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Physics Flashcards

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1
Q

What is a magnetic field?

A

The region around a magnet in which a piece of iron or steel will be attracted to it.

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2
Q

What are lines of force or magnetic field lines?

A

Iron fillings placed near a magnet will form a pattern of lines that loop from one pole to the other.
These loops are the lines of force.

A plotting compass places placed in the magnetic field will always point along the field line.

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3
Q

What happens if two magnets are brought near each other?

A

Like poles will repel however unlike poles will attract each other.

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4
Q

What are electromagnets?

A

A magnet made by wrapping insulated wire around a piece of iron (called the core).

When a current flows through a wire, a magnetic field is produced around the wire.
When a current flows through the wire the iron becomes strongly magnetised.
When the current is switched off the iron loses it’s magnetism. This temporary magnetism makes electromagnets very useful.

Uses include scrapyard cranes, circuit breakers, electric bells and relays.

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5
Q

How do electromagnets work?

A

An insulated wire is wrapped around a piece of iron

When a current flows through a wire, a magnetic field is produced around the wire and the iron becomes strongly magnetised.

When the current is switched off the iron loses it’s magnetism.

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6
Q

What’s the motor effect?

A

The force experienced when a wire carrying an electric current is placed in a magnetic field.

The force is a maximum if the wire is at an angle of 90° to the magnetic field and zero if the wire is parallel to the magnetic field.

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7
Q

What’s Flemings left hand rule?

A

The rule to determine the direction of the force. (The thumb and first two fingers are held at right angles to each other)

  • the first finger represents the magnetic field (north to south)
  • the second finger represents the current (pointing positive to negative)
  • the thumb represents the direction of the force/movement
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8
Q

What increases the size of the force?

A

Increasing the strength of the magnetic field

Increasing the size of the current

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9
Q

How does an electric motor work?

A

It has a coil which turns when a current is passed through it.
When a current passes through the coil, it spins because
- a force acts on each side of the coil due to the motor effect
- the force on one side of the coil is in the opposite direction to the force on the other side.

The speed of the motor is increased by increasing size of the current.
The direction of the motor can also be reversed by reversing the direction of the current.

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10
Q

What does a split-ring commutator do?

A

It reverses the direction of the current around the coil every half-turn.

Because the sides swap over each half-turn the coil is always pushed in the same direction.

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11
Q

What is electromagnetic induction?

A

The process of creating a potential difference using a magnetic field.

When a potential difference is induced across the ends of the coil due to the movement of a magnet into the coil.

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12
Q

How is a pd potential difference induced?

A

If an electrical conductor ‘cuts’ through magnetic field lines, a pd is induced across the ends of the conductor.

A pd is only induced if there is movement - if direction of movement of the wire or coil is reversed or the polarity is reversed, the direction of the induced or is also reversed.

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13
Q

What increases size of the induced potential difference within electromagnetic induction?

A

The speed of movement
The strength of the magnetic field
The number of turns on the coil

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14
Q

What does a transformer consist of?

A

Two coils of insulated wire - the primary and secondary coil.
These are wound on to the same iron core.
When an alternating current passes through the primary coil, it produces an alternating magnetic field in the core. This field continually expands and collapses.

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15
Q

How does a transformer work?

A

Two coils of insulated wire are wound on to the same iron core (primary and secondary coil.)

When an alternating current passes through the primary coil, it produces an alternating magnetic field in the core. This field continually expands and collapses.

The alternating magnetic field lines lass through the secondary coil and induce an alternating potential difference across its ends. - if the secondary coil is part of a complete circuit and alternating current is produced.

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16
Q

Why are the coils of wire used for transformers insulated?

A

So the current doesn’t short across either the iron core or adjacent turns of wire, but flows around the whole coil.
The core is made of iron so is easily magnetised.

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17
Q

How are transformers used in the national grid?

