2.25 explain some uses of electrostatic charges, e.g in photocopiers and inkjet printers

Photocopiers

In photocopiers, there is an image plate which is positively charged, an image of what you are photocopying is then projected onto it. The charge leaks away from the white/light parts of what your copying as they make light fall on the plate. The dark/black parts of what your copying remain charged and attract negatively charged black power, which is transferred onto positively charged paper. Next, the paper is heated to stick the powder down.

Inkjet printers

In an inkjet printer, tiny droplets of ink are forced out of a fine nozzle, this makes them electrically charged. The droplets are then deflected as they pass between two metal plates. A voltage is applied to the plates, one is negative and the other is positive. The droplets are attracted the the plate of the opposite charge and repelled from the plate with the same charge - the size and direction of the voltage across each plate constantly changes so that each droplet is deflected to hit a different place on the paper.

2.23 explain electrostatic phenomena in terms of the movement of electrons

This is quite confusing and may need a few reads to fully understand...

The movement of electrons is responsible for all phenomena relating to current electricity. However, electrostatic phenomenon, such as electrostatic induction , is the transfer of electrons from one body to another.

For example, polythene and cloth. When they are rubbed together, the rod will gain electrons and become positively charged and the cloth will lose electrons and become negatively charged.

2.24 explain the potential dangers of electrostatic charges, e.g when fuelling aircraft and tankers

Problem

As fuel flows out of a filler pipe (the pipe thing that you put in your car, etc), static electricity can build up. This can easily lead to a spark, which can cause a fire in dusty/fumy places.


Solution

Make the nozzle of the filler pipe out of metal so that the charge is conducted away instead of building up (as the building up of electrostatic charge will cause a spark).

It is also good to have earthing straps between a feeling tank and the fuel pipe, to earth the charge, avoiding a build up of the charge.

2.22 understand that there are forces of attraction between unlike charges and forces of repulsion between like charged

- Two materials with opposite electrostatic charges are attracted to each other
- Two materials with the same electrostatic charge will repel

NOTE: like magnets, these forces get weaker the further apart the two materials are

2.21 explain that positive and negative electrostatic charged are produced on materials by the loss and gain of electrons

When two insulating materials are rubbed together, they become electrostatic ally charged. This is because the electrons are transferred from one material onto the other. For example, when polythene rod (this has a negative charge) is rubbed against a cloth, (this has a positive charge), all the electrons move from the cloth to the polythene rod. This leaves the polythene rod negatively charged (as it has gained electrons) and the cloth positively charges (as it has lost electrons).

A way of remembering this is with the anagram, NIGPIL (yes i made it up, if you remember it tough, it does work!). It stands for...
Negative Is Gain - as the material that gains electrons becomes negatively charged
Positive Is Loss - as the material that loses electrons becomes posotively charged

NOTE: Which material transfers which electrons will depend on the two materials involved - it will not always be the cloth losing electrons/the rod gaining electrons.

2.20 describe experiments to investigate how insulating materials can be charged by friction

When two insulating materials are rubbed together, electrons will pass from one insulator onto the other insulator.

Rub a cloth on a polythene rod. Electrons will move from the duster to the rod. The rod will become negatively charged and the cloth will become equal positively charged. To check, hold the rod and the cloth near a pile of small bits of paper,  the bits of paper will be attracted to the rod/cloth as they are electrically charged.

Alternatively, you could use an acetate rod and cloth, it involves the same process and, therefore, the same outcome will occur. The only difference is that the acetate rod will be positively charged and the cloth will become negatively charged.

2.19 identify common materials which are electrical conductors or insulators, including metals and plastics

Materials that are electrical conductors conduct charge easily (basically, a current can flow through them), they are usually metals (although not always). Examples include copper and silver

Electrical insulators do not conduct charge well at all (basically, a current can't flow). Examples include plastic and rubber.

2.18 understand that: voltage is the energy transferred per unit charge passed, the volt is a joule per coulomb

Voltage is the energy transferred per unit charge passed, it is measures in volts. One volt is one joule per coulomb.

2.17 know that electric current in solid metallic conductors is a flow of negatively charged electrons

Electric current is the rate of flow of electrical charge (in amperes, A) around a circuit, in solid metal conductors (for example, a copper wire), charge is carried by negatively charged electrons.

