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Moments. A ‘moment’ is the turning effect of a force – it can be calculated by multiplying the force (N) by the perpendicular distance from the pivot (m).
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Moments A ‘moment’ is the turning effect of a force – it can be calculated by multiplying the force (N) by the perpendicular distance from the pivot (m). The moments in, for example, a crane, must be balanced to prevent it tipping; counterweights are added and the necessary distances are carefully calculated.
Centre of Mass The centre of mass is the point where all the mass of an object is concentrated; it must remain inside the edge of the base, or the object will tip over. Shorter, wider objects have a lower centre of mass and are therefore more stable.
Circular Motion If an object is moving in a circle, the velocity is always changing because it is always changing direction, but the force remains towards the centre of the circle. This force is called the centripetal force. It’s size depends of the mass and speed of the object, and the radius of the circle – or it’s distance from the centre point. (Think of the Earth’s distance from the sun). As mentioned, centripetal force can be found in regards to planetary orbits, in cars going around bends or on fairground rides. (Also in swinging ropes. As long as you don’t let go, especially if your name is Alan Donald.)
Gravity and Planetary Orbits The larger the masses of two objects, the greater the gravitational force between them. The gravitational force is the centripetal force. Planets always orbit stars, and the orbits are elliptical – like slightly squashed circles. The greater the distance of a planet from the sun, the longer it’s orbit takes, because the force of gravity is less (planets which are closer need to move faster so they don’t get sucked in).
Gravity and Planetary Orbits Continued Geostationary satellites... • Are in high orbits over the equator. The orbits take 24 hours to complete, so they are always above the same point. • They are used for telephones and TV. Low Polar Orbit Satellites... • As the name would suggest, these are in relatively low orbits around the poles, so they sweep over the whole surface of the Earth. • They are used for weather forecasts and spying.
Images The difference between a real image and a virtual image is that a virtual image is not there and therefore cannot be projected. When light reflects off anything, the angle of incidence is equal to the angle of reflection. Refraction is the light bending as it changes speed. This happens when is crosses a boundary between two medium – i.e. air and water.
Mirrors Three rules for a plane mirror – • The image is the same size as the object. • The image is the same distance behind the mirror as the object is in front. • The image is virtual because it is formed bydiverging rays.
Mirrors Curved mirrors; the facts. The centre of curvature (C) is the centre of the circle which would be formed. The centre of the mirrors actual surface is called the vertex. Halfway between C and the mirror is the focal point(F). The line down the middle, on which the vertex, the centre of curvature and the focal point lie, is the axis.
Mirrors Ray diagrams for concave mirrors; the rules. • An incident raywhich is parallel to the axis will pass through the focal point when it is reflected. • An incident ray which passes through the focal point will reflect parallel to the axis.
Mirrors Ray diagrams for convex mirrors; the rules. • An incident ray which is parallel to the axis will reflect so it appears to pass through the focal point. • An incident ray which passes through the focal point reflects parallel to the axis.
Lenses Refraction in a converging lens; the rules. • An incident ray which is parallel to the axis refracts and passes through the focal point. • And incident ray which passes through the focal point refracts parallel to the axis. • An incident ray passing through the centre of the lens does not refract.
Lenses Refraction in a diverging lens; the rules. • An incident ray which is parallel to the axis refracts so it appears to have come from the focal point. • An incident ray which passes towards the focal point refracts parallel to the axis. • Like the converging lens, an incident ray passing through the centre of the lens does not refract.
Uses of These Lenses Magnifying glasses use convex lenses, but the object must be closer than the focal point to work. The image is virtual. In cameras, the light refracts through the lens and forms an image on the photographic lens.
The Magnification Formula Magnification = Image Height / Object Height
Sound Waves Sound waves, like light, can reflect and refract; hard, flat surfaced reflect sound best, which is easily noticed in an empty room. This is why carpets make such a difference. Humans hear sound in the frequency range of 20 to 20 000 Hz, but some people, often as they get older, can’t hear sounds at the higher end. Sound does not travel in a vacuum, as there are no particles to vibrate, i.e. space.