A

Step up - makes pd in the secondary coil greater than that of the primary coil. It’s secondary coil has more turns than the primary.

Step down - makes pd in the secondary coil LESS than that of the primary coil. It’s secondary coil has more turns than it’s primary.

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18
Q

What’s a switch mode transformer?

A

It’s lighter and smaller than an ordinary transformer and operates at high frequency.

It also uses very little power when there’s no device connected across its output terminals

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19
Q

What happens if a 1.5V cell is used as the supply for the primary coil?

A

The transformer will not work as it requires an ac supply.

A cell supplies dc.

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20
Q

What is the equation to relate the pd across and the number of turns on the primary and secondary coil?

A

Vp / Vs = np / ns

Vp = pd across primary coil (volts)
Vs = pd across secondary coil (volts)
np = number of turns on the primary coil
ns = number of turns on the secondary coil
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21
Q

How efficient are transformers?

A

Almost 100%

For 100%, VpIp = VsIs

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22
Q

A transformer has 100 turns on the primary coil and 400 on the secondary coil.
The pd across the primary coil is 2V. What is the pd across the secondary coil?

A

8 V

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23
Q

How can physics and potential difference apply to hospitals (for both diagnosis and therapy)?

A

ECG or electrocardiogra, is used to measure the pd generated by the heart.

Electronic devices measure blood pressure

Digital thermometers are used to measure temperature

An endoscope containing bundles of fibre optics is used to look inside the body without making large incisions.

X-rays are used to take pictures of suspected broken bones.

CT scanners are used to build up digital pictures of a cross-section through the body

MR scanners use radio waves to produce detailed pictures of the body.

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24
Q

What is a moment?

A

The turning effect of a force

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25
Q

What is the equation to identify the size of the moment?

A

M = F * d

Moment (Nm) = force (N) * perpendicular distance from the line of action of the force to the pivot (m)

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26
Q

How can a moment be increased?

A

Increase the force

Distance to the pivot must increase

It’s easier to under a wheel-nut by pushing on the end of a long spanner than a short one as the long spanner increases the distance between the pivot and the line of action of the force b

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27
Q

What is the load?

A

The force we are trying to move.

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28
Q

What is effort?

A

The force applied to the lever in a moment.

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29
Q

What is the centre of mass?

A

The centre of mass of an object is the point where it’s mass can be thought to be concentrated at a single point.

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30
Q

What is equilibrium?

Physics

A

The state of an object when at rest

Any object that is freely suspended will come to rest with its centre is mass directly below the point of suspension. The object is then in equilibrium.

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31
Q

How can the centre of mass of a thin irregular sheet of material be found?

A

Suspend the thin sheet from a pin held in a clamp stand. Because it is freely suspended it is able to turn.

When it comes to rest have a plumbline (string with a small weight on the end) from the same pin

Mark the position of the plumbline against the sheet

Hang the sheet with the pin at another point and repeat the procedure

The centre of mass is the weather lines that marks the precision of the plumbline cross

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32
Q

What is the position of the centre of mass depend on?

A

The shape of the object
It sometimes lies outside the object

For a symmetrical object, it’s centre of mass is along the axis of symmetry.
If the object has more than one axis of symmetry,the centre of mass is where the axes of symmetry meet

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33
Q

How do moments in balance work?

A

If an object is in equilibrium, it is balanced and not turning.

We can take the moments about any point and you’ll find that the total clockwise moment and the total anticlockwise moment are equal, seen daily e.g. By seesaws and balance scales.

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34
Q

How do you calculate the force needed to stop an object turning?

A

For an object in equilibrium,

The sum of the anticlockwise moments about any point = the sum of the clockwise moments about that point.

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35
Q

What is the resultant moment?

A

If the line of action of a weight lies outside the base of an object, there will be a resultant moment and the object will tend to topple over.

An object topples over is the resultant moment about its point of turning is zero.

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36
Q

How is stability of an object increased?

