Sec 2 - Electricity
" 2.3 understand the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances "
Insulation is covering a live wire with a material that won't conduct the electricity. Double insulation is a precaution that makes sure the live wire cannot touch the casing (so no shock can be conducted) usually by putting extra insulation round that wire. Double insulation can also mean that the casing of an object is plastic so even if the wire touches it, it wont conduct. An earth wire is touching the case so that if a current is in the case, it will be directed through the earth wire, this will then take the current to the earth. Additionally the surge of electricity in the wire may break the fuse. Fuses are sections of wire in the circuit that melt if too high a current goes through them. They come with different maximum currents. Circuit breakers have an electromagnet that is activated if the current goes above a certain limit. the electromagnet pulls an iron switch towards it, this opens the switch and breaks the circuit.
" 2.12 describe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperature "
An LDR is a light dependent resistor. Its resistance changes with the intensity of light: the brighter it is the less resistance; the less light the more resistance. Thermistors are temperature dependent resistors. In hot conditions there will be less resistance where as in the cold the resistance is high.
" 2.4 understand that a current in a resistor results in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts
As a resistor slows down the movement of electrons, the kinetic energy that was moving them is converted into heat energy. This can be used, for example, in hair dryers or heaters.
" 2.15 understand that current is the rate of flow of charge "
Current is the rate at which charge is flowing through a circuit. 'It is like the flow of water through a set of pipes'
" 2.7 understand the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery. "
Direct current flows in one direction only. It is supplied by cells and batteries. It comes out as a straight line on an oscilloscope. Alternating current changes from one direction to another rapidly. Mains electricity is alternating (interestingly this is because the electricity has to go through transformers on the national grid which only work on ac current, although that's not relevant here!)
" 2.17 know that electric current in solid metallic conductors is a flow of negatively charged electrons.
Electric current is a flow of electrons, so when there is an electric current in a metal, the electrons in the metal are flowing.
d) Electric charge 2.18 identify common materials which are electrical conductors or insulators, including metals and plastics. "
Electrical conductors are materials that allow a current to pass through them. To do this they need to have 'free' electrons, because current is a flow of electrons. Metals have free electrons because of the way they are bonded (atoms and electrons within a lattice) this means they are good electrical conductors. Plastics are polymers which are bonded in a way that means electrons aren't free and so can't move. No flow of electrons means no electric current so they are insulators.
" 2.13 know that lamps and LEDs can be used to indicate the presence of a current in a circuit
For an LED to light up there must be a current in a circuit. If a LED is in a circuit but not emitting light then there must be no current. If an LED is illuminated then it will have a current flowing through it. By this we know that if the LED in our circuit is shining then there is a current, if it isn't then we don't.
2.10 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally "
If you increase the resistance the current will decrease. resistors, metal filament lamps and diodes all create resistance in a circuit and so will decrease the current. This can be investigated using an ammeter and measuring the current with and without these components, or with different voltage levels (measured by voltmeter.)
" c) Energy and potential difference in circuits 2.8 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting "
In a series circuit everything is connected on one line. This means that the voltage is shared out between every component: this makes it useful for supplying low power things like fairy lights. In a parallel circuit different components are connected separately to the supply. This means that of one component breaks the others can continue being powered as the whole circuit is still functioning, this makes it practical to use. It is also good for charging higher power things as the potential difference is equal all over a parallel circuit so each component receives the full voltage.
b) Mains electricity 2.2 understand and identify the hazards of electricity including frayed cables, long cables, damaged plugs, water around sockets, and pushing metal objects into sockets "
In frayed cabling the insulation has worn down exposing live wires, electricity can be conducted from these. Longer cables are at a higher risk of being damaged and there is more resistance with longer wires making them more at risk of over heating. Damaged plugs create a risk that some of the safety features may be broken. Water conducts electricity and can cause energy from the circuit to flow trough it creating a fire and electrocution risk. Metal objects in sockets have the same dangers.
" 2.11 describe the qualitative effect of changing resistance on the current in a circuit "
Increasing the resistance will decrease the current. This can be achieved by adding more components or ones with higher resistance. Decreasing the resistance will increase the current. This can happen if components are removed or replaced by those with lower resistance.
" 2.9 understand that the current in a series circuit depends on the applied voltage and the number and nature of other components
The current in a series circuit is the same through out all parts of the circuit. It is worked out using the equation I= V/R. So its the total of the voltages received by the components divided by the total of all the components resistances.
" 2.16 know and use the relationship between charge, current and time: charge = current × time; Q = I × t "
charge = current × time Q = I × t
" 2.6 use the relationship between energy transferred, current, voltage and time: energy transferred = current × voltage × time ; E = I × V × t "
energy transferred = current × voltage × time E = I × V × t n.b this is the same thing as saying power x time
2.5 know and use the relationship: power = current × voltage; P = I × V and apply the relationship to the selection of appropriate fuses "
power = current × voltage P = I × V
2.14 know and use the relationship between voltage, current and resistance: voltage = current × resistance V = I × R "
voltage = current × resistance V = I × R
