Additional Physics

The force acting on an object can change how it moves.

When one object exerts a force on a second object exerts an equal and opposite force on the first object.

Resultant forces:

If 2 or more forces are acting in the same straight line or parallel they can be added together to find the resultant force.

The accerleration of an object can be calculated if you know its mass and the resultant force acting on it.

The acceleration of ab object can be found using : a= F / m

The force can be calculated by: F= m x a

a= Acceleration ( M/S ^2 )

F= The resultant force acting on an object (N)

m= The mass of the object (Kg)

Distance time graphs give information about the motion of an object.

A distance time graph shows how far an object has travelled from a starting point at various times.

Acceleration is the rate of velocity (or speed in a straight line).

Acceleration can be calculated using: a= v- u / t

a= Acceleration (m/s^2)

u= Initial velocity( start velocity (speed)) (m/s)

v= Final velocity (m/s)

t= Time taken for the change in velocity to happen (s)

Acceleration can also be calculated from the gradient of a velocity time graph.

When a vehicle is travelling at a steady speed the driving force on the vehicle is balanced by the resistive or drag forces acting in the opposite direction. Most of the resistive forces are caused by air resistance.

Stopping distance = Thinking distance + Breaking distance.

Thinking distance is the distance travelled in the time it takes for the driver to react. The greater the speed of the vehicle, the further it will travel whilst the driver is reacting.

Breaking distance is the distance travelled while the breaks are applying a force to slow the vehicle.

The greater the speed of the vehicle, the greater the breaking force is needed to stop in particular distance. For a constant breaking force the breaking distance increases with speed.

The brakes produce a frictional force between the brake and wheel. The kinetic energy of the vehicle is reduced. Energy is transferred to the brakes and the temperature of the breaks increases.

Breaking distance is increased by:

The condition of the car's tyres and breaks.

The road conditions.

Thinking distance is increased when:

The driver is tired.

The driver has taken drugs.

The driver is distracted.

The resultant force acting on a falling object depends on its weight and the air resistance.

The weight of an object is the force exerted on it by gravity:

W= m x g

W= is the weight of the object (N)

m= The mass of the object (Kg)

g= The gravitational field strength (N/kg)

An object starts to fall through a fluid such as water. The weight of the object makes it accelerate. Weight always acts downwards. The resistance of the fluid causes an upward force.

As the falling object accelerates, the resistance force increases.

When the resultant force is zero, the object falls at a constant velocity called terminal velocity.

Velocity time graphs show the changing forces on a falling object.

The graph shows the stages in the fall of a skydiver:

1- AB: The force of gravity pulls the skydiver down. Air resistance is so small that the resultant force is large. The skydiver accelerates.

2- BC: As the velocity of the skydiver increases the air resistance increases. Weight remains constant. The resultant force gets smaller and the acceleration decreases.

3-CD: The air resistance is equal and opposite to the weight of the skydiver. The resultant force is zero so acceleration is zero. The skydiver has reached terminal velocity.

4-DE: The skydiver opens the parachute. The large surface area increases the air resistance so that it is bigger than the weight. The resultant force is upwards. This means the acceleration downwards is negative. The skydiver's velocity reduces. As the velocity reduces, the air resistance decreases until it is again equal to the weight.

5-EF: The skydiver falls to the ground at a new lower terminal velocity and hts the ground at F and stops.

A force acting on an object may cause it to change shape. This includes streaching, compressing. bending or twisting in to a new shape.

When a force moves, a part of the object it does work. Doing work transfers energy.

If the object is elastic then some of this trasferred energy is stored as elastic potential energy. An object is elastic if it recovers its origional shape when the force is removed.

The extension is the change in length of the object when it is stretched. The extention of an elastic object is directly proportional to the stretching force applied to it. This is shown in the equation:

F= k x e

F= The force applied (N)

k= The spring constant for the object (N/m)

e= The extension of the object (m)

The equation applies up to a certain extension. This is called the limit of proportionality. For greater extensions, the equation does not apply.

Force, work, energy and power are all quantities related to moving objects.

Work: Work done is when a force causes an object to move through a distance. Energy is transferred from one form to another when work is done.

