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Physics > Magentism > Flashcards

Magentism Flashcards

1
Q

What is a magnetic field?

A

A magnetic field is a force field surrounding a magnet or current carrying wire and acts on any other magnet or conductor placed in the field.

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2
Q

In a bar magnet where is the field the strongest?

A

It is strongest at the end of the poles as the lines are the closest and the most concentrated.

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3
Q

What is the definition of magnetic field line or line of force?

A

This is the path which a north pole would take if placed in the magnetic field.

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4
Q

How are the poles of a magnet determined?

A

When a magnet is free to align with the earth’s horizontal component of its magnetic field whichever direction it points in is the pole.

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5
Q

What is the motor effect?

A

This is when a magnetic field exerts a force on a current carrying conductor placed in the field.The direction of the force is perpendicular to the wire and the field lines.

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6
Q

How to calc size of force on wire in magnetic field?

A

Use F=BIL B is flux density,I is current and L is length of conductor in the field.Can add sin theta which represents angle between wire and field lines.B in Tesla or Nam^-1.

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7
Q

When is the force on a wire max and lowest?

A

Max when theta equals 90 and 0 when theta equals zero or parallel.

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8
Q

What does the force do to the wire?

A

It pushes it out of the field.

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9
Q

How can you reverse direction of force?

A

Reverse current or magnetic field direction.

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10
Q

What condition must be met so you can use F=BIL?

A

B and I must be perpendicular.

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11
Q

What is the definition of magnetic flux density?

A

It is the strength of the magnetic field.It is defined as the force per unit length per unit current on a current carrying conductor in a magnetic field.

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12
Q

Why does a current carrying wire in a magnetic field feel a force?

A

It feels a force because the electrons in the wire are pushed towards one side.

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13
Q

What is the formula for the force on a charged particle in a magnetic field?

A

F=BQv and can be expanded to involve sin theta.This is if the moving particle is at 90 degrees to the field lines.

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14
Q

When using left hand rule to charged negative particles what do you have to be vary of?

A

The direction of the current is in the opposite direction to the motion of the particles.

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15
Q

Why are charged particles deflected in a circular motion?

A

This is because the magnetic force is always perpendicular to the velocity of the particle.This means no work is done and the force acts towards the centre causing a centripetal acceleration.Instead the force changes the direction of the velocity only.

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16
Q

Describe how a cyclotron works?

A

Consists of two hollow semicircles called dees.A uniform magnetic field is applied perpendicular to the planes of the dees.An alternating voltage is applied in between the dees.A particles enters a dee from the centre and follows a semicircular path as a force is being exerted on it.The voltage is reversed and the particle is attracted towards the other.This cycle continues to happen until the radius of the particles orbit is equal to the radius of the dees.At this point the particle exits the accelerator.Everytime the particle crosses the dees its speed and radius increase.Since frequency is independant of speed then the particle will spend same amount of time travelling in the dee.A dee is an electrode.

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17
Q

What is electromagnetic induction?

A

Electromagnetic induction occurs when there is relative motion between a magnet and a wire.Moving either of these two relative to the other will induce an emf across the wire.If the circuit is complete a current will also be induced.This is because a wire cutting magnetic field lines gets a force exerted on it which pushes its electrons to one side.

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18
Q

How can you increases the size of the induced emf?

A

Move the magnet or wire faster,use a stronger magnet or use a wire with more coils.

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19
Q

When in no emf induced?

A

This occurs when the wire is parallel to the field lines and doesn’t cut across them.

20
Q

Is the induced emf constant?

A

No it is not constant,this is because the induced emf ceases when there is zero relative motion between the magnet and wire.

21
Q

What is Lenz’s law?

A

It states that the direction of the induced current is always such to oppose the change that created the current.

22
Q

What happens when a magnet is pushed into a coil and a current is induced?

A

The induced current creates its own magnetic field that opposes the field of the magnet.This means if an n pole is pushed in,the current will create an n pole to oppose this magnet.This is because energy must be conserved and if the field of the current was a south pole this would increase the induced current.

23
Q

What is the formula and definition for magnetic flux?

A

Phi=BxACostheta B is field strength and A is area swept by the conductor.Magnetic flux is the total magnetic field passing through a given area.Magnetic flux is also the number of field lines.

