Physics

Moments 1.1

A moment is the turning effect of a force (defined by the equation, Moment of a force [in newton metres] = force [newtons] x perpendicular distance from the pivot to the line of action of the force).

We can increase the moment by...

• increasing the size of the force
• using a spanner with a longer handle

Get it right!

Make sure that you know the definition of a moment well! In calculations, make sure that your units are consistent.

Centre of Mass 1.2

The centre of mass of an object is the point where its mass may be thought to be concentrated. It is very easy to find the centre of mass of a symmetrical obect/ shape as it is the point at which the planes of symmetry meet.

Experiment:

In order to find the centre of mass for an irregular shape it must be suspended from a point by a pin. A plumbline can be used to draw a straight line from the pin, downwards. The process is then repeated to a point of intersection.

Key point-

1. When an object is in equilibrium, its centre of mass is directly beneath the point of suspension.

Moments in balance 1.3

The moments of the forces acting on an object when it is stationary can be defined as equal and opposite. This can be seen on a seesaw that is balanced, and it is an example of the Principle of Moments.

For example, we know that these moments must be equal hence we can work out the missing value.

 0.5 x 5 = 0.25 x F

Stability 1.4

The factors that affect the stability of an object are simple, a) the base width and b) the height of the Centre of Mass. Ultimately an object with a wide base and low Centre of Mass will be the most stable.

When an objects's centre of mass goes beyond its base (or pivot) it will topple. For example if the CoM goes beyond the dotted lines on the diagram below, the object will topple.

Circular motion 1.5

For any object moving at a constant speed (in a circle), the object accelerates continously towards the centre (of the circle). This translates into the resultant force being in the direction of the circle's centre, we call this force Centripetal.

Key Point:

1. Centripetal force increases with speed and circle radius or distance.

Get it right!

Centripetal force isn't a force in its own right; it is always provided by another force such as gravitational force or electric force.

Gravitational attraction 1.6

The force of gravity is an attractive force that gets bigger the greater the mass of each object is, it also increases as the distance between the object decreases.

Worked example;

If a space probe is midway between the Earth and the Moon the force of gravity on it due to the Earth is bigger than that due to the Moon. This is simply because the Earth has greater mass and greater size.

Planetary orbits 1.7

The centripetal force on the planets is due to the force of gravitational attraction between it and the Sun. To stay in this orbit, at a specific distance, a planet must move at a particular speed around the Sun.

The further a planet is away from the sun, the less its speed is as it moves around the Sun. In addition, the further the planet is from the Sun, the longer it takes to make a complete orbit.

Satellites 1.8

Launching satellites - if the satellite's speed is too low, it will fall to the ground; if the initial speed is too great, it will fly off into space; and finally, at the 'correct' speed, it orbits the Earth.

The period of a Satellite is the time it takes to make one complete orbit.

Key Points:

1. A satellite in a geostationary orbit has a period of 24 hours and stays at exactly the same position over Earth's equator.

2. These are used for communication satellites.

3. Monitoring satellites are usually in low polar orbits, this allows us to see more detail on the Earth's surface.

Refraction 2.3

Refraction is the change of direction that a wave makes as it enters a new medium. It can happen to any wave, including sound or light. For light rays; if the speed is reduced then the wave bends towards the normal, and if the speed is increased, then it will bend away from the normal.

Musical sounds 2.7

Frequency - the number of complete wave cycles passing a point each second, measured in Hz.

Amplitude - the maximum disturbance of the particles in a substance (eg air) when a sound wave passes through it.

Key Points;

1. The loudness of a note depends on the amplitude of the sound wave.

2. The pitch of a note depends on the frequency of the sound wave.

Ultrasound 2.8

Ultrasonic waves are those above the human hearing range ie greater than 20 000 Hz in frequency.

They can be used in the human body to scan organs or babies as they are non-ionising. For each pulse sent out, a probe picks up the refected pulses from the path of the transmitted pulse. Computers can process this data and form an image. As they are partially reflected at different tissue types they can be used, with great success, to scan organs.

We can also use them to detect flaws in metals using the same principle. A calculation can be done by using the known distance and reading from the display the location of the internal boundary.

