# Physics AQA Unit 3

Set of revision notes. These are long, so hopefully nothing has been missed. But if you think something should be included, that hasn't been, could you let me know?

Just a warning, lenses aren't my forte. So when it comes to describing how to draw them, it may sound a little muddled. :S

HideShow resource information
• Created by: Bexie
• Created on: 25-05-12 10:12

## Moment

Moment is the turning effect of a force. (e.g using a spanner, turning a tap)

Load is what you are trying to move.

Effort is the force you are applying to move the load.

Pivot is the point around which the lever is moving or rotating.

REMEMBER: Moment (Nm) = Force (N) x Distance from force to pivot (m)

So he moment is bigger if the force is bigger or is the distance is increased. Any turning effect is a moment.

Moments can also act in pairs, Clockwise and Anti-Clockwise. When the Clockwise moment is the same as the Anti-Clockwise moment, then the turning effects are balanced.

1 of 21

## Centre of Mass

The centre of mass (or gravity) of an object is the point at which all of its weight is concentrated. For an object hanging freely, it will come to rest with the centre of mass below the point it is suspended from.

To find the centre of mass of a symmetrical body then it is along the axis of symmetry. Where the lines cross, is where the centre of mass is. (This is why you can balance a ruler one the end of your finger if you position it correctly.)

For an unsymmetrical or irregular shaped objects, you can find the centre of mass by freely suspending the object from a point.

If you use a 'plumbline' (a mass on a piece of string) you can draw a line from the suspension point along the plumb line. Now suspend the object from another point and do the same thing. Where the two lines cross, is the centre of mass.

If the centre of mass is low, or an object has a wide base. Then the object becomes more stable. If the centre of mass is acting outside of the base, then the object will topple over.

2 of 21

## Centripetal Force

Centripetal force = The force acting towards the centre of a circle, when an object is moving in a circular motion.

Consider a mass on the end of a string being swung around in a circle. The tension in the string is providing the centripetal force. This mass is constantly accelerating towards the centre of the circle. When this object is spinning around its direction is constantly changing, which means its velocity is constantly changing.

The centripetal force can be increased if;

1. The mass of the object is increased.

2. The speed of the object is increased.

3. The radius of the circle decreases.

This force applies for any object moving in a circle (e.g planets, planes, cars).

3 of 21

## Centripetal Force in Space

The centripetal force is being provided by gravity. Planets that orbit a star, and moons that orbit plants, are in orbit due to gravity. So the centripetal force for planets is provided by the gravitational force.

Planets in our solar system orbit the Sun in elliptical orbits. In order for Earth to remain in orbit at a particular distance around the Sun, they need to move at a particular speed. If the Earth was orbiting too quickly or too slowly, it would drift into the Sun or out into space. This is also true for moons orbiting our planet.

The force of gravity between two objects will increase if;

1. The mass of the object is bigger.

2. The objects are closer together.

The time it takes for a planet to orbit the Sun depends on how far away it is from the Sun. The time to orbit is often referred to as the period,

4 of 21

## Artificial Satelites

A satellite is an object that orbits the planet (e.g. Our moon). We also have artificial satellites such as communication satellites. These artificial satelites are placed in different orbits around the Earth, depending on their function.

Communications = Satelites in geostationary orbit. This means that the satellite always stays above the same point on Earth and takes a day to complete an orbit. The satellite orbits the planet at th espeed as the planet rotates on its axis. Examples are; TV and Phonte Satelites.

Monitoring = Satelites with a lo polar orbit and may scan around Earth several times a day. So as the planet rotates on its axis, the satelite can get images of several different parts of the planet. Examples are; Weather and Spy Satelites.

5 of 21

## Light and Reflection

Light is an electromagnetic wave.

Reflection = When light strikes a shiny surface and gets reflected.

When the light hits a plane (flat) mirror we can measure the incoming incident ray from the normal. This is a line that comes out perpendicular (right angles) to the surface of the mirror. For any reflected ray, the angle of incidence is equal to the angle of reflection.

Forming Images = images formed in a plane mirror are;

1. Same size as the object.

2. Upright

3. Same distance behind the mirror as the object is in front.

4. Laterally inverted (left is right, right is left).

5. Virtual (an image not made from real light rays. Cannot be projected onto a screen).

Images must be described by these key points; (Real/Virtual, Bigger/Smaller, Upright/Inverted)

6 of 21

## Curved Mirrors

Curved mirrors can be described as either concave [curved outwards. )] or as convex [curved inwards (].

Curve mirrors have a principal focus (labelled F in the diagrams). The focal length is the distance from the mirror to the focus.

In a concave mirror, when light rays come in parallel to the optical axis they get reflected through the principal focus. The light converges or comes together at that point.

