Physics GCSE OCR Gateway P4 Revision Cards

• The Higher the resistance in a circuit, the harder it is for current to flow.
• Variable resistors can be changed 
• Resistance = voltage / current

• Live brown wire
• Neutral blue wire
• Earthed green/ yellow wire

If a fault occurs:

• A big current is sent down the live wire through the case and cuts out the earthed. The surge blows the fuse. This isolates the whole appliance making it impossible to get an electric to get a shock and prevents fires.
• Fuses should be rated.
• Metal cased objects are earthed.

• Caused by friction build up.
• Electrons move across material.
• Positive charges never move, just the -ve charges.
• positive and negative attract, others repel but it attracts dust, clings, crackles, can cause electric shocks and sparks.
• Objects can be earthed or insulated to prevents sparks e.g. fuel tankers are earthed and operating theaters are earthed due to the large concentrated of Oxygen.
• Anti-static sprays provide an easy path for charges to move away.

Uses:

• Get an even covering of paint when paint spraying.
• Dust precipitations when cleaning up emulsions.
• Defibrillator- Restarting hearts.

• Current (amps A) only flow when there is a Voltage across the circuit.
• Voltage (vols V) force that pushes the current.
• Resistance (Ohms) slows the flow down.
• Voltage increases- current increases.
• Resistance increases- current decreases (more voltage will be needed).
• If you break a current the current stops flowing.

• Sound= longditudal wave
• Ultrasound= high frequency wave which you can't hear.

Uses of Ultrasound:

• Measuring blood flow.
• Breaking down kidney stones: High energy waves turn into sand-like particles, then they pass through the system, it's painless and doesn't use surgery.
• Prenatal foetus scans: ultrasound waves pass through the body and bounce back differently. The times and distributions of these waves are preprocessed and produce an image.

X-rays would be dangerous to a foetus, they look ate hard bone where ultrasound looks at soft tissue. Ultrasound doesn't use ionising radiation. 

X-rays are used in hospitals, they pass easily through flesh but not bone or metal. They could cause cancer. Radiographers must wear lead aprons and stand behind lead screens. Alpha can't penetrate the skin unlike Gamma and Beta.

X-rays and gamma rays are both electromagnetic waves. They're similar lengths  therefore have similar properties. Gamma rays are released when atoms decay. X-rays can be made by firing high speed electrons into heavy metals such as Tungsten. They are easier to control than Gamma rays.

• Radiotherapy- cancer treatment uses gamma rays to kill living cancer cells; gamma rays focus on the tumour using on the wide beam, minimise the exposure of normal cells radiation and reduces the chances of more damage.
• Tracers in Medicines- Must be gamma/ beta so it isn't absorbed by the body, Iodine- 131 is used to see if Iodine is absorbed by the Thyroid gland properly.
• Sterilisation of surgical instruments- Gamma rays will kill microbes, doesn't damage temperature sensitive instruments like thermometers and plastic instruments like boiling would. 

• Tracers in industry e.g. finding leaks 
• Smoke detectors
• Radioactive dating of rocks and other artefacts. Radioactive carbon dating uses Carbon-14 to calculate how old articles are by looking at how much radioactive material is still in there.

• Half Life is the time taken for half of the radio active atoms now present to decay. 
• Long life= activity falls more slowly. 
• Short life= activity falls more quickly.
• It is shown as a curve on a graph, use a step-by-step method to work out the half life if a graph has not been given.

• Nuclear Fission:- Splitting Uranium Atoms which release energy, this is used to form heat, this boils water and creates steam which spins a generator to form electricity. 
• Li-235 becomes unstable Li-236 which splits to become K-90 and Barium-144 which are radioactive.

• H= Alpha =Big, heavy, doesn't penetrate much, Helium nuclei .
• e= Beta= Fast, small, electrons.
•   Gamma= Electromagnetic radiation, no mass or charge. 

Radioactivity is random: unstable nuclei decay and given out radiation change into new elements. Atomic numbers must be balanced.

Comes from unstable isotopes in food, air, building materials, rocks, cosmic rays, fall out from.

The level of background radiation changes depending on where you are:

• High altitudes mean more exposure to cosmic rays.
• Underground mines due to rocks surrounding you.
• Rocks such as granite and Radon gas getting trapped in peoples homes.

Radon gas:- exposure= more lung cancer, low concentration= safe, 1/20 lung cancer deaths are because of Radon gas, more harmful than smoking, but new homes are being built with better ventilation to help stop the build up of it in high concentrations at home.