- Electrostatic effects are due to the movement of electrons
- The law of electric charges: Like charges repel, Unlike attract
- A negative charge is due to an excess of electrons
- A positive charge is due to a lack of electrons
- When a polythene rod is rubbed with a duster, electrons are transferred from the duster, making the polythene rod negatively charged.
P4- Electric shocks
- If a person touches something high voltage, large amounts of electric charge may flow through the body to earth
- Current is the rate of flow of charge
- Static electricity may be a nuisance but not dangerous: dust and dirt are attracted insulators, such as television screens and clothes made from synthetic materials often cling to each other and to the body
- Electric shocks can be avoided by connecting an object that's likely to become charged to the earth pr standing on a rubber mat
- Anti- static spray carries away electric charge, this prevents a build up of charge.
- Defibrillation is a procedure to restore a regular heart rythm by delievering an electric shock through the chest wall to the heart
- Two paddles are charged by a high voltage supply, then placed firmly on the patient's chest to ensure good electrical contact, electric charged is passed through the patient to make their heart contract.
- Great care is taken to ensure that the operator does not recieve an electric shock
P4- Paint sprayers
- Static electric is used in paint spraying
- The spray gun is charged, all the paint particles become charged with the same charge, as like charges repel the paint particles spread out giving a fine spray, the object to be painter is given the opposite charge to the paint, opposite charges attract so the paint is attracted to the object and sticks to it, the object gets an even coat with limited paint wastage
P4- Dust Precipitators
- A dust precipitator removes harmful particles from chimneys of factories and power stations that pollute the atmosphere
- A metal grid (or wires) is placed in the chimney and given a large negative charge from a high-voltage supply, as the soot particles pass close to the wires the soot particles become negatively charged, like charges repel ao the soot particles are repelled away from the wires, they are attracted to the positively charged plates and stick to them, when the particles get big enough they fall back down the chimney
P4- Electric circuits
- An electric current is a flow of electric charge
- Charge is carried by negatively charged electrons
- The current is measured in amperes (A) using an ammeter connected in series
- The current in a circuit is due to an electron movement: Electrons are pushed around the circuit by the battery, the bump into atoms in the resistor giving them energy, their energy makes the atoms vibrate more so the resistor gets hotter and its resistance increases
P4- resistors and resistance
- Resistance- the amount by which a conductor (e.g. resistor) reduces an electric current
- A variable resistor changes the resistance
- The potential difference (pd) between two points in a circuit is the difference in the voltage between the two points. It's measured in volts, using a voltmeter connected in parallel.
- For a fixed resistor, as the pd difference across it increases, the current increases
- For a fixed power supply, as the resistance increase, the current decreases
- The formula for resistance is resistance= pd/ current (R=V/I), it's measured in ohms.
P4- Live, neutral and earth wires
The live wire carries a current that alternates between a + and - voltage as has to cope with different plugs, it's usually brown.
The neutral wire completes the circuit, it's lept at a zero voltage, providing a return path for the current. It's usually blue.
The earth wire is a safety wire that's connected to the case of an appliance to prevent it becoming live.
P4- Insulation and fuses
- A fuse contains wire which melts, breaking the circuit if the current becomes to large
- No current can flow, preventing overheating and further damage to the appliance
- Earth wires and fuses stop a person recieving an electric shock if they touch a faulty appliance. As soon as the case becomes 'live', a large current flows in the eatyj and live wires and fuse 'blows'
- A re-settable fuse (circuit breaker) doesn't need to be replaced to restor power; it can be re-set
P4- Longitudinal waves
- All sounds, including ultrasounds, is produced by vibrating particles that form a longitudinal waves
- The features of longitudinal sound waves are: they can't travel through a vacuum, the denser the medium the faster a wave travels; the higher the frequency or pitch, the smaller the wavelength, the louder the sound or the more powerful the ultrasound the more energy is carried by the wave and the larger its amplitude. In a longitudanal wave the vibrations are same direction as wave
- Ultrasound is sound of a higher frequency than humans can hear, it travels as a pressure wave- compressions and rarefactions
P4- Uses of ultrasounds
- When ultrasound is used to break down kidney stones: a high-powered ultrasound beam is directed at the kidney stones, the energy breaks the stones down into smaller pieces, the tiny pieces are the excreted from the body in the normal way
- When ultrasound is used in a baby scan, a pulse of ultrasound is sent into the body. At each boundary between different tissures some ultrasound is reflected and the rest is transmitted.The returning echoes are recorded and used to build up an image of the internal structure
- Ultrasounds can be used for body scans instread of X-rays because it differentiates between soft tissue better and it doesn't damage living cells
P4- using radiation
- Alpha is absorbed by the skin or paper so is of no use for diagnosis or therapy, but can be dangerous when in body as cannot get out of body.
