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Science 3 > Physics b > Flashcards

Physics b Flashcards

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0
Q

What is the formula for a moment ?

A

Force x perpendicular distance from the line of action of the force of the pivot

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1
Q

What is a moment ?

A

The tuning effect of a force

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2
Q

Describe moments using a spanner as an example

A

A spanner can pivot around a nut. The force acting on the spanner causes a turning effect on the nut. The moment is the turning effect

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3
Q

How does a larger force affect the moment ?

A

The more force the more the spanner will turn, therefore a larger moment

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4
Q

How does a longer spanner affect the moment ?

A

If the same amount of force is exerted on a longer spanner the moment will be greater.

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5
Q

In which direction should the force be to maximise the moment ?

A

Perpendicular to the spanner. This maximises the distance which increases the moment

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6
Q

What is the centre of mass ?

A

The point at which the whole mass of the object is concentrated

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7
Q

If you suspend and object at a random point, what will happen ?

A

The object will swing until it’s centre of mass is vertically below the point of suspension

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8
Q

How do you find the centre of mass of any shape ?

A
  1. Suspend the shape and a plumb line from the same point
  2. Draw along the plumb line
  3. Repeat the process, but at a different pivot point
  4. The centre of mass is where the two lines cross
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9
Q

What is an easier way of finding the centre of mass on symmetrical shapes ?

A

Draw the lines of symmetry on the shape. The centre of mass is where the lines cross

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10
Q

What is needed for a seesaw to balance ?

A

The moments on each side of the pivot need to be equal

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11
Q

An object won’t turn when …

A

anticlockwise moment = clockwise moment

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12
Q

What do levers do ?

A

They use the idea of balanced moments to make it easier for us to do work, like lifting an object

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13
Q

How do levers make lifting objects easier ?

A

The levers increase the distance from the pivot. If the distance is further, less force is needed to get the same moment.

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14
Q

What are levers also known as ?

A

Force multipliers - they reduce the amount of force needed to get the same moment

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15
Q

If the moments acting on an object aren’t equal, what happens ?

A

There is a resultant force - the object will turn

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16
Q

What type of objects tip over ?

A

Unstable objects

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17
Q

What do stable objects have ?

A

A wide base and a low centre of mass

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18
Q

When does an object tip over ?

A

When the centre of mass moves beyond the edge of the object’s base

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19
Q

How does an object tip over ?

A

It’s because of moments. If the line of action of weight is outside the base of the object a resultant moment is formed.

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20
Q

How are objects designed to stop them toppling over ?

A

Wide heavy bases give a low centre of mass. Because it is wide, it is hard for the line of action of weight to leave the base.

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21
Q

What is a simple pendulum ?

A

A weight suspended on a piece of string. When you pull a pendulum and let it go, it will swing back and forth

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22
Q

What is the time period ?

A

The time taken for the pendulum to swing from one side to the other and back again

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23
Q

What formula calculates time period ?

A

1 divided by frequency

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24
Q

If the length of the pendulum increases, what happens to the time period ?

A

The longer the pendulum, the greater the time period

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25
Q

What property of liquids allow them to be used in hydraulics ?

A

Liquid are virtually incompressible

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26
Q

What formula finds the pressure of the water in a hydraulic system ?

A

Force divided by cross-sectional area

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27
Q

What are hydraulic systems a form of ?

A

Force multiplier

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28
Q

What are force multipliers ?

A

They use a small force to produce a bigger force

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29
Q

How does a simple hydraulic system work ?

A

Two pistons are involved. The first has a small cross-sectional area. Pressure is exerted on this piston using a small force, this is transmitted to the next piston. This piston has a large cross-sectional area, which creates a larger force

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30
Q

What are hydraulics used for ?

A
  • Hydraulic Car brakes
  • Hydraulic car jacks
  • landing gear on some aircraft
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31
Q

What is velocity ?

A

Both the speed and direction of an object

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32
Q

When an object is moving in a circle, what happens to the velocity ?

A

The object is constantly changing direction, therefore the velocity is constantly changing. The object is therefore accelerating towards the centre of the circle

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33
Q

What is the name of he force that keeps something moving in a circle ?

A

Centripetal force

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34
Q

What three forces can act as centripetal forces ?

A

Friction
Tension
Gravity

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35
Q

What centripetal force acts when a car goes around a bend ?

A

Friction - between the car’s tyres and the road

36
Q

Why centripetal force keeps water in a bucket when it is spun around ?

A

Tension in the rope

37
Q

What centripetal force keeps the planets in orbit ?

A

Gravity

38
Q

If an object is moving in a circle at a high speed, what happens to the force ?

A

A higher centripetal force is needed to keep the object in a circle

39
Q

Why does the centripetal force have to be higher if the object is large ?

A

To stop the object from flying off, to keep it in the circle

40
Q

What happens when the circle is smaller ?

A

A higher centripetal force is needed because the object has to turn at a steeper angle

41
Q

What is a magnetic field ?

A

A region where magnetic materials and wires carrying current experience a force acting on them

42
Q

What direction do arrows on magnetic field lines go ?

A

From the North Pole to the South Pole

43
Q

What happens when a current flows through a wire ?

A

A magnetic field is created around the wire in the form of circles

44
Q

What is a solenoid ?

