Physics, Chemistry and Biology: Post your doubts here!

[fatma123]

thnx so much for helping me guys

 

[Hamnah Zahoor]

If this book comes with a CD then u need to check it, the answers to the end of chapter questions are usually there. Otherwise, if the question reference is given the then you can always open up the marking scheme for it
Thanks to hamnah, for finding it out ^

You are welcome .....I found it in physics pastpapers code: 0625
oct/nov 2010/22 Question:3

 

[fatma123]

If this book comes with a CD then u need to check it, the answers to the end of chapter questions are usually there. Otherwise, if the question reference is given the then you can always open up the marking scheme for it
Thanks to hamnah, for finding it out ^

yea it does come with CD but i dont have my book at the moment bcuz its in the locker

 

[Shawn Lee]

Can anyone please explain why the answer for d is vertically downwards?

 

[Shawn Lee]

Can anyone please show me a sample circuit for ci) , I couldnt really understand the question.

 

[ar47saeed]

View attachment 63049
Can anyone please show me a sample circuit for ci) , I couldnt really understand the question.

Did you check the mark scheme?

 

[Asim Shah]

Physics Notes O level by my A* student available. Free . just open my Facebook account.

 

[Getsuga juujisho]

salamu alaykum
i dont understand what to choose in 0625_w17_qp_21 D or A?

 

[dangerdolan02]

Yo, does anyone have the grade boundaries for all three sciences for the 2016 n 2017 papers?

 

[Hussein Yasir]

which

salamu alaykum
i dont understand what to choose in 0625_w17_qp_21 D or A?

which question?

 

[Hussein Yasir]

yo

Yo, does anyone have the grade boundaries for all three sciences for the 2016 n 2017 papers?

you can find them in "dynamic papers"

 

[Hussein Yasir]

Atomic Physics Latest Syllabus Notes
(Please Like only If you believe this was helpful)



Introduction about Atoms

All Substances are made up of atoms. Atoms are the smallest building blocks of matter.

Atoms contain particles, some of which are charged:

Electrons are negatively charged, protons are positively charged, and neutrons are neutral.

The atom’s nucleus contains the protons and neutrons, while electrons revolve around it.

In a neutral atom, the charge of the protons is equal to the charge of the electrons. This differs when an ion is formed. An ion is an atom with a net electrical charge

Ions form when a neutral atom gains/loses an electron.

The number of protons in the nucleus of an atom is referred to as the atomic number.

The total number of protons and neutrons in the nucleus of an atom is the mass number.

The mass number is usually on top of the element’s symbol, while the atomic number is below.


For Example:

8O16 This means that the element ‘O’ (Oxygen) has a mass number of 16 and an atomic number of 8.

In this case, oxygen is a neutral atom, therefore, its atomic number gives the number of protons as well as electrons in the atom.

So, 8O16 has 8 protons and 8 electrons

The number of neutrons is obtained by subtracting the atomic number from the mass number. For oxygen, this would be: 16 – 8 which equals 8.

Therefore, a neutral atom of oxygen has 8 protons, 8 neutrons, and 8 electrons.

 

[Hussein Yasir]

Atomic Physics Latest Syllabus Notes (continued)
(Please Like only If you believe this was helpful)

Isotopes

Some forms of elements are rather unstable, they a carry a heavier load than usual in their nucleus. An isotope is an example.

Isotopes are forms of the same element which have the same atomic number but a different mass number, caused by the difference in the number of neutrons.

Isotopes have the same chemical properties as they have the same number of electrons, but have different physical properties.

Hydrogen has three isotopes, each having 1 proton but a different number of neutrons.

(Hydrogen, Deuterium, Tritium)

 

[Hussein Yasir]

Rutherford’s Scattering Experiment

In 1911, Ernest Rutherford carried out an experiment that lead to a new model of the atom. In that experiment, alpha particles were fired from a Radium source at a thin gold foil (10 mm thick).

Detectors were used to find how the alpha particles were scattered by the gold atoms.

The observations were as follows:
-About 99% of the alpha particles pass straight through the foil.
-Some of the alpha particles were deflected sideways.
- A very small number of the alpha particles rebound off gold foil.

Because of the results of this experiment, Rutherford showed that:
-The nucleus of an atom has a relatively small diameter compared with that of the atom.
-The alpha particles were repelled by a positive nucleus.
-The positive nucleus is a very heavy central body that can scatter back alpha particles.

 

[Hussein Yasir]

Radioactive Decay

Most Nuclei are stable because they contain the right balance between protons and neutrons. Nevertheless, not all combinations of protons and neutrons are stable. Some have heavy nuclei that decay in time, emitting radiations needed to reach the balance needed for stability.

Radioactive decay is a random (completely unpredictable) and spontaneous (self-starting) disintegration through which the parent unstable nucleus emits radiations and transforms to the more stable daughter nucleus, forming a new element.

 

[Hussein Yasir]

Types of Radiations

Three types of radiations can be emitted during radioactive decay:

1. Alpha Particles 2α4

An alpha particle is a nucleus of Helium – 4. It consists of 2 protons and 2 neutrons.

When a nuclide decays by alpha emission, the atomic number decreases by 2 and the mass numberdecreases by 4.
ZXA Z – 2YA – 4 + 2α4 + energy

1. Beta Particles –1β0

A beta particle is a high-speed electron. It carries one negative charge and has a negligible mass.

