National 4 & 5 Physics Portree High School

Electricity and Energy 1 – Summary Notes

Electrical Charge

We know that atoms are made up of 3 particles:

Protons (positive charge)

Electrons (negative charge)

Neutrons (no charge)

In an electrical conductor, electricity flows because

it is the electrons that move through the wire.

When electrical charges don’t move, this is called static charge. Static charges have many applications:

1. Photocopying

2. Pollution Control

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3. Paint spraying

Car manufacturers can save money by using charged paint spray guns. They work because like charges repel and unlike charges attract.The spray gun is charged positively, which causes every paint particle to become positively charged. Like charges repel and the paint particles spread out. The object to be painted is given a negative charge and so attracts the paint particles. The advantages of using this system are that less paint is wasted, the object receives an even coat and the paint covers awkward ‘shadow’ surfaces that the operator cannot see.

The Coulomb – a measure of electrical charge

Electrons are the basic carriers of electrical charge. The problem is that the charge on one electron is so tiny, it doesn’t really do much. This is a bit like thinking about sugar; one grain of sugar doesn’t give you much sweetness, however a whole bag of sugar ... mmm!

So, many electrons are grouped together (just theoretically) to form a ‘packet’ of charge that is large enough to be measured. This ‘packet’ of charge is called a COULOMB. There are 6.25 x 1018 (lots and lots) of electrons in one Coulomb.

Electrical Current

When charges start to flow, or move in a conductor, we get an ELECTRICAL CURRENT. The more charges that pass a point in a given time, the greater the current. Electrical current is measured in AMPERES, or Amps for short. If one coulomb of charge passes a point in one second, then the current is one Amp.

Alternating Current (ac) and Direct Current (dc)

In dc, the electrical charges flow one way only round a circuit.

A battery gives dc.

In ac, the electrical charges first flow one way, then the other

round the circuit. Mains power is ac.

Voltage (or Potential Difference)

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This is the energy given to each coulomb of charge in a circuit. This energy comes from a battery or power supply of some sort. Voltage is measured in Volts.

The Effect of an Electrical Field on a Charge

If an electron is released between the two charged plates as shown, it will accelerate towards the positive plate.

The energy it gains in moving through a

potential difference, V, is given by:

W = Q x V W = Work Done (Joules)

Q = Charge of particle (Coulombs)

V = Potential Difference (Volts)

Electrical Resistance

RESISTANCE is a measure of how difficult, or easy, it is for the charges to flow in a circuit. Resistance is measured in Ohms ().

A large resistance means that the charges find it difficult to get through, so not many of them make the journey (small current), and they use up a lot of energy getting through (large voltage).

Factors that Affect Resistance of a Wire

1. Thickness of wire (Thin wire = High R)2. Length of wire (Long wire = High R)3. Temperature of wire (High temp = High R)

Ohm’s Law

HIGH resistance = large voltage

= small current

LOW resistance = small voltage

= large current

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This is probably the fundamental electrical engineering formula linking voltage, current and resistance.

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Example:

Calculate the total resistance in the parallel circuit above.

R1 = 2 Formula is 1/RT = 1/R1 + 1/R2 + 1/R3

R2 = 3 RT = ½ + 1/3 + 1/4

R3 = 4 RT = 0.5 + 0.33 + 0.25 = 1.08

RT/1 = 1/1.08 = 0.93

Measuring Voltage and Current

It is very important to flip both sides head over heels at this stage.

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Current is measured using an AMMETER.

Voltage is measured using a VOLTMETER.

Circuit Symbols

Real Life Parallel Circuits

The voltmeter is placed across the component.

No need to break the original circuit.

The ammeter is placed in the circuit.

A ‘hole’ needs to be made in the original circuit, and the ammeter then placed in the ‘hole’.

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Almost all household wiring is in parallel; lighting and sockets. This is to keep everything else working when there is a fault with one unit.

Real Life Series Circuits

You may think that series circuits have no application in the real world because they are too simple. If you have a basic on/off torch, you will have a series circuit. In any safety application where you want two switches to operate at the same time for a piece of machinery to work, you will have these switches in series.

Electrical Power

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Power is a measure of the energy used each second. Power is measured in Watts (w).

Electricity companies charge us for every ‘Unit’ of electricity we use at home. One ‘Unit’ of electricity is when 1Kw (1000W) is used for one hour. This is one kilowatt-hour (Kwh).

What can be done with 1 Kwh of electrical energy...

Energy saving measures

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With all of these ideas, the energy used to do the job is less.

Energy = power x timeUsing this formula, it is clear that to reduce the energy used, we need to either reduce the time or power.

1. Buying energy efficient appliances.

When you buy a new fridge, dishwasher, kettle, etc.,

you should pay attention to the energy efficiency rating.

Efficient appliances may cost more to buy, but they

will reward you with cheap running costs.

2. Energy Saving Light Bulbs .

This 20w energy saving light bulb, costing £4.00 gives out as much light as an

old fashioned 100w light bulb costing £0.50.

Assuming a unit price of £0.18 for our electricity, we can

work out how much it would cost for us to have each light

switched on permanently for one week. Remember, to do this

we need to calculate how many Kwh each bulb uses. To do this we need the

power in Kilowatts, and the time in hours.

3. Switch off Appliances .

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Having appliances on standby may be easy and convenient, but the appliances still use up energy:

Logic Gates

Figuring out logic Circuits

Example: Draw the truth table for the following circuit:

Step 1: Mark points in the circuit that will help you:

Step 2: Start truth table by putting in all the possible input combinations, remember to be logical and count up using binary:

Appliance When on (watts)

Standby (watts)

Stereo 22 12TV 100 10Video recorder 13 1DVD recorder 12 7Digital top box 6 5Computer + peripherals

130 15

Computer monitor 70 11Laptop computer 29 2Broadband modem 14 14Answering machine 3 3Battery charger 14 1Mobile phone charger

5 2

Total 418 83

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Step 3: Fill in any other points that you’ve created:

Step 4: Have a look at the circuit and identify what points in the circuit determines the output, then go ahead and finish the truth table: