Saturday, 31 March 2012

Forces, movement, shape and momentum

(as momentum is for Single Science, I won't be talking about it unless you want smth on it, anything for Single Science is labelled SS in front of the specification point.)

1.8 express a force as a push or pull of one body on another

A force is a push or pull that one object exerts on another
3 things a force can do to an object:

  • change its speed
  • change is shape
  • change its direction
1.9 identify various types of force (for example gravitational, electrostatic etc.)

various types of force:
  • push/pull (contact force)
  • tension-the pull at both ends of a stretch spring, string or rope
  • compression
  • thrust/upthrust
  • load
  • effort
  • *weight/gravitational
  • *electrical/electrostatic
  • *magnetic
*the last three forces are non-contact, they can act without touching an object

SS 1.10 distinguish between vector and scalar quantities 
Scalars have magnitude (size), Vectors have both magnitude and direction. 
Weight/tension/compression à forces
Field strength – magnetic/electrical/gravitational

SS 1.11 appreciate the vector nature of a force
Forces have magnitude and direction, it is therefore a vector. For example, weight is a force with magnitude, and it acts downwards. It is a vector, and you would normally use an arrow pointing downwards with its magnitude ("xN" N being the unit-Newtons) to represent it in a diagram.

SS 1.12 add forces that act along a line
The forces act along a line, meaning they are collinear. You can add the forces together like scalars. For instance if two forces are acting on a box, both pushing towards the right side, one with a force of 4N and the other with 6N, then 4N+6N=10N (resultant force). So a total of 10N is acting on the box, pushing it to the right. 

However, if one of the forces was acting to the left with say, 2N and the other to the right with 7N, then the resultant is 5N to the right. The 2N to the left cancels out 2N from the right.  You can think of the force acting to the left as a negative value, like the reverse direction, so 7N + (-2N) = 5N. It's good to use diagrams to help you out. 

1.13 understand that friction is a force that opposes motion
Friction is a force that always opposes motion between 2 surfaces in contact.

1.14 recall and use the relationship between unbalanced force, mass and acceleration:

force=mass x acceleration
F= m.a

1.15 recall and use the relationship between weight, mass and g:

weight= mass x g
W= m x g
g=gravitational field strength, this is the gravitational force exerted per unit mass at a point in the field, it is a vector quantity. unit= N/kg
on earth, we consider g as 10N/kg, or g = 9.8 m/sbut we use g = 10 m/sthis is how an object in the Earth's gravitational field would accelerate if it was free fall-without friction (this doesn't happen in the real world). 
( to explain how we get to this:
a= F/m = mg/m = g ) 

1.16 describe the forces acting on falling objects and explain why falling objects reach a terminal velocity

2 forces act on falling objects: 
  • Earth's gravity-Weight (W): has direction (vector quantity), pulls object downwards towards the centre of the Earth
  • air resistance/drag (D)--upwards force, pushes object upwards
Air resistance increases with speed. When an object first starts falling, their weight is greater than air resistance. (W>D) Hence it falls faster, accelerating towards the ground. However, as it gains speed, the air resistance increases until it eventually equals the object's weight. (W=D) Acceleration then becomes zero (a=0) and the object will have reached terminal velocity. 

1.17 describe the factors affecting vehicle stopping distance including speed, mass, road condition and reaction time

vehicle stopping distance= *( reaction time (a.k.a thinking time) x constant speed ) + breaking distance 
vehicle stopping distance= thinking distance + braking distance
*d=s x t, so it becomes thinking distance
thinking distance: how far the car travels at constant speed before the driver reacts by applying the car brakes
braking distance: distance travelled by the car as it decelerates to a stop

  • as speed increases, stopping distance increases
  • as mass increases, force needed to stop car increases (F=ma)
  • dry weather=more friction, rainy=less friction, as water acts as lubricant
  • as reaction time increases, stopping distance increases (If driver was drunk, their reaction would be slower, so reaction time increases, hence stopping distance increases)
SS 1.18 recall and use the relationship between momentum, mass and velocity:
momentum = mass x velocity
p= m x v

SS 1.19 use the ideas of momentum to explain safety features

SS 1.20 use the conservation of momentum to calculate the mass, velocity or momentum of objects

SS 1.21 use the relationship between force, change in momentum and time taken:
force= change in momentum/time taken

SS 1.22 understand Newton's third law
Newton's Third Law of Motion:
For every action, there is an equal and opposite reaction, and these forces act on mutually opposite bodies. 
i.e. forces occur in pairs.
Fab = -Fab

E.g. when an object is falling, not only is weight acting on it downwards, but there is also air resistance pushing it upwards -- the forces act in opposite directions on the same object.
Or for instance, when a car is driving, the driving force pushes the car forwards, but there is also friction acting in the opposite direction, if the driving force wasn't bigger than friction, the car would be slowing down...