Sound Waves The greater the amplitude of a wave, the louder the sound. The higher the frequency of a wave, the higher the pitch. The quality of the note depends on the waveform – see the next slide for diagrams.
Sound Waves A pulse wave makes a reedy sound. A saw-tooth wave makes a brassy sound.
Sound Waves A triangle wave makes a weak, mellow sound. A square wave makes a hollow sound.
Ultrasound Ultrasound is sound with a frequency higher than humans can hear, and is partially reflected at a boundary between two media, so you can use it for various purposes, including quality control and getting images of a foetus. If the ultrasound wave is passed through metal casing, for example, when the wave reaches a crack it some of it will reflect back and some will carry on until it reaches the other side. The echoes are processed by a computer to calculate the precise distance of the crack from each side. It works the same way for a foetus.
Ultrasound Ultrasound can also be used for cleaning mechanisms too delicate to take apart. The waves cause both the dirt and the object to vibrate, and the dirt falls off. This technique is also used by dentists to clean teeth.
Magnetic Fields The definition; A magnetic field is a region where magnetic materials and wires carrying currents experience a force acting on them. Iron, steel, nickel and cobalt are magnetic.
Magnetic Fields The magnetic field around a wire (carrying current). The magnetic field around a solenoid (coil).
The Motor Effect Remember Fleming’s Left Hand Rule. First finger = Field SeCond finger = Current ThuMb = Motion
The Motor Effect The force increases if either the current or the magnetic field is increased. The current runs from positive to negative; the field runs from north to south.
Simple Electric Motor Four things will speed the current; • More current. • More turns on the coil. • A stronger magnetic field. • A soft iron core. Because the forces act both up and down, the motor rotates. The split-ring commutator is used to swap the contacts every half turn in order to keep the motor rotating in the same direction. The direction can be reversed by changing either the magnetic poles or the polarity of the DC supply.
Electromagnetic Induction Four factors affect the size of the voltage induced; • Strength of the magnet. • Turns on the coil. • Area of the coil. • Speed of movement. To induce a voltage, either a magnet must be moved in a coil of wire or a conductor must be passed through a magnetic field. This ‘cuts’ the field.
Generators Generators induce a current in a coil by rotating it in a magnetic field. Generators have slip rings and brushes so that the contacts don’t swap every half turn and the wires don’t twist.
Transformers Transformers alter AC voltages. The primary coil produces a magnetic field which remains within the iron core – almost all of it then passes through the secondary coil, with hardly any loss. The voltage must be AC as it needs to be constantly changing to be detected by the secondary coil. Whether or not the transformer is ‘step up’ or ‘step down’ depends on the relative number of coils – if the primary has more than the secondary, it is a step down, and vise-versa.
The Transformer Equation Can be used either way up. Primary Voltage/Secondary Voltage = Turns on Primary/Turns on Secondary
Stars and Galaxies Star form from clouds of gas and dust which are pulled together by gravity; as they are compressed, enough heat is produced to set off nuclear fusion reactions – this then leads onto the star releasing light and other radiation. Originally, the only element in existence was hydrogen; these formed helium via nuclear fusion. Three helium nuclei can form one carbon nucleus, and this can then go on to combine with more helium to form other elements like oxygen.
A Star’s Life Cycle • Clouds of dust and gas... • ...become a protostar... • ...when the temperature gets high enough, they become a main sequence star... • ...the hydrogen runs out and the star swells into a red giant... • ...a small star will cool and shrink to a white dwarf, then fade to a black dwarf... • ...a big star will expand and contract until it explodes – a supernova... • ...the core left behind by a supernova becomes a neutron star, or a black hole... • ...the dust and gas thrown out can become a new solar system, with a second generation star...