24
Q

What is magnetic flux linkage and what is its formula?

A

It is the product of the magnetic flux and number of turns of a conductor.This represents the magnetic field through each turn.It is found by BANcostheta and its unit is weber.Cos Theta is the angle between the normal to the plane of the coil and the field lines.

25
Q

What is Faraday’s law of em induction?

A

It states that the induced emf in a ciruit is equal to the rate of change of magnetic flux linkage in that cirucit. and its negative due to the opposite direction.

26
Q

How can induced emf be related to velocity?

A

e=BLV.

27
Q

What is an alternator?

A

It is a generator of alternating current.It converts KE into electrical energy.It contains slip rings and brushes.It induces a current by rotating a coil in a magnetic field.The output voltage and current change direction with every half turn.

28
Q

What is the formula for changing flux linkage in an AC generator?

A

Nphi=BANxcos(omegat).

29
Q

What is the formula for the induced emf in an AC generator?

A

e=BANxomegaxsin(omegat).

30
Q

What does a motor do?

A

It converts electrical energy into kinectic energy.

31
Q

What is back emf?

A

This is emf induced in the spinning coil of a motor as it is experiencing a change in its flux linkage.It acts in the opposite direction to the pd applied to the motor in accordance with lenz’s law.V-e=IR I is current through coil,R is circuit resistance and V is pd applied to coil.

32
Q

What changes with the speed of rotation of the coil?

A

The emf is proportional to the speed of rotation of the coil.As the speed rises current falls.

33
Q

What is the peak to peak value?

A

Its the difference between the two max peak values of current in opposite directions.Twice the peak value.

34
Q

What is an alternating current?

A

It is a current that changes direction over time.

35
Q

What is the rms voltage?

A

It is the square root of the mean of the voltages squared in once complete cycle.It is the equivalent direct current that produces the same heating effect.Constant voltage.

36
Q

How can you make a wire vibrate?

A

If you place it in a magnetic field and alternate the current the force direction will keep reversing making the wire vibrate.

37
Q

Describe an experiment to how force varies with current?

A

Pass a length of wire through two magnets which are clamped above a top pan balance.The wire should also pass through a wooden slab on the top pan balance.Connect the wire to a dc supply using crocodile and variable resistor.Vary current and measure the reading on the top pan balance.Make sure to zero the balance in between readings.Plot a graph of F against I and you should see a straight line graph.Calc F using F=mg.

38
Q

What does the radius of circular motion depend on for a charged particle?

A

Its mass and velocity, increasing these 2 increase its radius and it is deflected less by the field.

39
Q

What is the frequency of roatation dependant on for charged circular motion?

A

Not on the velocity of the particle it depends on B charge and mass.

40
Q

What is the formula for magnetic flux?

A

Phi=BxA where A is area perpendicular to magnetic field

41
Q

Describe an experiment to measure the effect of varying the angle between the normal to plane and field lines?

A

Connect an Ac power supply and resistor to a stretched metal spring.Place a search coil into the spring and connect the coil to an oscillscope.Initally place the search coil so that the normal to the plane of the coil makes an angle of 0 degrees with the field lines.Slowly increase the angle of the search coil in 10 degrees intervals until the normal to the coil makes a degrees of 90 degrees with the field lines.Plot a graph of emf against cos theta.Should get a downwards sloping curve.Oscillscope should have time base off.

42
Q

What is a dynamo/genearator?

A

It converts ke into electrical energy.

43
Q

How do eddy currents arise and how do they cause heat loss?

A

They are looping currents that arise in the core of a transformer due to its changing magnetic field.The current flows on the core and causes heat loss as the current does work against the resistance of the magnetic material.Laminate to reduce them.They create their own magnetic field opposing the cores.

44
Q

How doe a transformer work?

A

AC voltage applied across primary coil creates alternating magnetic field.Induces magnetic field in core.This induces magnetic field in secondary coil.

45
Q

How to increase efficiency of transfomer?

A

Laminate core
Low resistance wires
Close spacing of coils so flux can pass through both.
Soft magnetic material so not much energy is required to magnetise and de magnatise.

46
Q

Formula for transformers?

A

Np/Ns=Ip/Is=Vp/Vs.

Physics (19 decks)

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