Work done= Force x distance

W= F x d

W= The work done (J)

F= The force acting on the object (N)

d= The distance moved by the object while the force is acting (M)

The distance must be measured in the same direction as the force acting.

Power is the amount of work done or energy transferred in a given time.

Power= Energy / Time

P= E/T

P= Power (W)

E= Energy transferred (J)

t= The time taken (S)

A power of 1 Watt is a joule per second.

The amount of energy transferred is equal amount of work done.

Kinetic energy (KE) and gravitational potential energy (GPE) are two forms of energy.

Gravitational potential energy is the energy an object has because of its position in a gravitational field.

Lifting an object does not work against the gravitational force (Weight). As a result, the object gains gravitational potential energy.

Gravitational potential energy = mass x gravitational field strength x heigh difference

Ep = m x g x h                                              When an object falls, its GPE energy is transferred in

Ep= is gravitational potential energy (J)       to kinetic energy.

m= Mass (Kg)

g= Gravitational field strength ( N/kg)

h= Height above a given level (M)

Kinetic energy is the energy an object has when it is moving. It depends on the mass and speed of the object.

Kinetic energy= 1/2 x mass x velocity^2

Ek= 1/2 x m x v^2

Ek= Kinetic energy (J)

m= Mass of the object (Kg)

v= The speed of the object (m/s)

The direction the object moves is not important in calculating kinetic energy.

The momentum of an object depends on both its mass and its velocity:

momentum= mass x velocity

p= m x v

p= Momentum of an object (Kg m/s)

m= Mass of the object (Kg)

v= Velocity of the object (M/s)

Like velocity, momentum has a specific direction.

In a closed system, the total momentum of all objects before an event is the same as the total momentum after the collision.

A closed system is one in which no objects or energy are added or taken away.

Some materials can be given a charge of static electricity.

Some materials become electrically charged when rubbed by another material. These charges are fixd or static because the materials used are insulators. The electrons cannot travel materials.

The material that gains electrons becomes negativly charged. The material that loses electrons becomes positivley charged.

An electric current is a flow of electical charge.

An electric current will only flow through a conductor such as a metal.

The size of the eletrical current is the rate of the flow of electrical charge.

Current= Charge / Time

I = Q / T

I= Electrical current (A)

Q=The amount of electric charge (C)

t= Time (s)

A current of 1 amp means that 1 coluomb of charge passes a point every second.

When an electric charge flows it does work and energy is trasferred. The potential difference is the amount of energy transferred by each coulomb of electric charge between two points.

Potential difference= Work done / Charge

V= W/Q

V= Potential difference (V)

W= Work done (J)

Q= Electric charge (C)

A potential difference of 1 volt means that 1 J of work is done by each coulomb of charge.

A resistor is an electrical component that reduces the current in a circuit.

Current and potential difference:

A current potential difference graph shows how the current through a component varies as the potential difference across it is changed.

Resistance:

The potential difference across a component depends on the current and the resistance.

Potential difference = Current x Resistance

V= I x R

V= Potential difference (V)

I= Current (A)

R= Resistance (Ohms)

The same current flows through all the components.

The potential difference of cells arranged in series is the sum of the potential difference of each cell.

This is only true if the cells are set up the same way around so that the + terminal of one cell is connected to the - terminal of the next.

The total resistance in a series circuit is the sum of the reistance of each of the components.

In a parallel circuit the current divides to pass through the different parts of the circuit.

The potential difference across each component is the same.

The current through some components varies in different ways.

Filament bulbs:

In a metal, the outer electrons become delocalised, leaving behind metal ions. The electrons can flow and carry charge.

When the potential difference across the thin wire in a filament bulb increases, the electrons move rapidly.

The electrons colide with the ions that make up the structure of the metal filament.

The electrons transfer energy to the ions so the temperature of the filament rises.

The ions vibrate more and obstruct the electrons, so resistance increases.

A current can only flow through a diode in one direction.

The resistance of the diode is very high in the reverse direction.

A light emitting diode (LED) will only give out light when the current flows in the forward direction above a small threshold value.

The resistance of a light dependent resistor (LDR) falls as the intensity of light falling on it increases. This means that an LDR allows more current to flow in the light compared with in the dark.

They are used to turn on street lights.

Thermistor:

The resistance of a thermistor decreases as the temperature rises. This means that the current through a thermistor rises as the temperature rises.

Thermistors are used in thermostats to control temperatures.

Cells and batteries supply DIRECT CURRENT (D.C). This is an electric current that always travels in the same direction.

The oscilloscope trace for d.c from a battery is a horizontal line. The potential difference can be read off the vertical scale.

The potential difference supplied to a circuit can be shown on an oscillioscope screen.

Mains electricity supplied to homes and businesses is an ALTERNATE CURRENT (a.c).

Alternating current is an electric current that changes direction regularly and its potential difference is constantly changing.

The outer case of the plug is made of an insulating plastic. This stops electrical energy from being conducted to the outside of the plug.

The 3 pins are made of a metal such as brass, which is a good conductor.

The inner cores of the wires are fixed to the 3 pins, usually by screw.

Cables may be 3 or 2 core (2 or 3 wires).

Only 3 core cables contain an earth wire.

The cable grip holds the cable firmly in the plug. This stops the wires from being pulled from the connectors.

Electrical appliances have safety features to protect uses from electric shocks and fires.

Fuses:

A fuse contains a wire made of a metal with a low melting point. It is connected tp the live pin in a plug.

When a current above the rating of the fuse flows through it, the wire becomes hot and melts. The circuit is broken so no harm can occur.

If the current rating of the fuse is too low it will melt when the normal current flows. The fuse has to be replaced. This is a nuisance.

If the current rating of the fuse is too high then it may not protect the appliance from a larger than normal current.

This could result in overheating that could cause a fire or damage the appliance.

Earthing:

The earth wire connects the metal casing to the earth pin in the plug.

If a fault connects the casing to the electric current, the earth wire conducts the current directly to the ground.

A large current flows, and this causes the fuse to blow or circuit breaker to operate.

Appliances with cases made of a non conducting material are said to be double insulated because they have an extra layer of insulation. They do not need an earth wire.

When an electrical charge flows through a resistor, the resistor gets hot. All the components in an electrical circuit, including the wires, act as resistors.

The energy transferred in a resistor is related to the potential difference across it and the charge that is flowing.

Energy = Potential difference x charge

E = V X Q

E= Energy (J)

V= Potential difference (V)

Q= Charge (C)

Power is the rate at which energy is transferred.

Power= Energy / Time

P = E / T

P= Power (W)

E= Energy (J)

t= Time taken (S)

The power of an electrical appliance is related to current and potential difference by the equation ...

P= I x V

P= Power (W)      I= Electrical current (A)         V= Potential difference across the appliance (V)

The nuclear atom:

Atoms have a tiny nucleus which has a positive charge. It is made up of protons and neutrons and is surrounded by electrons.

Proton =  +1 charge , 1 mass.

Neutron = 0 charge , 1 mass.

Electron= -1 charge, very small mass.

The number of protons is the same as the number of electrons, so an atom does not have an overall charge.

Atoms can lose or gain electrons to form ions.

When electrons are gained -> The ion has a negative charge.

When electrons are lost -> The ion has a positive charge.

We are constantly recieving radiation from our surroundings. Some substances give out radiation from their unstable nuclei. These substances are said to be radioactive. The process is not affected by temperature or by chemical reactions.

Radioactive decay is a random process. This means it is impossible to predict when a particular nuclus will give out radiation.

Most cosmic rays are stopped by the atmosphere.

Most of the sources of background radiation are natural, but a few are man made (Such as radiation from medical proceedures and nuclear power).

Types:

Nuclear power       Cosmic rays     Food and drink       Medical treatments      Buildings

Randon gas: This is released into the air by certain types of rock.

The nuclei of radioactive substances are unstable and give out various types of radiation.

There are 3 common types of radiation given off by radioactive nuclei.

Alpha particles: Made up of 2 protons , 2 neutrons (Same as helium nucleus). They are positively charged. They have a mass number of 4 and an atomic number of 2.

Gamma radiation: Is electromagnetic radiation with a short wavelength.

Beta particles: They are electrons so they have a negative charge. They have a mass number of 0 and an atomic number of -1.

An unstable nucleus can give out different types of radiation:

Alpha particles:

• Will travel a few cm in the air.
• Very ionising.
• Can be stopped by a sheet of paper.

Beta particles:

• Will travel a few m in the air.
• Moderately ionising.
• Can be stopped by 3mm thick aluminium.

Gamma rays:

• Will travel km in the air.
• Weakly ionising.
• Need thick led to stop them.

The activity of a radioactive material decreases with time.

The half life of a radioative isotope is the average time taken for the number of nuclei of the isotope in the sample to half.

Alternatively, if we measure the count rate of a sample containing a radioactive isotope, the half life is the time taken for the count rate to fall to half its initial level.

The count rate is the number of nuclear decays that happen every second.

Radioactive isotopes are used as TRACERS. In hospital, they are injected into a patient and carried to cancer cells.

The radiation can be detected by a gamma camera and shows where the cancer cells are in the patcients body.

The half life of the radioactive isotope must be long enough so that there is time for it to reach the cancer cells before it decays.

The half life must be short enough so that soon there is very little radioactive material left in the body. A half life of a few hours is suitable.

Risks from radiation: Alpha is the least penetrating and cannot pas through skin but as it is the most ionising it could cause most damage to cells if the source is inside the body.

Gamma is the least ionising so causes the least damage but it is the most penetratrating so is harmful when the source is outise the body. 

Beta falls between alpha and gamma in penetration and ionising strength.

A source of alpha particles is used in smoke alarms. It gives off a constant stream of alpha particles.

Alpha radiation produces ions in the air that complete the electric circuit. Smoke particles absorb the alpha particles so the circuit is broken and this sets off the alarm.

Beta radiation is used to monitor the thickness of paper and to control the filling of cartons. 

When paper is too thick not as many beta particles pass through the paper to reach the detector. A processor increases the force on rollers to make the paper thinner. If the paper is too thin, more beta particles get through.

In cancer treatment, powerful beams of gamma rays are sent from various difections to focus on the cancerous cells, so these cells recieve more radiation than the surrounding healthy cells.

Radiation from a radioactive isotope was used to investigate the structure of the atom. 

Until the 1890s people thought of atoms as hard balls. The discovery of the electron suggested that atoms had structure.The first theory was that atoms were like plum ruddings or currant buns.

Rutherfords: Rutherford realised that the plumb pudding model could not explain the new evidence. 

He suggested instead that most of the atom was empty space. Right at the centre of the atom was very small but very dense positiveky charged nucleus. 

Rutherford and Marsden:

In 1909, Rutherford and Marsden repeated an experiment which alpha particles were passed through a gold leaf. Unlike other scientists they checked to see if any alpha particles were deflected by large angles. They did not expect to find anything new, but they did find that a few alpha particles bounced almost straight back from the gold atoms. 

Nuclear fission releases energy that is used in nuclear power stations.

In nuclear fission the nucleus of an atom splits two smaller nuclei and energy is released.

 Energy is also released.

Nuclear fusion is the process which makes stars shine.

Nuclear fusion is the joining together of 2 atomic nuclei to form a single larger atomic nucleus. 

Nuclear fusion reactions release a lot of energy. 

Fusion produces atoms of heavier elements.

Hydrogen nuclei were formed shortly after the Universe formed in the big bang.

In stable stars, nuclear fusion reactions form all the elements to join up to iron.

Some stars explode in a supernova at the end of their life. 

This is when nuclear fusion reactions form the elements heavier than iron.

The life cycle of stars lasts millions or billions of years.

The size of the star decides what will happen to it during its life cycle.

Stars like the sun remain stable for billions of years. The gravitational forces pulling the matter in are balanced by other forces that try to make the star expand.

Fusion reactions turn hydrogen into other, heavier elements, step by step. These fusion reactions keep the star shining for billions of years. Stars are so massive they have enough matter to last for their lifetime. 

The birth of stars:

• Gases and dust in space are attracted together by gravity.
• If the ball of matter is big enough, fusion reactions start and a protostar is formed. This becomes a main sequence star.
• Small masses may also form and be attracted by a larger mass to form planets.