The motor effect 3.1

We can increase the size of the force by; increasing the current, using a stronger magnet or finally placing the wire perpendicular to the magnetic field.

We can reverse the direction of the force by reversing the current. For electric motors we use split ring commutators that are fixed to the coil, this allows the current to reverse every half turn of the coil meaning that it continues to move in the same direction.

We use the motor effect to to make loudspeakers work, a diaphragm is attatched to the coil, which vibrates as an alternating crrent passes through. This created particle movement, and consquently sound waves are produced.

Electromagnetic induction 3.2

The dynamo effect - the effect in which a potential difference is generated in a wire or coil when the wire or coil cuts across the lines of a magnetic field.

How can we induce a potential difference across the ends of a conductor?

When a wire cuts the lines of a magnetic field, potential difference is induced in the wire. If the wire happens to be part of a complete circuit then the induced potential difference causes a current to flow in the circuit. It can be increased by: increasing the speed of the wire or using a stronger magnet.

What are the purposes of the slip rings and the brushes in an a.c. generator?

The slip rings and brushes allow the current to switch every half-turn whilst maintaining the motion in one direction. The pair are made of a conducting material so that there can be a flow of electricity.

Transformers 3.3

Transformer - electrical device used to change an alternating voltage.

How do transformers work?

A transformer is made of two coils of insulated wire, both wound around the same iron core. When an alternating current passes through the primary coil, an alternating p.d. is induced in the secondary coil. This is because the alternating current (passing through the primary coil) produces an alternating magnetic field. The lines of this field pass through the secondary coil and induce an alternating p.d. across it. If the transformer is connected to a complete circuit this will then flow as an electric current.

Why don't they work with d.c.?

As with direct current there is no alternating magnetic field, and so the secondary voltage is zero.

In order to increase efficiency lamenated iron is used, this reduced the eddy currents within it, greatly improving overall efficiency.

Transformers and the National Grid 3.4

National Grid - the network of cables and transformers used to transfer electricity from power stations to consumers.

Why are transformers used in the National Grid?

They are used to step-up the voltage produced by power stations as, the higher the p.d., the greater the efficiency of transferring electrical power through the grid. This means that they greatly reduce the heat loss and subsquently energy wastage.

This is due to, Power = Voltage X Current (so one can be reduced whilst maintaining the other, to create equal power flow)

What is the difference between a step-up and a step-down transformer?

A step-up transformer increases the p.d. and so has a greater number of coils on the right (or secondary) these are used outside power stations for example. By contrast a step-down transformer has fewer turns on the right than the left, as it is used to reduce the p.d. These are commonly found near homes, they make electricity safer to use.

Galaxies 4.1

What is a Galaxy?

A galaxy is a series of Stars and their Solar Systems. There are many billions of them dotted around the Universe, they provide hot spots in an otherwise extremely cold and dark place.

Which force is responsible for their formation?

The force of gravity is resposible for pulling and pushing matter into stars/ galaxies.

Key point;

1. As the Universe cooled, it expanded. Uncharged atoms attracted each other (via gravitational attraction) and Stars and Galaxies formed from them.

The life history of a star 4.2

Eventually the Sun will move from being a main sequence Star to being a red giant. In its current state it is able to exert enough force outwards to prevent it being squashed (by gravity). This enery/ force is produced by fusion of light nuclei and is equal to that of the inward pressure.

Low mass star:

Prostar ---> Main stage ---> Red giant ---> White dwarf ---> Black dwarf

High mass star:

Prostar ---> Main stage ---> Red giant ---> White dwarf ---> Supernova ---> Neutron star ---> Black whole (if enough mass!)

The larger the star the shortest its overall life. The largest stars exploded long ago, created new planetary nebulae and providing material for new stars and solar systems.

How the chemical elements formed 4.3

How were the heavy elements formed?

Elements heavier than iron were formed and scattered in Supernova explosions. These happen when a massive star collapses as a Supernova. Hydrogen and hydrogen fuse together to form Helium, this fuses together in different quantities to form other light Nuclei.

Light elements are formed as a result of fusion in stars.

Elements from old supernova explosions gather together and form new Stars and galaxies. The more cycles material goes through, the greater the chance that life will form.