A

A coil of wire

45
Q

What is the magnetic field like inside a solenoid ?

A

Strong

Uniform- straight and ordered

46
Q

What is the magnetic field like outside of a coil of wire ?

A

Just like a bar magnet - odd and less powerful

47
Q

How do increase the strength of the magnetic field around a coil of wire !

A

Add a magnetically ‘soft’ iron core through the middle of the coil. This is an ELECTROMAGNET

48
Q

What is a ‘magnetically soft’ material ?

A

A material that magnetised and demagnetises very easily

49
Q

Why are electromagnets useful ?

A

They can be switched on and off

50
Q

How do you turn an electromagnet off ?

A

The current supplied creates the magnetic field. If you stop the current, the magnetic field stops

51
Q

How are electromagnets used ?

A

Cranes - magnets can be used to pick up metals in a scrap yard. If it was a normal magnet the crane wouldn’t be able to let go of the magnet

52
Q

What is the motor effect ?

A

The force experienced by a current-carrying wire in a magnetic field

53
Q

How do you increase the force in the motor effect ?

A
  • increase the current

* stronger magnet

54
Q

At what angle does the maximum force act in the motor effect ?

A

90 degrees to the magnetic field and the current

55
Q

What happens if the magnetic field and current is travelling parallel to each other ?

A

Then no force acts on the object

56
Q

How can we work out the direction of the force ?

A

Flemings left hand rule

57
Q

How do you do Fleming’s left hand rule ?

A

Using your left hand, point your first finger in the direction of the magnetic field (North - South) and the second finger in the direction of the current. Wherever the thumb is pointing is where the object will move

58
Q

How does a simple electric motor work ?

A

The force acting on one of the ‘side arms’ causes it to rise. The current is in the other direction along the other ‘side arm’ so the force acts downwards. This causes the ‘side arms’ to spin

59
Q

Why do the side arms spin ?

A

They are attached to a spindle and forces act one up and one down, so it rotates

60
Q

Why are split-ring commutators used ?

A

It is a clever way of swapping the contacts every half turn to keep the motor rotating in the same direction

61
Q

How do you change the direction I the motor ?

A
  • Change the direction of the current

* swap the magnetic poles

62
Q

Where are electric motors used ?

A

Any electrical appliance that has rotating parts

63
Q

How do you use an electric motor to make things spin ?

A

Link the coil to an axle, the axle will spin around

64
Q

What is the axle attached to on a…
• food mixer ?
• CD players ?

A
  • The blade

* The part the CD is placed on

65
Q

What is electromagnetic induction ?

A

The creation of a potential difference across a conductor which is experiencing a change in magnetic field

66
Q

What does ‘induction’ basically mean ?

A

Creation

67
Q

How can you ‘induce’ voltage ?

A

Moving a magnet in a coil of wire

68
Q

What happens if you move a magnet back and forward inside a coil of wire ?

A

You induce a potential difference that keeps swapping direction - this produces alternating current

69
Q

how do generators work ?

A

If you spin a magnet inside a coil of wires a current will be induced.

70
Q

Why does spinning a magnet inside a coil of wires induce a current ?

A

As you turn the magnet, the magnetic field through the coil changes - this change in the magnetic field induces a current, which can make a current flow in the wire

71
Q

How is AC current induced in a generator ?

A

If you keep turning the magnet in the same direction, the magnetic fields direction will reverse every half turn. This causes the voltage to reverse - like in alternating current

72
Q

What are dynamos an example of ?

A

Generators

73
Q

What are dynamos and where are they used ?

A

They are use in lights on bikes. A cog attached to the wheel causes a magnet to spin inside a coil of wires, inducing an AC current

74
Q

What do transformers do ?

A

They change the AC potential difference (not DC)

75
Q

What are the two types of transformers you need to know ?

A
  • Step up - increase voltage

* Step down - decrease voltage

76
Q

How do the amount of coils change in different transformers ?

A
  • Step up - more turns on secondary coil

* Step down - less turns on secondary coil

77
Q

How do transformers work ?

A

Electromagnetic induction

78
Q

What are the two coils in a transformer connected by ?

A

An iron core

79
Q

How do transformers work ?

A
  1. Primary coil produces a magnetic field which stays in the iron core
  2. It I alternating current so the field changes direction constantly
  3. This change is felt by the secondary coil
  4. The changing field induces an ‘alternating potential difference’ across the secondary coil
80
Q

How do the amount of coils affect the potential difference induced ?

A

The more turns on the secondary coil, the more voltage induced

81
Q

Why do transformers only work with alternating current ?

A

There would be no induction in the secondary coil because the magnetic field is travelling in the same direction

82
Q

What does the iron core do ?

A

Transfers the magnetic field. It doesn’t carry any current

83
Q

Why does power in = power out on a transformer ?

A

They are nearly 100% efficient

84
Q

When are switch mode transformers used ?

A

They operate at higher frequencies than traditional transformers - between 50Hz and 200Hz (mains is 50Hz)

85
Q

What is the advantage of working at high frequencies ?

A

They can be small and light compared to traditional transformers

86
Q

Where are switch mode transformers used ?

A

Mobile phone chargers and power supplies

87
Q

Why can you leave phone chargers turned on when phones aren’t connected ?

A

They produce very little power when they’re switched on with no load

Science 3 (6 decks)

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