Convex mirrors use the same kind of principal. When light rays come in parallel to the optical axis, they reflect off the mirror but don't focus at a point. Rather they spread out from each other or diverge. You draw the reflected ray by using a ruler going from the focus point to the point where the incident ray struck the mirror.

7 of 21

## Concave Mirrors

To find where an image is produced in a concave mirror, you should draw a line from the top of the object, travelling parallel to the optical axis. This will then reflect through the focus point. Then draw another line going through the focus and reflecting off parallel to the optical axis. Where these two lines cross is where the top of the image is.

If the object is in front of the principal focus then the procedure is the same, a line parallel from the top of the object and a line from top through the principal focus. However, the rays that get reflected will never meet as they are diverging (moving away from each other).

So what we do is follow the reflected rays back behind the mirror, as that is where the image would appear to be coming from, just like what we would do for a plane mirror.

8 of 21

## Convex Mirrors

In a convex mirror, we do a similar technique to that for concave. We draw a line from the top of the object which moves parallel to the optical axis. When it strikes the mirror we use the optical axis behind the mirror to determine where it goes. We then draw a ray from the top of the object and draw it as if it was going to go through the focus point. This ray then gets reflected off parallel to the optical axis. The reflected rays diverge from each otherso they will never meet, so we draw the line behind the mirror to determine where the image is formed,

As you have seen, some images are bigger or smaller than the object. In order to work out the amount of magnification we use this formula;

MAGNIFICATION = IMAGE HEIGHT / OBJECT HEIGHT.

If the magnification is less than one, then the image is smaller than the object.

If the magnification is more than one, then the image is larger than the object.

If the magnification is equal to one, then the image and object are the same height.

9 of 21

## Refraction

Refraction = When a wave changed direction when entering a more/less dense medium.

Using the example of light, when the ray enters the Perspex block from air it gets slowed down as Perspex is denser. This also causes the ray to change direction (bends towards the normal). When the light is leaving the block it speeds up as air is less sense. The ray will then bend away from the normal line.

If the lit enters along the normal line (so perpendicular to the objects surface) then no refraction occurs. The light will still be slowed down, as it is travelling through a denser material, but the light will not change direction.

When light enters a prism we can see the entire spectrum due to an effect called dispersion. The different colours of light have different wavelengths, therefore different speeds in the medium. Red is refracted the smallest angle, because of having the longest wavelength.

10 of 21

## Lenses

Lenses use the effect of refraction to form images. Like mirrors, there are two types of lenses;

• Convex (Converging Lenses). - Focuses parallel light to one point.
• Concave (Diverging Lenses). - Makes the parallel light spread out.

On a ray diagram, a converging lens is often shown as an arrow. A diverging lens is often shown as a double-head arrow.

A ray diagram for a lens is similar to that for a mirror. Two rays are used.

Ray one is parallel to the axis and is refracted through the Focal.

Ray two passes straight through the centre of the lens.

11 of 21

## Converging Lenses

Converging lenses can produce different images depending on where they are placed. A second point is often labelled on these ray diagrams, which is twice the focal length (2F).

If an object is placed outside of 2F then the image produced is real, diminished and inverted. This can help your eye to focus on the retina. Also used in cameras to focus an image.

If an object is placed between 2F and F then the image produced will be real, inverted and magnified. A use for this is in projectors.

If an object is placed at F, then the rays of light will never meet. This is used for spotlight.

if an object is placed between the principal focus (F) and the lens, then a virtual, upright and magnified image can be produced. Used for magnifying glasses.

12 of 21

## Diverging Lenses

Diverging lenses always produced the same type of image;

• Virtual
• Upright
• Smaller (Diminished)

Diverging lenses re commonly used in spectacles for people who are short sighted.

13 of 21

## Sound Waves

Sound is a longitudinal wave and travels by particles vibrating. Sound can not travel in a vacuum because there are no particles to carry the sound. Sound will travel faster in denser material (e.g faster in metal than air). Sound travels at about 340m/s in air.

Longitudinal waves oscillate parallel to the direction of travel. (e.g sound waves)
Transverse waves oscillate perpendicular to the direction of travel. (e.g light waves)

Frequency = The number of waves that occur every second, measured in Hertz (Hz).
Frequency determines the pitch of a sound. So high frequency is equal to higher sound.

The hearing range for humans is 20Hz to 20,000Hz.

Amplitude = How loud the sound is. (Taller the waves, means louder the sound/amplitude)

Wavelength = The distance between peak to peak, or trough to trough. (measured in metres).

Like light waves, sound waves can be reflected (an echo) and refracted.

