6.18 Explain the use of step-up and step-down transformers in the largescale generation and transmission of electrical energy

Transformers in the national grid are used to stop the wires from overheating - a very high current flowing through very long wires would produce a lot of heat, because the wires have some resistance.

• Step up transformers increase the voltage, decrease the current to stop energy waste through heat when passing through long wires

• In households, wires are shorter so there's less heat wasted, so step down transformer decreases voltage and increases current to make it more useful to us. 

Questions 5 - Electricity

1. When correctly connected to a 9 volt battery the wire has a current of 0.30 amperes flowing through it.

(i) Give the equation that links current, resistance and voltage. (1)

V = I x R (or R = V/I, same thing)

(ii) Calculate the resistance of the wire.  Show clearly how you work out your answer and give the unit. (3)

Tip: use the 123 method...always...but especially when it's 3 marks!

1. I = 0.30 A       2. R = V      3. R = 9 ÷ 0.30

    V = 9 V                     I              = 30  Ω (ohms)

(iii) When the wire is heated, the current goes down to 0.26 amperes.  State how the resistance of the wire has changed. (1)

The resistance has increased to about 34 Ω (ohms), whereas before it was only 30. It goes up.

2. An adaptor can be used to connect up to four appliances in parallel to one 230 V mains socket. The adaptor is fitted with a 13 A fuse. The table gives a list of appliances and the current they draw from a mains socket.

Appliance	Current
computer	1 A
hairdryer	4 A
heater	8 A
iron	6 A
television	2 A

(i) What current will flow to the adaptor when the television, computer and hairdryer are plugged into the adaptor? (1)

2 + 1 + 4 = 7 A (always mention unit.)

(ii) Write down the equation which links current, electrical power and voltage.(1)

P = I x V 

power = current x voltage

(iii) Calculate the electrical power used when the television, computer and hairdryer are plugged into the adaptor. Show clearly how you work out your answer and give the unit. (2)

Tip: use 123 method

1. I = 7 A          2. P = I x V      3. P = 7 x 230

    V = 230 V                             = 1,610 W (watts)

(iv) What would happen to the fuse if the heater is also plugged into the adaptor?

Give a reason for your answer. (2)

Current is greater than 13A, so the fuse would melt. (in fact it would be 15A)

Note: the stuff that's underlined is more or less where you'd get the marks. In some cases, because of formatting, I was unable to underline some stuff, so here it is below.

In question 1 (ii), you get marks for the correct re-arranging of the equation, the units (ohms) and the right answer. Note that in question 2 (iii), you can change the answer to 1.61 kW (kilowatts) and still get the mark.

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Questions - not answered

1. A set of Christmas tree lights is made from twenty identical lamps connected in series. 

a) Each lamp is designed to take a current of 0.25 A. The set plugs directly into the 230 V mains electricity supply.

(i) Write down the equation that links current, potential difference and resistance. (1)

(ii) Calculate the resistance of one of the lamps. Show clearly how you work out your final answer and give the unit. (4)

(iii) What is the total resistance of the set of lights? (1)

(b) How does the resistance of a filament lamp change as the temperature of the filament changes? (1)

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2.  Burning fuels produce smoke particles and waste gases. An electrostatic smoke precipitator can remove the smoke particles from the waste gases.

The following sentences A to E describe how an electrostatic smoke precipitator works. The sentences are in the wrong order. Arrange the sentences in the right order. Start with sentence C. (3)

A The smoke particles stick to the positively charged collecting plates.

B The smoke particles are given a negative charge.

C The waste gases and smoke pass through a negatively charged metal grid.

D The collecting plates are knocked so the smoke particles fall and can be taken away.

E The smoke particles are repelled from the grid and attracted to the positively charged collecting plates.

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3. The underground storage tanks at petrol stations are filled from tankers. A static electric charge can build up on the tanker as the petrol flows through the pipe to the storage tank. This could be dangerous. 

Why is the static electric charge dangerous and what should be done to stop the charge building up? (2)

No copyright infringement intended - I did not write any of the questions above. All rights go to the appropriate author and owner. However, all answers are completely 100% totally written by me. I may have modified the question number to fit formatting but that's it.

2.25 Explain some uses of electrostatic charges, eg in photocopiers and inkjet printers.

Photocopier

BBC Bitesize has a very clear diagram (above) that explains how this works. Steps are as follows:

• Original document placed on photocopier
• Light goes through
• Image of it is projected onto a positively charged plate/surface
• Where the light touches the plate, the electrical charge is gone (poof.) so that only the writing/image/whatever is left
• Negatively charged toner (ink) particles are sprayed or rolled onto the thingie
• The ink sticks to the positive areas
• Paper is placed over this and the ink is transferred to the paper
• It is heated so the ink sticks
• Photocopy is taken off the photocopier and turned over - 'TIS DONE!