A

The wider the base of an object and the lower it’s centre of mass, the further it has to tilt before the life of action of the weight moves outside the base.

Therefore the stability is increased by making its base wider and it’s centre of mass lower.

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37
Q

What is the equation in order to work out pressure?

A

P = F / A

Pressure (Pa or N/m2) = force (N) / cross sectional area at right angles to the direction of the force (m2)

38
Q

How do hydraulic pressure systems work?

A

They use the pressure in a fluid to exert a force

Liquids are virtually incompressible and the pressure in a liquid is transmitted equally in all directions.
This means that a force exerted at one point on a liquid will be transmitted to other points in the liquid.

39
Q

What does the force exerted by a hydraulic pressure system depend on?

A

The force exerted on the system

Area of the cylinder on which the force acts

Area of the cylinder that exerts the force

40
Q

What are hydraulics usually used for?

A

They’re usually used as a force multiplier so that a small effort can be used to move a large load.
(So force produced should be more than the effort applied).

41
Q

What is centripetal acceleration?

A

The acceleration towards the centre of the circle of an object that is moving around in a circle.

When an object moves in a circle it is continually changing direction, so it is continuously changing velocity / it is accelerating - this acceleration is known as centripetal acceleration

42
Q

What is the centripetal force?

A

The force acting upon an object which is only accelerating.

If this force stops acting, the object will continue to move in a straight line at a tangent to the circle.

43
Q

What increases the centripetal force to make an object perform circular motion?

A

Increased mass of the object

Increased speed of the object

Decreased radius of the circle

44
Q

A student is whirling a cooker around on a piece of string in a horizontal circle. What force provides the centripetal force?

A

Tension in the string

Should the string break, the cooker will fly off at a tangent to the circle.

45
Q

What is an example of an oscillating motion?

A

A pendulum moving to and fro along the same line.

46
Q

What does the time period of a simple pendulum depend on?

A

It’s length (only)

47
Q

What is a simple pendulum?

A

A pendulum consisting of a mass (the bob) suspended on the end of a string.
When the bob is displaced to one side and let go. The pendulum oscillates back and forth through the equilibrium position.

(Equilibrium position is the position of the pendulum when it stops moving)

48
Q

What’s the amplitude of an oscillation of a pendulum?

A

The distance from the equilibrium to the highest position on either side.

49
Q

What’s the time period of a pendulum oscillation?

A

Time taken for a complete cycle:

  • time from the highest position on one side to the highest on the other and back or
  • time between successive passes in the same direction through equilibrium position

We measure the average time for 20 oscillations and divide by 20

50
Q

What is the frequency of pendulum oscillations and what equation relates it to time period?

A

Frequency of oscillations is the number of complete cycles of oscillation per second

T = 1/f

Time (s) = 1 / frequency (Hz)

51
Q

What is the time period of a pendulum that completes 20 oscillations in 5.0 seconds?

A

20 / 5 = 4.0 oscillations in 1 second so frequency = 4Hz

T = 1/f
T = 1 / 4
T = 0.25

0.25 seconds.

52
Q

What are the ends of a magnet called?

A

Magnetic poles (there’s a north and south)

53
Q

What are X-rays?

A

They’re part of the magnetic spectrum.
They have a high frequency and a very short wavelength.
They’re used in hospitals to make images and CT scans and to destroy tumours at or near the surface of the body.

They can damage living tissue when they pass through it.

They’re used to form images of bones on photographic film.

54
Q

What are properties of X-rays?

A

They affect a photographic film in the same way as light.

They are absorbed by metal and bone and some soft tissue.

They are transmitted by healthy tissue.

They cause ionisation and can damage living tissue when they pass through it so precautions must be taken when using them. (Workers wear film badges and lead screens for protection).

They may be used for therapy e.g. To treat cancerous tumours at or near the body surface.

55
Q

What are CCDs and CT scanners?

