All of P3 - revision cards

What is refration?

Refraction is when light changes direction as it passes from one medium to another (eg. air to water). The change in direction is caused by the boundaies of the two mediums being different densities and having different refractive indexes, whcih effects the speed of the light rays.

Angle of incidence: This is the angle between the normal and where the light hits the medium. This always bends away from the normal.

Angle of refraction: The angle between the light in the material and the normal. This is always less than the angle of incidence. This ray always bends towards the normal.

Refractive Index: This is the ratio of how much a substance can refract a light ray. The equation for refractive index is (this will be on your physics equation sheet):

REFRACTIVE INDEX = Sin (ANGLE OF INCIDENCE) / Sin (ANGLE OF REFRACTION)

or n= sin i/sin r. You can rearrange this equation to find other things too.

Visable light can be sent down glass optical fibres. In medicine, they're used to look inside the body without surgery.

Total internal reflection can only happen when the angle of incidence is greater than the critical angle. The critical angle is when the angle of incidence produces a refraction along the boundary (of the two materials). The equation (on the physics equation sheet) is:

REFRACTIVE INDEX= 1 / SIN (CRITICAL ANGLE). Rearrange to find the critical angle.

Uses for total internal refraction:

Endoscopes allow surgeons to perform keyhole surgery for internal observation, without physically cutting open the patient. It contains two bundles of optical fibres- one is used to shine light, the other has a lens to carry an image back to be viewed.

Laser light is used as an engergy source in endoscopes. It can also cut, burn and destroy tissue, seal off leaking blood vessels and it also makes surgery on the retina possible.

Different lens shapes:

• Converging (convex) lens: makes parallel rays of light focus at the focal point. Its uses range from cameras, magnifying glasses and to correct long sighted people
• Diverging (concave) lens: makes light spread at a virtual focal point. It is used to correct short sighted people.

How lenses form a REAL IMAGE: This is only possible if its a converging lens, and the object is further away than the focal point.

How lenses form a VIRTUAL IMAGE: This is where the image is formed where the rays appear to be coming from. Its always virtual in a diverging lens, but the image will be virtual if the object is closer than the focal point (in a converging lens)

Magnifaction:

MAGNIFACTION = IMAGE HEIGHT / OBJECT HEIGHT. If the image is larger than the object, the magnifaction will be greater than 1.

Converging lens:

• If the object is closer than the focalpoint, the image is virtual, magnified and upright.        
• If the object is further away than the focal point the image is real and flipped.

Diverging lens

The Cornea: This is where light enters. It also helps protect and focus light onto the Retina.

The Retina: A layer of light sensitive cells at the back on the eye. These send signals of what image youre seeing to the brain.

The Iris: a coloured ring of muscle. This adjusts how much light enteres the eye.

The Pupil: central hole which allows light to enter the eye.

Lens: Focuses light onto the retina to form a sharp image.

Cilary Muscles: Change the tickeness of the lens. They're attatched via the suspensory ligament.

SHORT SIGHTED: The image formed by the lens is formed infront of the retina. This is caused by a lens which is too powerful, or if the eye is too long. It can be fixed with a converging lens.

LONG SIGHTED: The image is formed behind the retina.This may be caused by a lens which is too weak or an eye which is too short. It can be fixed with a diverging lens.

A moment is another word for turning effect. A moment can be increased by increasing the size of the force (the effort pushing down), or by increasing the distance between the force and the pivot.

You can calculate the moment using: MOMENT(nm) = FORCE(n) X DISTANCE(m)

On a 'seesaw' type question/diagram, you need to work out the moment for both sides of the pivot, and see whether the momet would be clockwise or anticlockwise. If the seesaw is equilibruim, the force x distance would be the same for each side.

The stability of an object can be increased by

• having a a centre of mass as low as possible
• a base wide enough to prevent toppling over.

When an objects line of action (of its centre of mass) is outside of its base, it will fall over.

Liquids are incompressible. The pressure in a liquid acts equally in all directions, this is why forces exerted onto one part of the liquid are transmitted to other parts.

You can calculate the pressure using this equation: PRESSURE(pa) = FORCE(n) / AREA(m^2)

A simple hydraulic system has two pistons, with one having a smaller cross sectional area. Pressure is the same at both pistons. The second pressure has a larger area, so as force = pressure x area, it will have a larger force.

The uses include things such as landing gear on aircraft, car breaks and car jacks.