When a nuclide decays by beta emission, the atomic number increases by one and the mass number remains unchanged.

ZXA Z+1YA + –1β0 + energy

1. Gamma Radiation γ

Gamma radiations are electromagnetic radiations with a very high energy (short wavelength). They cause no change in mass or atomic number.

ZXA ZXA + γ + energy

 

[Hussein Yasir]

Detecting Radioactivity

Radioactivity can be detected using several ways:

1. Photograph Film

A photograph film can be blackened by radiations

1. The Gold Leaf Electroscope

Dry air is normally a good insulator, so a charged electroscope will stay that way, as the charge cannot escape.

When an electroscope is charged, the gold leaf sticks out, because the charges on the gold repel the charges on the metal stalk.

When a radioactive source comes near, the air is ionised, and starts to conduct electricity. This means that the charge can “leak” away, the electroscope discharges and the gold leaf falls.

1. Geiger-Muller Tube

When a particle enters the tube, it pulls an electron from an Argon atom. The electron is attracted to the central wire, and as it rushes towards the wire, the electron will knock other electrons from Argon atoms, causing an “avalanche”. Thus one single incoming particle will cause many electrons to arrive at the wire, creating a pulse which can be amplified and counted. This gives us a very sensitive detector.

This does not differentiate between different types of radiations.

1. The Cloud Chamber

It is a chamber through which the track of ionising particles can be made visible. The radioactive particles ionise the vapour so it condenses into droplets which fall on the track of the ionising particles.



Alpha particles form straight tracks as they are massive, so they do not get deflected. In addition, their tracks are thick as they have a strong ionizing power.








Gamma rays have very short and faint tracks due to their negligible mass and ionising power.



Background Radiations

A (G-M) tube connected to a rate-meter will always give a reading, even with no radioactive source present. This is due to the radiations that are essentially in nature, such as:

1. Cosmic rays from from outer space
2. Naturally occurring radioactive materials in the Earth’s surface.
3. Nuclear weapon testing

Therefore, to ensure accuracy in an experiment to measure the activity of a radioactive substance the background radiations should be subtracted off the measured count. To do that:

• The radioactive sources are removed a fair distance while placed in their lead containers. Then, the background radiations are measured and recorded.
• The experiment with the source is now carried out, and its respective reading is recorded.
• The background reading is subtracted from the total reading to find the source’s intensity only.

 

[Hussein Yasir]

Radioactive materials in magnetic fields:

In a magnetic field that is going into the page (away from the viewer), Fleming’s left hand rule is used to determine the deflection of the charged particles.
Alpha particles are deflected upwards (to the left), while beta particles are deflected downwards (to the right).



Radioactive materials in electric fields:

In an electric field, beta particles are attracted towards the positive plate as they are negative and alpha partciles are attracted towards the negative plate as they are positive.

Notice that beta particles get deflected a greater deal than alpha particles as they are much lighter.

Being electromagnetic waves, gamma rays are electrically neutral, so they do not get deflected by either.



Half-life

The half-life time is the time taken for half the number of radioactive atoms of a sample to disintegrate. I.e. the time taken for a sample’s activity to be reduced to half.

After one half-life, a sample’s activity is reduced to half. After two half-lives, a quarter, and so on..

It is entirely random and not affected by any factors such as temperature.

In a decay curve, it is crucial to subtract background radiations before finding the half-life time.

Uses of radio isotopes

Gamma rays are used in:

• Sterilising medical instruments (as they do not make the instruments radioactive)
• Killing cancer cells
• Tracers

A small amount of the radioactive isotope is injected in the body, and then it is traced using a detector.

Usually Technetium – 99 is used as it only emits gamma radiation which can penetrate the body and has a low ionising effect, in addition to the fact that it Technetium – 99 has a short half-life so it decreases quickly in the body.

Alpha particles are used in:

• Smoke Detectors

Radiation from the alpha source falls on a detector. Since alpha is charged, a small current flows in the detector. Smoke absorbs this alpha radiation, causing no current to flow to the detector, so the alarm is sounded.

Beta particles are used in:

• Monitoring the thickness of materials

The beta source is placed on one side of a moving sheet of material and a (G-M) counter on the other side. The count rate decreases if the thickness increases and vice-versa.

Alpha particles would be fully absorbed by the paper and gamma rays would penetrate the paper, only beta particles’ count would have a significant change when the thickness changes.





Radioactive isotopes are also used in radioactive dating and measuring fluid flow in pipes.
For the radioactive dating of carbon – 14 for example, the activity of a sample can be measured using a G-M tube or the number of carbon-14 atoms can be counted using a mass spectrometer



Handling radioactive materials

It is necessary to be careful with radioactive materials as they can cause cancer, infertility, and blindness.

Therefore, it is crucial that:

• Gloves are worn and radioactive sources are held by the forceps.
• Hands are washed after any handling of radioactive materials.
• All radioactive materials should be stored in a lead box.
• Long tongs are used when handling them to keep the sources as far from the body as possible.

 

[Hussein Yasir]

All this sent was Atomic physics
Please only like if you found this useful
thanks.

 

[Hussein Yasir]

What method can be used to show that crystalisation point has been reached?