A

Charge-coupled devices (CCDs) are used to form electromagnetic images of X-rays.

CT scanners use X-rays to produce digital images of a cross-section through the body.

Some body organs made of soft tissues e.g. The intestines can be filled with a contrast medium that absorbs X-rays so that they can be seen on an X-ray image.

56
Q

What are ultrasound waves?

A

Waves which are above 20,000Hz.

It can be used for diagnosis and treatment.

Electronic systems can produce ultrasound waves. When a wave meets a boundary between two materials, part of the wave is reflected.
The wave travels back through the material to the detector.
The time it takes to reach the detector can be used to calculate how far away the boundary is. The results may be processed by a computer to give an image.

The scan is non-ionising so is safer than X-rays e.g. For scanning unborn babies and soft tissue. It may be used in therapy e.g. To shatter kidney stones into small pieces.

57
Q

How do you calculate the distance travelled by an ultrasound pulse?

A

s = v * t

S is the distance traveled in metres
V is the speed of the ultrasound wave in metres per second
T is the time taken in seconds

58
Q

What’s refraction?

A

The change of direction of light as it passes from one transparent substance into another.

This takes place because waves change speed when they cross a boundary.
The change in speed causes a change in direction unless the waves are travelling along a normal.

59
Q

What is the refractive index?

A

The refractive index, n, is a measure of how much a substance can refract a light ray.

60
Q

How do you calculate the refractive index?

A

N = sin i / sin r

N is the refractive substance of the substance
sin i is the sin of the angle of incidence
sin r is the sin of the angle of refraction

61
Q

What is the critical angle?

A

It is the angle of incidence of a light ray in a transparent substance which produces refraction along the boundary.

62
Q

What equation relates the critical angle to the refractive index?

A

n = 1 / sin C

N is the refractive index
C is the critical angle

63
Q

How does total internal reflection occur?

A

When the angle of incidence of a light ray in a transparent substance is greater than the critical angle or when the angle of reflection is equal to the angle of incidence.

64
Q

What’s an endoscope?

A

A device used to look inside a a patient’s bodywork out cutting it open or when performing keyhole surgery.
The endoscope contains bundles of optical fibres. These are very thin, flexible glass fibres.
Visible light can be sent along the fibres by internal reflection.

Laser light may be used in an endoscope to carry out some surgical procedures e.g. Cutting, cauterising and burning.
The colour of the laser light is matched to the type of tissue to produce maximum absorption.

Eye surgery on the retina can be carried out by using a laser light that passes straight through the cornea at the front of the eye but is absorbed by the retina at the back.

65
Q

What does a converging lens do?

A

It focuses parallel rays to a point called the principal focus.
Parallel rays of light that lass through a converging (convex) lens are refracted so that they converge to a point.

Because light can pass through the lens in either direction, there is a principal focus on either side.

A camera uses a converging lens to form a real image of the object on a film or array of CCDs.

A magnifying glass is a converging lens that is used to form a virtual image of an object.

66
Q

What is the focal length?

A

The focal length is the distance from the centre of the lens to the principal focus

67
Q

What’s the principal focus (focal point)?

A

The point where light rays parallel to the principal axis of a lens are focused (or, in the case of a diverging lens, appear to diverge from).

68
Q

How does the distance of an object from a converging lens affect the image produced?

A

In an object is further away from the lens in either direction, there is a principal focus on either side of the lens.

Of the object is nearer to the lens than the principal focus, an upright, virtual image if formed behind the object.
The image is magnified - the lens acts as a magnifying glass.

69
Q

How is magnification calculated?

A

Magnification = image height / object height

70
Q

What does a diverging lens do?

A

A diverging lens makes parallel rays spread out as if they came from a point called the principal focus.

Parallel rays of light that pass through a diverging (concave) lens are refracted so that they diverge away from a point (the principal focus).

Because light can pass through the lens in either direction, there is a principal focus on either side.