When an object moves in a circular motion, its velocity changes constantly as its changing diretion.Velocity is tangent to the circle. As the velocity increases, the centripetal force increases.

The resultant force which causes acceleration of circular motion is called the centripetal force and is always directed towards the centre of the circle.  Things which could provide the centripetal force is friction (for cars etc) or tension (for string etc).

If you want to increase the speed, you can:

• Decrease the radius
• Increase the mass

Pendulums

The time period for oscillating pendulum (how long it takes to do one oscillation) is:

HOW MANY SWINGS (USUALLY 1) / FREQUENCY OF OSCILLATIONS (hz)

the frequency is equal to the number of complete cycles of oscillations per second.

Electromagnets are made of insulated wire around an iron core. When current flows through the wire, a magnetic field is produced, and te iron core is magnitised strongly.

Steal and other materials aren't suitable for electromagnetism as they keep the magnetism once the electromagnet is turned off. Some uses for electromagnets are:

Circuit Breaker:

This is sued for safety within electrical appliences. It is a switch in series- and the switch is held in place with an iron cap. When too much current is passed through the electromagnet, the switch is pulled open (via the electromagnet magnetism) and the circuit is broken.

Electric Bell:

Once connected to a battery, the iron armature is pulled towards an electromagnet (and hits the bell). This however opens the switch and the circuit is broken, so the armature springs back- but this powers the circuit again, and the electromagnet attracts the armature again. The cycle repetes.

A Relay:

A relay is used to switch on/off electrical appliances.

First, current passes through an electromagnet. The magnetism attracts an iron armature on a pivot. As a result, the armature closes the switch gap and completes the circuit. This turns on the appliance.

This is the effect which occours when a current carrying wire is put in the presence of a magnetic field experiences a force. The force will be greatest when the wire is perpendicular to the field lines. Force does act when the wire is parallel to the field lines.

The size of the force can be Increased by turning up the current, or increasing the magnetic field strength. The direction of the force will always be at right angles to the field lines. It will be reversed if the current or magnetic field is reversed.

Flemmings left hand rule: 

You can use this rule to find which way the movement (force) will be.

• thuMb = Movement (force)
• First finger = Field direction
• seCond finger = Current direction (+ to -)

The electric motor uses the motor effect.

• The motor has a split ring communictor, which is connected to two brushes.
• The ring spins and brushes a current onto the wire, making it have an electromagnetic field, but only for a moment of time.
• The same poles are next to each other, so the side is repelled, making the wire turn.
• When the current stops, the momentum is already set in so the wire carries on.
• When the current is brushed back on, the poles repel again, making the motor spin.

You can make the motor spin faster by increasing the number of coils, increasing the magnetic field or increasing the current.

A transformer has a primary coil and a secondary coil so they can transform one voltage to a higher or lower voltage. They use an alternating current because otherwise, there would be no change in the magnetic field- so the second voltage would be 0.

An alternating current produces an alternating magnetic field, which produces an alternating voltage across the secondary coil. Step up transformers up the voltage in the second coil, while step down lower the voltage. You can use this equation (you will need to rearrange):

primary voltage / secondary voltage = no. of coils in primary / no. of coils in secondary.

Switch mode transformers are a type of transformer which is used in a mobile phone charger. These are smaller and lighter than other transformers.

In grid cables (where transformers are used), the higher the voltage the greater the efficiency. The lower the voltage, the greater the current has to be to achieve the same power. This is because of waste energy. Transformers are assumed to be 100% efficient, so you can use:

primary voltage x primary current = secondary voltage x secondary current.

X-Rays: X-rays are part of the electromagnetic spectrum with a very short wavelength (and high frequency) of 0.01-10 nanometres (3x10^16 to 3x10^19 Hz)

They work by a flat pannel detector placed behind the patient, and x-rays being shot at the body part they need to be. The x-rays pass through soft tissue, but they're absorbed by teeth and bones.  The film the x-rays reach becomes dark- this is why bones appear light.

However, a danger is that high doses can cause ionisation, which kills living cells. However, this can be useful for destorying tumors and other things inside the body.

CT scanners use x-rays, but they produce a high resolution 3D image, and can tell the difference between tissue types. However, they cost more and give a higher radiation dose.

Ultrasound: You can use ultrasound for prenatal scans, as there are no known side effects and its not ionising. However, they produce a grainy, low quality image. You can use this equation to find out how far away something in the body is: 

DISTANCE = ( SPEED X TIME ) / 2 (divided by 2 because it is there and back- and you only need 'there')