This page is particularly useful - it is a link to BBC Bitesize and it has various uses for electrostatic charge. (If the hyperlink doesn't work, copy and paste the link below into your browser)

http://www.bbc.co.uk/schools/gcsebitesize/science/add_gateway_pre_2011/radiation/electrostaticsusesrev1.shtml

2.24 Explain the potential dangers of electrostatic charges, eg when fuelling aircraft and tankers

Electrostatic charges can sometimes be dangerous. For example, when fuelling a car.

The problem
The tube that the fuel runs through is rubber. This is an insulator. The fuel rubs against the rubber, creating friction so the fuel is charged. The static build-up can cause a spark - the car is made of metal, and the electrons have nowhere to go, so they try to 'jump' onto the metal, which creates a spark. When fuelling, you'll often notice that a strong smell and a tiny bit of gas comes out. This is highly flammable. When there is a spark, this gas will catch fire and could harm the person(s) near it. In a more extreme case, it might even cause the car to explode :( 

The solution
Earthing the excess charge - this means having a metal rod that carries the excess electrons down to the earth to the charge is no longer a hazard :) Note that this must be done before fuelling starts. 

Everyone needs a bit of Star Wars in their life <3

2.23 Explain electrostatic phenomena in terms of the movement of electrons

Electrostatic phenomena is when static has a specific effect. For example, when you get a static shock (you know, like if your friend is somehow charged and then you touch them and like ow it feels like someone just pinched you...it's that).

Electrons can move from one material to the other, and their ultimate goal is to reach the earth. When you touch your friend (let's say it's a she and her name is Agoriel), the electrons jump from whatever is charged (eg a wooly sweater) to you and then down to the earth.

A better example: your friend Agoriel is touching a Van der Graaff generator (look it up if you don't know what it is) - her hair is beginning to stand on end and she is negatively charged. She is standing on an insulator. If you stick out a finger to touch her finger, there will be a spark, you'll probably go "owch!" and the electrons jump to you and then travel down to the earth.

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

When two insulators rub against each other, the electrons move from one to the other. The one that has gained electrons is now negatively charged, and the one that has lost electrons is now positively charged.

Note that only electrons move - protons are still (hence the name)

This is charge by friction.

Materials can also be charged by induction. 

This is when a electrons are pushed away. Let's say that (for some reason) you are negatively charged. You place your hand in front of a can and it rolls towards you. How does this work? Because your hand is negatively charged, and like charges repel, it repels the electrons at the front of the can. This leaves the protons, which are positively charged. Therefore, the can is now charged positively by induction. Because opposite charges attract, the can rolls towards your hand.

To demonstrate this visually (sorry about the bad drawing skills, that was me making the diagrams)

Key - purple dots are electrons and red dots are protons (note that on both diagrams, the hand is supposed to have electrons on it but I didn't quite get the same shade of purple the second time through)

Figure 1 - Step 1

Figure 2 - Step 2

Questions 3 - Electricity

Sample questions
2.2 - Why should you not touch a mains appliance with wet hands?
Water is a conductor therefore it may cause the electric charge to flow to the water instead of the appliance and you could be electrocuted.

2.3 - The normal amount of current supplied to a device is 2.7 A. Which fuse rating would be suitable for this device?
a) 1A
b) 3A
c) 5A

Answer: B. If it was a 1A fuse, the device wouldn't work -when you turned it on, the fuse would melt and you'd have to replace it. If it was a 5A fuse, if there was an electricity surge, the device would be damaged and fried by excess current under 5A (difference between 2.7 and 5 is too big)

2.5 - Find the power of an appliance that transfers 1000 J in 30 seconds.

• E = 1000J
• T = 30 sec
• P = E / T (note - this equation is not on spec. but you are expected to know it)
• P = 1000 / 30
• P = 33.33333....
• P = 33.3 W (3 s.f.)

2.6 - How much charge flows when a current of 3A runs for 5 mins?

• I = 3A
• T = 5 mins 300 sec
• Q = I x T
• Q = 3 x 300
• Q = 900 C

 Questions (not answered)

• 1. How much energy is transferred when:
• a)  200 C flows through a resistor that has a potential difference of 230 V?
• b) a current of 5A passes through a 12 V lamp for 20 mins?

• 2. State one advantage of a circuit breaker compared with a fuse.

• 3. An appliance is supplied with 3A when it functions normally. 
• a) Should you use a 3A, 4A or 5A fuse?
• b) Explain your choice.

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

Examples of conductors include:

• copper
• iron
• nickel
• gold
• steel (that's a metal alloy)
• aluminum/aluminium
• pretty much any metal

Examples of insulators (non conducting materials):

• plastic
• Styrofoam
• rubber
• cloth
• pretty much any non metal

Explanations :)

Conductors are materials that allow an electrical current to be passed through them. They need to have 'free' electrons because current is a flow of electrons - this is why metals are conductors. Some materials will have more free electrons. The more of them they have, the less resistance, and therefore they are a better conductor.

Non conducting materials do not allow an electrical current to be passed through them. They do not have free electrons which means they cannot move. No flow of electrons therefore no current.