1.23 recall and use the relationship between moment of a force and its distance from the pivot

moment= force x perpendicular distance from the pivot
moment= F x d

1.24 recall that the weight of a body acts through its centre of gravity

SS 1.25 recall and use the principle of moments for a simple system of parallel forces acting in one plane

SS 1.26 understand that the upward forces on a light beam, supported at its ends, vary with the position of a heavy object placed on the beam

1.27 describe how extension varies with applied force for helical springs, metal wires and rubber bands
they all obey Hooke's Law up to their elastic limit
-thanks to a correction by an anonymous person (see comments), it apparently should be:
helical springs and metal wires do obey Hooke's Law, but rubber bands do NOT follow Hooke's Law  and the extension is NOT directly proportional to the force causing it. 

1.28 recall that the initial linear region of a force-extension graph is associated with Hooke's Law

Hooke's Law: The extension is directly proportional to the stretching force. 


Hooke's Law only applies to the straight part of the graph-initial linear region, up to the limit of proportionality. 
Elastic limit-if a spring is taken beyond this limit, it won't return to its old shape=deformed.

1.29 associate elastic behaviour with the ability of a material to recover its original shape after the forces causing deformation have been removed

Deformation from the limit of proportionality to the Elastic limit is still reversible; but 
Beyond the elastic limit, deformation is irreversible=permanent deformation or plastic deformation. 

Movement and position

1.1 Units: kg, m, m/s, m/s2, N, s, N/kg

a) Movement and position
1.2 understand and use distance-time graphs

Distance -time graphs

The gradient of a distance-time graph=speed
Distance isn't increasing-gradient=0, hence speed=0---->object is stationary (not moving)

Distance time graph from BBC Bitesize
The steeper the graph, the greater the speed as the gradient is steeper. 
If the graph is curved, then the speed must be changing. If it is curving upwards (blue line), the speed is increasing, if it is curving downwards, the speed is decreasing. So the gradient of the graph also tells us how the speed is changing.

Instantaneous speed=How fast an object is moving at a particular instant.
The gradient of the tangent at a point on the distance-time graph gives us the instantaneous speed.

1.3 recall and use the relationship between average speed, distance moved and time:

average speed=distance moved/time taken

1.4 recall and use the relationship between acceleration, velocity and time:

acceleration=change in velocity/time taken
where v=final velocity
          u=initial velocity

1.5 interpret velocity-time graphs

1.6 determine acceleration from the gradient of a velocity-time graph

the gradient at a point on the velocity-time graph gives you the acceleration

1.7 determine the distance travelled from the area between a velocity-time graph and the time axis

basically, the area under a velocity time graph = distance travelled

Sunday, 11 March 2012


Please note that not all of the specification points for Radioactivity (Section 7) are answered here. But if there are any questions, just comment. :)

Investigating radioactivity
  • We can use a Geiger counter to detect  radioactivity. The counter clicks each time a particle of radiation from a radioactive substance enters the Geiger tube. 

7.9 recall the sources of background radiation

Radioactivity around us 
When we use a Geiger counter, it clicks even without a radioactive source near it. This is due to background radiation from radioactive substances found naturally all around us. 

Background radiation is: 
  • ionising radiation from space (cosmic rays)
  • from devices e.g. X-ray tubes 
  • from radioactive substances in environment-some are present due to nuclear weapons testing and nuclear power stations, but most of it is from substances in the Earth (e.g. radon gas is radioactive and is a product of the decay of uranium in the ground)

Why are some substances radioactive?
Every atom has a nucleus made of protons and neutrons, and electrons orbit around in the space surrounding the nucleus. 

Most atoms have a stable nucleus that doesn't change. But some are unstable, and they become stable by emitting alpha, beta or gamma radiation. An unstable nucleus is said to decay when it emits radiation. We can't tell when an unstable nucleus will decay. It is a random event that happens without anything being done to the nucleus.