- Beta radiation passes through the skin, but not bone. Its medical application are limited but it's used, for example, to treat the eyes.It's blocked by thin aluminium.
- Gamma radiation is very penetrating and is used in medicine. Cobalt-60 is a gamma-emmiting radioactive material that's widely used to treat cancer. It's stop by thick lead.
- When nuclear radiation passes through a material it causes ionisation (the formation of ions- charged particles). Ionising radiation damages living cells, increasing the risk of cancer.
- Cancer cells can be destroyed by exposing the affected area to a large amount of radiation- radio theropy
P4- Comparing X-rays and gamma rays
- Have similar wave lengths but produces in different waves
- X-rays are made by firing high-speed electrons at metal targets. A hot cathode emits electrons that are attracted to a anode. When the electrons hit in the middle most of their kinetic energy is converted into heat but some is transferred to X-rays
- An X-ray machine allows the rate of production and energy of the X-rays to be controlled, but you can't change the gamma radiation emmited from a particular radioactive source
- When the nucleus of an atom of a radioactive substance decays it emits an alpha or a beta particle and loses ant surplus energy by emitting gamma rays
- A radioactive tracer is used to investigate inside a patient's body without surgery
- The radioactive tracer being used is mixed with food or dink, or injected into the body
- Its progress through the body is monitored using a detector such as a gamma camera connected to a camera
In addition tracers are used to: track the dispersal of waste materials, find leaks or blockages in underground pipe and track the route of underground pipes
When using a tracer to locate a leak in underground pips a very small amount of gamma emitted is put into the pipe, a detector is passed along the ground above the path of the pipe and an increase in activity is detected in the region of the leak and little or no activity is detected after this point
P4- Treating Cancer
- A radioisotope is used to destroy a tumour in the body
- Three sources of radiation are arranged around the patient with the tumour at the centre, the healthy tissue only recieves one-third of the dose which limits damage to healthy tissue
- Or each radiation source is slowly rotated around the patient. The tumour recieves constant radiation but healthy tissue recieves only intermittent doses
P4- Radioactive decay
- Radioactive substances decay naturally, giving out alpha, beta and/or gamma radiation
- It's a random process, it's not possible to predict when exactly a nuclues will decay
- Radioisotopes have unstable nuclei. The nucleur particles aren't held together strong enough
- The half-life of a radioisotope is the average time for half the nuclei present to decay. It can't be changed
- The nucleon is a particle found in the nucleus, so protons and neutrons are nucleons
P4- What are alpha and beta particles
When an alpha or beta particle is emitted from the nucleus of an atom, the remaining nucleus is a different element.
- Positively charged, has quite a large mass, is a helium nucleus, consists of two protons and two neutrons
- During decay mass number decreases by 4, nucleus has two less neutrons and two less protons, the atomic number decreases by 2
- Negatively charged, have a small mass, travels very fast, is an electron
- During decay mass number is unchanged, nucleus has one less neutron, nucleus has one more proton, atomic number incrases by one
P4- Background radiation
- Background radiation is due to: radioactive substances present in rocks (especially granite) and soil; and cosmic rays from space
- Most background radiation is from natural sources but some comes from human activity
Carbon-14 is a radioactive isotope of carbon that is present in all living things. By measuring the amount of carbon-14 present in an archaeological find, its approximate age can be found.
When an object dies, no more carbon-14 is produced. As the carbon-14 decays, the activity of the sample decreases. The ratio of current activity from living matter to the sample activity leads to a reasonably accurate date.