2.18 Understand that voltage is the energy transferred per unit charge passed and the volt is a joule per coulomb.

Voltage is the energy transferred per unit of charge.
The unit volt is simply a joule per coulomb - basically how many joules per coulomb (kind of like meters per second)

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

An electric current in solid, metallic conductors (meaning metals) is a flow of electrons, which are negatively charged.

• Current flows from positive (+) to negative (-)
• Electrons flow from negative (-) to positive (+)

(if you think that's silly, it is. But you need to know that, even if it's basically the same thing. You must know the difference. This silly thing is what happens when science changes over time...)

2.15 Understand that current is the rate of flow of charge

Self explanatory.

Current is the rate at which charge flows. 

Note: Charge is represented by Q in an equation while current is represented with an I. Don't get those two muddled.

2.14 Know and use the relationship between voltage, current and resistance:

IMPORTANT EQUATION!!

Voltage = Current x Resistance
V = I x R

Voltage measured in volts (V)
Current measured in amps (A)
Resistance measured in ohms (Ω)

You will need to re-arrange this equation a lot so keep this in mind:

Figure 1

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

A lamp or LED will not turn on unless there is a current in the circuit (and/or the circuit is complete). This means that is a current, the lamp/LED will turn on.

This can be used to determine whether there is a current.

Literally, this is all that matters:

• LED/lamp on = current
• LED/lamp off = no current

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

LDRs are light dependent resistors. This means that the brighter the light, the less resistance there is. (so the dimmer the light, the more resistance.)

Figure 1 - Circuit symbol for LDR

Thermistors are temperature dependent resistors.This means that the higher the temperature, the lower resistance there is. When it is cold, the resistance is high.

Figure 2 - Circuit symbol for thermistor

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

As resistance goes up, current goes down, as can be seen in Fig. 1.

Figure 1

As resistance goes down, current goes up.

This is proven - if you put numbers into the equation V = I x R (same as R = V/I) you will see this/these patterns emerge.

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

OKAY THIS IS REALLY CONFUSING BUT I'M GOING TO TRY.

In wires, the higher the voltage, the higher the current and vice versa. If there is virtually no resistance, they are directly proportional.

In resistors, if the current goes down, the voltage will go down, though they are not directly proportional. If resistance goes up, current goes down.

In filament lamps, as the temperature increases, the resistance will increase, therefore the current will go down as the temperature goes up. This also applies to the voltage, although, again, they are not directly proportional.

Diodes only allow current to flow through in one direction. So if it's not in the direction that the diode allows, the current will equal zero

Please help if you know anything about this point, thanks. :)

Note to self - add graphs in from book

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 series circuit, current is the same everywhere. This means that it can be worked out using the equation I = V/R . This means it is the total voltage supplied divided by the total resistance of all the components. 

Eg. if a battery supplies the circuit with 30V, what is the current? There are 3 bulbs and each has a resistance of 2, 4 and 7 ohms respectively.

So... let's name the bulbs. There are three. Let's say that...
Nimue bulb = 2 ohms
Esyae bulb = 4 ohms
Morthil bulb = 7 ohms

Nimue + Esyae + Morthil = 13 ohms total resistance
    (2)           (4)        (7)

The total voltage, we already established, is 30 V.

30 / 13 = 2.30769231
Round off to 3 s.f. or 2 d.p.
= 2.31 (3 sf)

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

In series, everything is connected on the same branch of wire, as can be seen in figure 1.

Figure 1

In series:

• Current is the same everywhere
• Voltage is shared
• If one component breaks, the others stop functioning
• 1 bulb supplied with 1 A will be brighter than two bulbs supplied with 1A (because current is the same!)

It is mainly useful for supplying things like fairy lights with electricity. It often means that lights connected in series are not particularly bright (they can be but that would require a lot of electricity and would be too expensive to be worth it)

In parallel, components are connected on different branches of wire, as can be seen in figure 2:

Figure 2

In parallel:

• Voltage is the same everywhere
• Current is shared
• If one component breaks, others continue working
• Bulbs tend to be brighter

This is useful for anything connected to mains electricity. This makes it practical and, when it comes to lighting, the lights are very bright. It also means that if, for example, your toaster stops working, the rest of the house will still have lighting.

2.7 Understand the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied

Direct current:

• Flows in one direction only (+ to -)
• Supplied by cells and batteries
• On an oscilloscope, shows a straight line

Alternating current:

• Varies between + and - 
• Changes direction very quickly
• Mains electricity is AC
• On an oscilloscope, shows a wiggly line

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

E = I x T x V
(way to remember it is E = ITV)
Energy transferred = Current x Voltage x Time

Energy is measured in Joules (J)
Current is measured in amperes (A)
Voltage is measured in volts (V)
Time is measured in seconds (sec)

Converting...
1 A = 0.001 kilo amps (divide by 1000)
1A = 0.000001 mega amps (divide by 1 million; 1000000)
1A = 1000 miliamps

(If you understand, don't look at the one below because it might confuzzle you)


In other words...
1 KILO amp = 1000 amps
1 MEGA amp = 1000000 amps (that's a million)
I MILI amp = 0.001

Same applies to all else :)