When an unstable nucleus decays, there are three ways that it can do so.
It may give out:-
  • an alpha particle (we use the symbol )
  • beta particle (symbol )
  • gamma ray (symbol )
As far as we know, there are four fundamental forces in the universe.
In order of increasing strength, they are:-
  • The Gravitational force 
        (very weak, only noticeable when large masses are involved)
  • The Electromagnetic force 
        (responsible for electrostatic attraction & repulsion,
         also the behaviour of magnets) 
  • The Weak Nuclear force 
        (actually pretty strong, but only operates over very short distances. Responsible for radioactive decay)
  • The Strong Nuclear force 
        (very strong, but very short range. Responsible for nuclear reactions such as fission).
Inside any nucleus, the protons and neutrons are held together by the incredibly powerful "Strong Nuclear Force", which overcomes the electrostatic repulsion between the protons.
A balance exists between these two forces.
This means that certain numbers of protons and neutrons can make a stable nucleus, whilst other groupings will be more or less unstable. These will eventually decay to produce a more stable arrangement.

The Stability Line

7.4 understand that alpha and beta particles and gamma rays are ionising radiations emitted from unstable nuclei in a random process
7.5 describe the nature of alpha and beta particles and gamma rays and recall that they may be distinguished in terms of penetrating power

Alpha particles
Alpha particles are made of 2 protons and 2 neutrons.
This means that they have a charge of +2, and a mass of 4
(the mass is measured in "atomic mass units", where each proton & neutron=1)
We can write them as , or, because they're the same as a helium nucleus, .
Alpha particles are relatively slow and heavy.
They have a low penetrating power - a sheet of paper will stop it. 
Alpha particles ionise other atoms strongly as they have a large charge. 

Alpha particles are made of 2 protons with 2 neutrons.
This means that when a nucleus emits an alpha particle, it loses 2 protons and so its atomic number decreases by 2.
Also, when a nucleus emits an alpha particle, its atomic mass decreases by 4 (2 protons + 2 neutrons)

Alpha-decay occurs in very heavy elements, for example, Uranium and Radium.
These heavy elements have too many protons to be stable. They can become more stable by emitting an alpha particle.
Alpha particles have a large charge(+2), so they easily ionise other atoms that they pass. Ionising atoms requires energy, so alpha particles lose energy rapidly as they travel. Thus they have a range of only a few centimetres in air.

In alpha decay: 
  • atomic number decreases by 2
  • atomic mass decreases by 4
Beta particles:

Beta particles have a charge of minus 1, and a mass of about 1/2000th of a proton. This means that beta particles are the same as an electron.
We can write them as  or, because they're the same as an electron, .
They are fast and light.
Beta particles have a medium penetrating power - they are stopped by a sheet of aluminium or plastics such as perspex.
Beta particles ionise atoms that they pass, but not as strongly as alpha particles do.

Under certain conditions, a neutron can decay, to produce a proton + an electron. The proton stays in the nucleus, whilst the electron is emitted at high speed.

This means that when a nucleus emits a -particle:
  • the atomic mass is unchanged (neutrons and protons have same mass)
  • the atomic number increases by 1 (addition of a proton)
This is because a neutron has changed into a proton (almost the same mass - we can ignore the tiny mass of the electron) and thus the number of protons has gone up.
Example: Strontium-90 undergoesdecay and forms Yttrium-90.
           a beta-deacy equation

(This isn't the whole story - an almost massless particle called an "anti-neutrino" is also emitted. Furthermore, we are only considering "beta-minus" emission (negatively-charged electrons).
There is another type of beta decay, called "beta-plus", where a positively-charged electron (called a "positron") is emitted, along with a neutrino. But that's beyond IGCSE..)

Beta decay occurs in very "neutron-rich" elements, for example, Strontium-90 and Iodine-130. These elements are typically created in nuclear reactors.
These elements have too few protons and too many neutrons to be stable. They can thus become more stable by emitting a beta particle.

Beta particles have a charge of -1, and weigh only a tiny fraction of a neutron or proton. As a result,  particles interact less readily with other atoms than alpha particles.
Thus beta particles cause less ionisation than alphas, and have a longer range, typically a few metres in air.

Remember, in Beta decay :
  • atomic number increases by one
  • atomic mass unchanged.

