Circuits and resistance
- Electrical current is the flow of electrons. The amount of charge carried by a current of: 1A flowing for 1s = 1 coulomb.- charge (C) = current (A) x time (s)
- Electrical energy is transformed to heat energy when charge flows through the resistor. --------------------- energy transformed (J) = potential difference (V) x change (C)
- A potential difference (voltage) exerts a force that pushes free electrons along.
- Electrical resistance in wires due to free electrons colliding with fixed, positive ions and their bound electrons.
- Resistance in wires depend on their length, thickness, temperature and type of material.
- Metals have free electrons and are good conductors with low resistances.
- Insultators do not have free electrons and so are poor conductors with very high resistances.
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Circuits and resistance 2
- Ohm's Law states that: resistance (Ω) = potential difference/current (A)
- The resistance of: a light-dependant resistor (LDR) decreases as light intensity increases, a thermistor decreases as the temperature increases, a filament lamp increases as the temperature of the filament increases.
- Components in an electrical circuit can be connected in the series and in parallel.
- In a series circuit: current through all components in the same at all points; total p.d. is shared between all components; total resistance = sum of resistances of components.
- In parallel circuit: total current is the sum of currents through the seperate components; the p.d. across all components is the same.
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- UK domestic electricity is an alternating current of frenquency 50Hz, average p.d. 230V. An oscilloscope shows the p.d. and frenquency of a.c and d.c.
- Fuses, circuit breakers and earth wires protect appliances from damage and people from injury or death. Three-pin plugs must be correctly wired and hold the correct fuse.
- The current through an appliance and the size of the fuse required can be calculated from: power (W) = current (A) x p.d. (V)
- The rate of which energy is transformed is called power: power (W) = energy transformed (J) / time taken (s)
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- Different forms of an element with the same number of protons but with a different number of neutrons in their atoms are called isotopes.
- Isoptopes have the same atomic number but different mass number.
- Isotopes with unstable nuclei emit energy ----> as particles or rays called radiation.
- A beta particle is a high energy electron.
- A gamma ray is an electromagnetic wave with no charge or mass.
- Background radiation comes mainly from rocks, soil, cosmetic rays, living things and medical X-rays.
- Using scattering experiments, Rutherford and Marsden revealed the structure of an atom: a small dense nucleus of protons and neutrons (held together by strong nuclear forces); fast moving electrons orbiting round the nucleus; and lots of empty space.
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Nuclear fusion and nuclear fission
- Nuclear fission = is the splitting of a nucleus.
- Neautrons are fired at unstable isotopes and energy is released. This produces more neautrons and sets up a chain reaction.
- Nuclear Fusion = two small nuclei join together to form a larger one, as happens in stars: 2 hydrogen nuclei --> 1 helium nucleus + ENERGY
- Uranium-235 and plutonium-239 are fissionable elements and are used to ----> produce nuclear energy in nuclear power stations.
- The minimum amount of pure uranium-235 needed to produce large amounts of energy is called critical mass.
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