2.16 know and use the relationship between charge, current and time.

Charge = current x time
    Q     =     I      x    t

Example...
A battery charger passes a current of 2.5A over a cell for a period of 4 hours. How much charge does the charger transfer to the cell altogether?

Firstly, cover 4 hours into seconds... 4 x 60 x 60 = 14,400 seconds.

now substitute into the equation...

Charge = 2.5 x 14,400 = 36,000 C (36 kC)

(charge is measured in coulombs, C)



Random fact that we need to know but doesn't really fit anywhere: the bigger the current the bigger the charge

2.15 understand that current is the rate of flow of charge

Charge cane positive or negative, and when it flows it is known as current. Therefore, current is the rate of flow of charge around a circuit, it will only flow through a component if there is a voltage across that component. The unit is Amps (or amperes), A.

2.14 know and use the relationship between voltage, resistance and current.

Voltage = current x resistance

V         =      I       x      R


Example..

a 4Ω resistor in a circuit has a voltage of 6V across it. What is the current through the resistor?


Firstly, rearrange the equation because we are finding I, the substitute what we know into it...

I = V/R

Therefore, Current = V/R = 6/4 = 1.5 A

Current = 1.5 A

2.13 know that lamps and LEDs can be used to indicate the presence of a current in a circuit

If there is a light in a circuit, it will be lit if there is a current in the circuit and not lit if there is no current. The same applies to LEDs (Light Emitting Diodes).

2.12 describe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperature

LDRs and Thermistors are components that can change their resistance (aka, it is not always the same).

An LDR (Light Dependant Resistor) changes its resistant depending on how much light falls on it. For example, in bright light the resistance is low and in darkness the resistance is at its highest.

A thermistor is a temperature-dependant resistor (its resistance changes depending on the temperature). For example, in hot conditions the resistance is low and in cooler conditions the resistance is high.

2.11 describe the qualitative effect of changing resistance on the current in a circuit

Increasing resistance (by adding components such as lamps or diodes) will decrease the current, decreasing resistance (removing components) will increase current.

2.10 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally

Resistors (e.g lamps and diodes) lessen the flow of current, whilst voltage increases the current. This can be investigated using an ammeter (measures current) and a voltmeter (measures voltage).

Set up a circuit like this (known as the 'standard test circuit')...

Interchange the component with different resistors for example a lamp or diode, and record the recordings on the ammeter.

Also, increase/decrease the voltage and note the change on the ammeter.

Compare and contrast results. Overall, increasing the voltage should increase the number on the ammeter (current) whilst resistors (the components) will lower the current.

2.9 understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

In a circuit, the voltage is the driving force.

Any component added into the circuit (for example, a lamp) will increase the amount of resistance in the circuit, this will decrease the current.

The voltage is trying to push the current around the circuit and the resistance is trying to slow it down, therefore, the relative sizes of the voltage and resistance decide how big the current will be.

If you increase the voltage - more current will flow
If you increase the resistance - less current will flow (alternatively, if you want the same amount of current to fly when the resistance is increased, you could increase the voltage).

2.8 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting

Series

In a series circuit, all components are connected on one line. This means that the voltage is shared between every component equally making it useful for supplying low power things, for example fairy lights. However, as the components are connected along one line, they can not be individually turned on, you can turn on all or none. Also, if one component breaks, the rest of the circuit will not work.

The circuit diagram above shows a circuit with two lamps connected in series. If one lamp breaks, the other lamp will not light.

Parallel

In a parallel circuit different components are connected separately to the supply. This means that, unlike in a series circuit, if one component breaks the other components can continue being powered as the whole circuit can still function. This makes it useful for powering objects that require a high power things as the voltage is equal throughout the circuit (therefore, each component receives the full voltage).

The circuit diagram above shows a circuit with two lamps connected in parallel. If one lamp breaks, the other lamp will still light.

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.

There are two different types of electricity: alternating current and direct current.

Mains electricity is alternating current (AC). Alternating current changes from one direction to another rapidly. Direct current is electricity supplied by cells and batteries, it flows in ones direction only.

2.6 use the relationship between energy transferred, current, voltage and time

2. energy transferred = current × voltage × time
   
   E=I×V×t 
   

(as power = IxV, E=IxVxt is the same as E=Pxt)