Gamma Rays

Gamma rays are waves, not particles.
This means that they have no mass and no charge-
Gamma rays have a high penetrating power - it takes a thick sheet of metal such as lead, or concrete to reduce them significantly.

Gamma rays do not directly ionise other atoms, as they have no charge, although they may cause atoms to emit other particles which will then cause ionisation.
We don't find pure gamma sources - gamma rays are emitted alongside alpha or beta particles. Strictly speaking, gamma emission isn't 'radioactive decay' because it doesn't change the state of the nucleus, it just carries away some energy.

Gamma rays () are electromagnetic waves, rather like X rays and radio waves.
(no mass and no charge.) 
After a nucleus has emitted an -particle or a -particle, it may still have too much energy: we say it is in an "excited state".
It can get rid of this energy by emitting a pulse of very high frequency electromagnetic radiation, called a gamma ray.

-particles and -particles pull electrons off atoms as they pass (we say they ionise the atoms), but rays don't. This means that they do not lose much energy as they travel, as they do not interact as much with the matter they pass.
Therefore, gamma rays have a high penetrating power, and a very long range.

There is no such thing as a pure -ray source. Gamma rays are given off by most alpha emitters and beta emitters. If we want a source of pure gamma rays, we can get it by using a substance that emits both beta and gamma, and simply keep it in an aluminium container that stops the beta particles.
Useful gamma sources include Technetium-99, which is used as a "tracer" in medicine. This is a combined beta and gamma source, and is chosen because betas are less harmful to the patient than alphas (less ionisation) and because Technetium has a short half-life (just over 6 hours), so it decays away quickly and reduces the dose to the patient.

Remember, in Gamma decay:-
  • atomic number unchanged
  • atomic mass unchanged.

Thursday, 8 March 2012

Physics Specification

This is a link to the specification, the last few pages are extremely useful with the equations and circuit symbols. But I have a post about the equations, as I added the symbols for them, which is better and easier to remember than words. Tell me if the link doesn't work.

Equations Glossary

These are most of the equations you need to know. If I have missed any out, please comment and let me know. Thanks!

(i) the relationship between average speed, distance and time: 
average speed= distance/time
(ii) the relationship between force, mass and acceleration: 
force = mass × acceleration 
acceleration = change in velocity/time taken
(iii) the relationship between density, mass and volume: 
(iv) the relationship between force, distance and work: 
work done = force × distance moved in direction of force 
(v) the energy relationships: 
energy transferred = work done 
kinetic energy = ½ × mass × speeds2
Ek= ½ x m x s2
gravitational potential energy = mass × g × height 
(vi) the relationship between mass, weight and gravitational field strength: 
weight = mass × gravitational field strength 
W=m x g
(vii) the relationship between an applied force, the area over which it acts and the resulting 
(viii) the relationship between the moment of a force and its distance from the pivot: 
moment = force × perpendicular distance from the pivot 
moment=F x d

(ix) the relationships between charge, current, voltage, resistance and electrical power: 
charge = current × time 
voltage = current × resistance 
electrical power = voltage × current 
(x) the relationship between speed, frequency and wavelength: 
wave speed = frequency × wavelength 
V(velocity)=f x λ
(xi) input (primary voltage)/output (secondary voltage)=primary turns/secondary turns
(xii) the relationship between refractive index, angle of incidence and angle of refraction: 
n=sin i/sin r
(xiii) the relationship between refractive index and critical angle: 
sin c = 1/n
(xiv) the relationship for pressure difference: 
pressure difference = height × density × g 
p = h x ρ x g

Conventional current vs. Electron flow

There's a lot of confusion about currents, and that's because there's the conventional current and the electron flow--the electron flow being the one that actually conducts electricity..

When electrons move, an electric current is produced. In short, electric current is formed by moving electrons.

Conventional current and electron flow-always confuzzles people

This is just a basic explanation of why one goes from positive to negative and the other is from negative to positive. Hope this clears things up! 

An electric current is actually caused by a flow of electrons moving from a negatively charged end to a positively charged end. This movement of electrons towards the positively charged end is known as electron flow. 

In the early days before the discovery of electrons, scientists thought that an electric current consisted of positive charges flowing from a positively charged end to a negatively charged end. 
This assumption is still widely held today because the discovery of electron movement did not affect the basic understanding of an electric current, which consists of moving charges. 
This idea is so strong that scientists today still adopt it and they have termed it conventional current flow (since this is the convention adopted by most).