14 of 21

## Sound Waves 2

Layers or air can be at different temperatures and refraction takes place at those temperature boundaries. During the day sound is refracted upwards because the ground is warmer than it is at night.
If you are a long way from a source of a sound (e.g a car alarm), it is easier to hear that sound at night than during the day because of this refraction.

The quality of the sound wave or note produced depends on the waves form. We can view this form by using an oscilloscope.

15 of 21

## Ultrasound

Ultrasound = Sound beyond the human hearing range. Certain animals can hear and produce ultrasounds but humans have to use electronic devices to produce them.

Ultrasounds get partially reflected when they meet a boundary between two different mediums.
An Ultrasound pulse needs to travel to the medium and back to the detector. So if a detector indicated that the Ultrasound took 10 seconds to return, this must mean that the object is 5 seconds away.

We use the equation; DISTANCE = SPEED x TIME. To work out how close the sound waves are.

Ultrasounds have several uses, including medical scans and cleaning devices.

16 of 21

## Electromagnetism

The direction of a magnetic field goes from the north pole to the south pole of the magnet,
When a current carrying wire is placed into a magnetic field it experiences a force. The size of this force can be increased by;

• Increasing the strength of the Magnetic Field.
• Increasing the size of the current.

If the wire carrying the current is parallel to the magnetic field then it will NOT experience a force.
The direction of the force can be determined by using your left hand;

• Index Finger = Magnetic Field.
• Second Finger = Current
• Thumb = Direction of Force.
17 of 21

## Electromagnetism Effects

The Motor Effect is used in several devices (e.g. electric drills, hairdryers, loudspeakers). A DC (Direct Current) Motor has a splint ring commutator. This allows the current in the coil of wire to change every half turn. This ensures the force is in the same direction and, as a result, the coil gets spun in the same direction each time.

A similar effect, called the electromagnet induction, is when a changing magnetic field induces a current in a wire and a potential difference. When the magnet is pushed into the coil, the current goes one way - positive current.
When the magnet is removed, the current goes in the opposite direction - negative current.
The potential difference also changes this way.

18 of 21

## Electrical Generators

Electrical generators produce this alternating current (AC). When a coil of wire is spun within a magnetic field (or a magnet spinning insider coil of wire), the alternating current and voltage is produced when the wire 'cuts through' the magnetic field lines. The slip rings stop the wires from getting tangled. The brushes are in contact with the slip rings and take the alternating current from the coil or wire and pass it into the circuit.

The size of potential difference produced an be increased by;

• Increasing the speed of rotation.
• Increasing the strength of the magnetic field.
• Increasing the number of 'turns on the wire'.
• Increasing the area of the coil.

Mains electricity is generated this way in a power station and travels to homes by the National Grid. Transformers are used in the National Grid in order to increase (step-up) the voltage and decrease (step-down) the voltage. The reason they are needed is because there would be too much energy lost due to heat aused by friction in the wires.

19 of 21

## Transformers

Transformers are made of a magnetic material (iron core) with coils of wire wrapped around them. In a transformer there are primary coils and secondary coils. The primary coil is the one that initially receives the unchanged voltage, the secondary coil is where the voltage gets changed.

When an alternating current passes around the iron core, a changing magnetic field is induced. The changing magnetic field produces an alternating current (and voltage) in the secondary coil. The number of turns around e coil will determine if he voltage is increased or decreased.

If the secondary coil has less turns than the primary coil, it is a step-down transformer.
If the secondary coil has more turns than the primary coil, it is a step-up transformer.
Transformers are governed by the following equation:

P.D ACROSS PRIMARY / P.D ACROSS SECONDARY = NUMBER OF TURNS ON PRIMARY / NUMBER OF TURNS ON SECONDARY.

20 of 21

## Stars and Space

Our sun is only one of billions of stars in our Galaxy (the Milky Way) and the universe is made up of billions of galaxies.

• Planets form, when lumps of rock get attracted to each other due to gravity.
• Stars form, when clouds of gas and dust from space get pulled together due to the gravitational attraction. The amount of gas build up (gets more concentrated) and forms a protostar. When the protostar gets denser and hotter, nuclear reactions (e.g. fusion) start which causes Hydrogen and other lighter elements to fuse together. During fusion energy gets released, which is what make the stars hot.
Main sequence stars form from Protostars when the forces within the star are balanced (gravitational force and radiation pressure from nuclear fusion). Our sun is a main sequence star. After this stage, the star can take two possible routes depending on their mass. (Red Giant or Red Supergiant).

Due to nuclear fusion in stars, elements were created. When the stars explode (go supernova) all of these elements are released. This means the elements that make up your body, were formed inside stars.

21 of 21

Report

really useful, thank you :)

Report

couple spelling mistakes but still helpful so thanks

Report

Very useful however endoscopes have not been included :(

## Similar Physics resources:

See all Physics resources »