Principles of Electricity
Principles of Electricity
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Principles of Electricity
This lesson centres on how electrical quantities are measured and how electron behaviour explains current, materials, and the effects of current.
Learning goal
I can explain standard SI units used in electrical work and describe how electron theory links to current, materials, and the effects of electric current.
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What words do you link with electricity and electrical measurement?
Use this as a prior-knowledge activation. Expect a mix of everyday and technical words such as current, voltage, wires, safety, power, units, copper, circuit, resistance and energy.
Why standard units matter
Electrical installation, maintenance and design depend on shared measurements.
If everyone uses the same unit system, values can be:
compared accurately
used safely in calculations
recorded clearly on drawings and test results
understood internationally
This standard system is the SI system.
Emphasise that standardisation is a safety and communication issue, not just a maths convention.
Base SI units in this topic
Common physical units
More physical units
metre (m) - length
square metre (m²) - area
cubic metre (m³) - volume
kilogram (kg) - mass
kilogram per cubic metre (kg/m³) - density
second (s) - time
degree Celsius (°C) - temperature
metre per second (m/s) - velocity
The handout groups these as units relevant to the electrical industry. Distinguish quantity from unit symbol clearly.
Electrical quantities and units
Electrical work uses derived SI units for important quantities.
Examples include:
current in ampere (A)
voltage in volt (V)
resistance in ohm (Ω)
power in watt (W)
energy in joule (J)
frequency in hertz (Hz)
Keep the distinction clear: quantity name, formula symbol, and unit symbol are not the same thing.
Further electrical units
Other electrical quantities also have standard SI units:
resistivity - rho (ρ)
impedance - ohm (Ω)
inductance - henry (H)
inductive reactance - ohm (Ω)
capacitance - farad (F)
capacitive reactance - ohm (Ω)
power factor - no unit
The handout lists resistivity with rho as the named unit notation; in delivery, clarify that rho is the symbol for the quantity resistivity. Power factor is dimensionless.
Which unit is used to measure electric current?
volt
ampere
watt
joule
Checks distinction between common electrical quantities and their units: current-ampere, voltage-volt, power-watt, energy-joule.
SI units in electrical work
Electric current is measured in amperes and has the formula symbol I.
The SI unit symbol for an ampere is A.
Potential difference, commonly called voltage, is measured in volts.
The formula symbol for voltage is V and its SI unit symbol is also V.
Electrical resistance is measured in ohms and has the formula symbol R.
The symbol used for the ohm is Ω.
Electrical power is measured in watts and has the formula symbol P.
The SI unit symbol for a watt is W.
Frequency is the number of cycles per second and is measured in hertz.
The formula symbol for frequency is f and its SI unit symbol is Hz.
Electrical energy or work is measured in joules.
The SI unit symbol for a joule is J.
Impedance has the formula symbol Z and is measured in ohms (Ω).
The electrical quantity represented by C is capacitance and its SI unit is the farad.
The SI unit symbol for capacitance is F.
Inductance has the formula symbol L and is measured in henries.
The SI unit symbol for a henry is H.
The Greek letter ρ (rho) is used as the formula symbol for resistivity.
All blanks are load-bearing terms taken directly from the units block.
Give four examples of conductors and insulators. What makes them different?
Expected elements: conductors have loosely attached outer electrons that can move easily; in copper this allows free electrons; a battery repels and attracts electrons; current only continues if the circuit is complete.
Glossary
SI units - the international system of standard units used for measurement
current - the flow of electric charge in a circuit
voltage - electric potential difference or electromotive force
resistance - opposition to the flow of electric current
power - the rate at which electrical energy is transferred or used
conductor - a material that allows electric current to flow easily
insulator - a material that strongly resists the flow of electric current
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Glossary
atom - a basic unit of matter made of a nucleus and electrons
proton - a positively charged particle in the nucleus
electron - a negatively charged particle around the nucleus
electromotive force - the driving force that moves electrons round a circuit
electrolysis - chemical change caused by electric current in a liquid
resistivity - a property that shows how strongly a material resists current
energy - the capacity to do work or transfer heat, measured in joules
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Structure of matter
All materials are made from atoms.
An atom has:
a central nucleus
protons in the nucleus
electrons around the nucleus
A molecule contains one or more atoms combined together.
Use the example from the handout: water is H₂O, made from hydrogen and oxygen atoms.
Electrical nature of atoms
Atoms contain charged particles:
protons are positive
electrons are negative
In a neutral atom, the number of protons equals the number of electrons.
If that balance changes, the atom becomes electrically charged.
Useful examples from the handout: hydrogen has 1 proton and 1 electron; helium has 2 and 2; uranium has 92 and 92.
Why copper conducts well
Copper has 29 protons and 29 electrons.
Its outer electron is loosely attached, so it can be detached easily. That makes copper a very effective material for carrying electric current.