The image produced by a diverging (concave) lens is always virtual.

71
Q

What symbols represent converging and diverging lenses?

A

Converging:

Diverging:
>——-

72
Q

What’s a ray diagram?

A

A ray diagram can be drawn to find the position and nature of an image formed by a lens.

The line through the centre of the lens and at right angles to it is called the principal axis.

73
Q

What does a ray diagram consist of?

A

3 construction rays from a single point on the object to locate the corresponding point on the image:

  • a ray parallel to the principal axis which is refracted through the principal focus
  • a ray through the centre of the lens which travels straight on without refraction
  • a ray through the principal focus which is refracted parallel to the principal axis
74
Q

Describe the image formed when an object is placed between a converging lens and F (focal length):

A 
The image formed is:
Virtual
Upright
Magnified
On the same side of the lens as the object
75
Q

How is the power of a lens calculated?

A

P = 1/f

P is power of the lens in dioptres D
f is the focal length of the lens in metres m

76
Q

How does light enter the eye?

A

Through the cornea. The cornea and the eye lens focus the light on to the retina.
The iris adjusts the size of the pupil to control the amount of light entering the eye.
The ciliary muscles later the thickness of the lens to control the fine focusing of the eye.
They are attached to the lens by the suspensory ligaments.
The optic nerve carries impulses from the retina to the brain

77
Q

What is the normal near point of the human eye?

A

25 cm. It has a far point of infinity so it’s range of vision if from 25 cm to infinity.

78
Q

What does it mean to be short sighted?

A

A person with short sight can see close objects clearly but distant objects are blurred because the uncorrected image is formed in front of the retina.

79
Q

What causes short sightedness?

A

By the eyeball being too long or the eye lens being too powerful.

It can be corrected using a diverging lens.

80
Q

What does it mean to be long sighted?

A

You can see distant objects clearly but close objects are blurred because the uncorrected image is formed behind the retina.

81
Q

What causes long sightedness?

A

The eyeball being to short or the eye lens being too weak.

It can be corrected using a converging lens.

82
Q

What determines the focal length of a lens?

A
  • the refractive index of the material from which the lens is made
  • the curvature of the two surfaces of the lens

The higher the refractive index of the glass used to make the corrective lens, the flatter and thinner the lens can be.

83
Q

What causes a resultant moment within stability?

A

If the line of action of the weight lies outside the base of an object, there will be a resultant moment and the object will tend to topple over.

The wider the base and lower the centre of mass of an object, the further is has to tilt before the line of action of the weight moves outside the base so stability is increased.

84
Q

Give similarities between an eye and a camera

A

• Both use a converging
lens
• Image formed is real
• Image is inverted
• Image in eye formed on retina, image in camera formed on film / CCDs
• Amount of light entering eye and camera can be controlled

85
Q

Turning the ignition key closes the ignition switch. Explain how this causes the starter motor to operate.

A

current flows through the coil / electromagnet and a magnetic field is produced.
The (short side of the) iron bar is attracted to the electromagnet and the contacts are pushed together (by iron bar)
The starter motor circuit is completed and current flows through starter motor

86
Q

Explain the advantage of a CT scan compared to an X-ray.

A

A CT scan gives a 3D image therefore the image can be observed from different directions

87
Q

Explain how a transformer works

A

An alternating input / current is supplied to the primary (coil)
This produces an alternating magnetic field
in the (iron) core.
This magnetic field links with the secondary coil
which induces an (alternating) voltage / p.d. across the secondary (coil)

88
Q

The wavelength of an X-ray is about the same as the diameter of ………………………………….

A

An atom

89
Q

What type of transformer is used in phone chargers?

A

A switch mode transformers.

90
Q

How do switch mode transformers differ from traditional transformers?

A 
Lighter
Smaller
Operates at a higher frequency
Uses very little power (when there is no device connected across its output terminal) 
More efficient

Further Additional Science (3 decks)

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