium mass star will become a white dwarf and fade away.

3. High mass stars will end up as either a Neutron star or a Black Hole.

Is that correct?
I thought Low/medium mass stars become white dwarfs too.

Super nova -> a Neutron star or if the star is big enough a black hole.


Even bigger stars become Black holes that release Gamma ray bursts and you can get Magnetars also, but I wouldnt write that.
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Ralphus J
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(Original post by M_I)
Just to make sure I've got this right:

You can have three outcomes of a star

1. Very low mass stars will stop after hydrogen shell burning and will fade away.

2. A low mass to medium mass star will become a white dwarf and fade away. Also do these stars undergo shell helium burning or not?

3. High mass stars will end up as either a Neutron star or a Black Hole.

Is that correct?
I dont know im about to do this, but i thought id let you know that i think this was the 6 marker in january so it shouldn't come up. Even so im going to do it.
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M_I
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(Original post by ChoYunEL)

(Original post by Oh my Ms. Coffey)
I thought Low/medium mass stars become white dwarfs too.

Super nova -> a Neutron star or if the star is big enough a black hole.


Even bigger stars become Black holes that release Gamma ray bursts and you can get Magnetars also, but I wouldnt write that.
OK I didn't even know what a black dwarf star was and I read about the very low mass star in the CGP book, it was in brackets and small print thought).

Also do stars that become white dwarfs undergo shell helium burning or not?
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m92singh
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does anyone have the key definition for this unit? please post thanks
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ChoYunEL
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G485

Electric field strength: The electric field strength at a point is the force per unit charge exerted on a positive charge placed at that point.
Magnetic flux density: Defined by the equation: ? = (F/I*L), where ? is the magnetic flux density, F is the forces experienced by a conductor carrying a current I and L is the length of the conductor.
Tesla: 1 Tesla is the magnetic flux density when a wire carrying a current of 1 Amp, is placed at right angles to the magnetic field, experiences a force of 1 N per metre of its length.
Magnetic flux: The magnetic flux, ?, through an area, A, is defined as: ?=?*A where
? is the component of the magnetic flux density perpendicular to the area A.
Weber: 1 Weber is equal to 1 Tesla metre squared, 1Wb = 1Tm2
Magnetic flux linkage: The product of the magnetic flux and the number of turns in a coil: Magnetic flux linkage = ?*A*N or ?*A*N*(cos ?) where ? is the magnetic flux density, A is the cross sectional area of a coil and N is the number of coils.
Capacitance: The capacitance of a capacitor is the charge stored per unit of potential difference across it.
Farad: 1 Farad is 1 Coulomb per Volt. 1F = 1CV-1
Proton Number: The proton number (or atomic number) is the number of protons in a nucleus. Symbol = Z
Nucleon number: The nucleon number (or mass number) is the number of protons and neutrons in a nucleus. Symbol = A
Isotopes: Isotopes are nuclei of the same element with a different number of neutrons but the same number of protons. They have the same proton number but a different nucleon number.
Binding energy: The minimum energy needed to pull a nucleus apart into its separate nucleons.
Binding energy per nucleon: The minimum energy needed to pull an individual nucleon from a nucleus. Calculated by dividing the Binding Energy by the nucleon number.
Activity: The activity, A, of a radioactive sample is the rate at which nuclei decay or disintegrate. Unit = Becquerel (Bq). 1 Bq = 1 s-1
Decay constant: The probability that an individual nucleus will decay per unit time interval. Symbol = ? and units are s-1, min-1, day-1 or any unit of time raised to power minus 1.
Half-life: The half life, t ½, of a radioisotope is the mean time taken for half the active (or undecayed) nuclei in a sample to decay.
Intensity: Intensity is the Power per unit cross-sectional Area.
Critical density: The density of the universe that will give rise to a flat universe, given by the equation: ?0 = (3*H02 / 8?G), where H0 is the Hubble constant and G is the Gravitational constant.
Astronomical Unit (AU): The average distance of the Earth from the sun.
Light-year (ly): The distance travelled by light through a vacuum in one year.
Parsec(Pc): The distance that gives a parallax angle of 1 arc second. Often a diagram is needed here, please refer to your book for this.

These definitions are used by the exam board and you must be able to quote them word for word.

Descriptions of the following experiments, among others, may be useful:

• Investigating Coulombs Law.
• Comparing Gravitational and Electric Fields.
• Flemings Left Hand Rule
• How a mass spectrometer works.
• How transformer work.
• How capacitors charge and discharge.
• The alpha scattering experiment.
• How nuclear power works.
• How to differentiate between different forms of radioactivity.
• How to measure the Half-life of a radioisotope.
• How Carbon Dating works.
• How X-Rays are generated.
• How to improve the contrast of X-Rays.
• How CAT Scanning works
• Contrast and Compare various diagnostic methods e.g. ultrasound and X-Rays.
• How the gamma camera works.
• How PET scanning works.
• How MRI scanning works and the advantages and disadvantages of this.
• How to produce Ultrasound waves – the piezoelectric effect.
• The difference between A and B Scans.
• The basis of the Cosmological principle.
• The life and deaths of stars.
• Describe Oblers’ Paradox.
• Describe red shift.
• How to convert between parsecs and SI units.
• The evidence for the Hot Big Bang model of the universe.
• The three fates of the universe.
• Ensure that you can use X=X0 e?t.

Please note that you have to be very precise when you answer long answer questions. Look through past exam papers and mark schemes to gain a sense of what you need to write.
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