The handout explains that the outer electron is far from the nucleus and affected by repulsion from inner electrons, so it is easier to free.
From free electrons to current
In a conductor, many free electrons move randomly.
When the material is connected across a battery:
negative terminal repels electrons → positive terminal attracts electrons → electrons move around the complete circuit
This electron flow is called electric current.
Stress that the battery provides electromotive force, but current only continues if the circuit is complete.
A complete circuit is essential
Even with an applied voltage, electrons cannot keep flowing unless there is a complete path.
If the circuit is broken:
current stops
electron flow stops
a switch can be used to control the circuit
This is a good place to connect theory to practical installation work: continuity matters.
What makes a conductor?
A conductor has outer electrons that can be moved or dislodged easily.
When one electron moves, it repels others, so the effect spreads through the material. This allows current to pass through the conductor.
Use the 'domino fashion' description from the handout to make the idea memorable.
What happens if a circuit is broken?
current stops
voltage doubles
resistance ends
nucleus moves
Tests understanding that a complete path is essential for continued electron flow.
From atom to current
A neutral atom has equal numbers of protons and electrons. In copper, a loosely attached outer electron can become free. When many electrons move round a complete circuit, the flow is called electric current.
Reinforces charge balance, copper's free electrons, and the definition of current.
Examples of conductors
graphite
sea water
dirty water
concrete
lemon juice
silver
gold
copper
aluminium
steel
iron
brass
bronze
platinum
mercury
Other conducting materials
Metals
Point out that most metals are good electrical conductors and also generally good heat conductors.
What makes an insulator?
An insulator offers very large resistance to the flow of electric current.
Its outer electrons are tightly bound, so there is essentially no electron flow through the material at ordinary voltages.
Link this back to resistance and material choice in installation work.
Examples of insulators
Common insulators
pure water
dry wood
dry cotton
dry paper
quartz
diamond
asphalt
Dry or pure materials
rubber
glass
plastic
oil
air
porcelain
ceramic
fibreglass
The contrast between pure water and dirty water is useful: impurities can change how well a material conducts.
Why material choice matters
Electrical systems need both types of material:
conductors to carry current
insulators to keep current in the right place
Safe installation depends on choosing the correct material for each job.
Tie this to familiar examples such as a copper conductor inside a plastic cable sheath.
Which material is an insulator in this lesson?
graphite
sea water
plastic
copper
Distractors are all examples named nearby, but only plastic is listed as an insulator.
What best explains the difference between a conductor and an insulator?
outer electrons
colour
mass
shape
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Choosing materials safely
Electrical systems need both conductors and insulators. Conductors allow current to flow, while insulators keep current in the right place.
Anchors the practical purpose of material choice in installations.
Effects of electric current
When current flows, it can produce different effects.
In this lesson, the focus is on:
thermal effects
chemical effects
These effects show that electrical energy can be changed into other forms.
The handout also names the magnetic effect, but says it will be covered later. Mention it briefly without developing it.
Thermal effect of current
When current passes through a conductor, the conductor heats up.
This happens because some electrical energy is converted into heat energy as electrons collide with each other and with atom nuclei.
This is the heating effect of current. Link it to resistance: heating becomes significant where current meets opposition.
Uses of the thermal effect
The heating effect is useful in devices such as:
electric lamps
electric irons
room heaters
water heaters
electric fuses
For fuses, note that heating is used as a protective effect: excessive current causes the fuse element to melt.
Chemical effect of current
When electric current passes through certain liquids, it can cause a chemical reaction.
This is called the chemical effect of electric current, or electrolysis.
Keep the focus on the definition and the idea that current can change substances chemically.
Electrolysis example
A clear example is acidified water.
Electric current through the liquid causes water molecules to split, producing:
hydrogen gas
oxygen gas
This shows that current does not only heat materials; it can also drive chemical change.
Electroplating
Electrolysis is used for electroplating.
A cheaper metal object can be coated with a more expensive metal, such as copper or silver, to improve its surface.
According to the handout, the object to be plated is the negative electrode, the coating metal is the positive electrode, and the electrolyte is a solution of the coating metal.
What is electrolysis?
magnetic force in air
heat from a fuse
chemical change in a liquid
light from a lamp
Checks the distinction between thermal and chemical effects of current.
Effects of electric current
When current passes through a conductor, some electrical energy becomes heat energy. In certain liquids, current can cause a chemical reaction called electrolysis. In acidified water, this produces hydrogen and oxygen.
Uses only examples explicitly taught in the current-effects subtopic.
How does electroplating show a useful chemical effect?
Expected elements: electroplating uses electrolysis to coat a cheaper metal with a more expensive metal such as copper or silver; this improves the surface/finish and is a practical use of the chemical effect of current.
What is one thing from this lesson you will remember?
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Which part of today's lesson felt hardest to understand?
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