20kV to 25kV

400kV

275kV / 400kV

132kV

400V 3-phase

230V (50Hz)

Generated by large power stations

Stepped up at power station

Bulk transmission over long distances

Distribution to towns and cities & industry

Light industry and commercial

Domestic supply

The National GridThe purpose of the National Grid is to transmit electricity across the

country with as little loss as possible.

11kV / 33kVDistribution to towns & cities and industry

EJC

Drax Power Station

Output power: 4 Gigawatts

Output current: 19,000 Amperes

Output voltage: 23,500 Volts

Provides 7% of the country’s electricity requirements.

One of the 660 Megawatt

alternators in the turbine

room of the power station.

EJC

Magnetism

Soft IronUsed in relays and transformers.

A relay is a device that contains a

solenoid (coil of wire) and is used to

activate a number of switches.

Steel, nickel & cobaltUsed to make permanent magnets

such as magnetic welding clamps.

CopperCopper is not strongly magnetic.

Copper is used in electrical cables.

EJC

Capacitor voltage ratings

Capacitors in parallelIf two capacitors are connected in

parallel then the maximum

working voltage is the lowest

value.

If two 10V capacitors are

connected in parallel then the

working voltage is 10V.

Working voltageA capacitor can be damaged if the

voltage applied to it is greater than the

working voltage.

If two capacitors have a working voltage

of 10V each then the overall voltage

rating is 20V.

Electrolytic capacitorsElectrolytic capacitors must be

connected to a d.c supply and in the

correct polarity. If the capacitor is

connected in reverse polarity then it can

become damaged.

+

+

10V

10V

10V10V

EJC

Capacitors in series

When all capacitance

values are the sameIf three identical capacitors are

connected in series then you can

easily calculate the total capacitance

by dividing the capacitance by the

number of capacitors in the circuit.

Example

If three 10uF capacitors are

connected in series:

10 ÷ 3 = 3.3 µF

10uF

10uF

10uF

EJC

Capacitors in series

100uF

200uF

300uF

The standard methodYou can calculate the total

capacitance of any series circuit using

this formula:

1

𝐶𝑇=

1

𝐶1+

1

𝐶2+

1

𝐶3+⋯

1

𝐶𝑇=

1

100+

1

200+

1

300= 0.0183

𝐶𝑇 =1

0.0183= 54.5µF

Example

If 100uF, 200uF and 300uF capacitors are connected in series:

You can use this formula for any

number of capacitors.

EJC

Capacitors in parallel

Just add them up!If capacitors are connected in parallel then to calculate

the total capacitance you simply add the values up.

Example

If three 10uF capacitors are connected in parallel:

10 +10 +10 = 30uF

10uF 10uF 10uF

EJC

Resistors in parallel

100Ω 100Ω 100Ω

When all resistance values are the

sameIf three identical resistors are connected in parallel

then you can easily calculate the total resistance by

dividing the resistance by the number of resistors in

the circuit.

Example

If three 100Ω resistors are connected in parallel:

100 ÷ 3 = 33.3 ΩEJC

Resistors in parallel

100Ω 200Ω 300Ω

The standard methodYou can calculate the total resistance of any parallel circuit

using this formula:

1

𝑅𝑇

=1

𝑅1

+1

𝑅2

+1

𝑅3

+⋯

1

𝑅𝑇

=1

100+

1

200+

1

300= 0.0183

𝑅𝑇 =1

0.0183= 54.5 Ω

Example

If 100 ohm, 200 Ohm and 300 Ohm resistors are connected in

parallel:

EJC

Resistors in series

100Ω

Just add them up!If resistors are connected in series

then to calculate the total resistance

you simply add the values up.

100Ω

100Ω

Example

If three 100Ω resistors are connected in

parallel:

100+100+100 = 300 Ω

EJC

RC Timing

Charging CapacitorsWhen a capacitor is connected in

series with a resistor it takes a

certain amount of time to charge up.

It charges quickly at first and

then slows down as it reaches

maximum.

The time it takes the capacitor

potential difference (pd) to reach

63% of the supply voltage is

calculated with:

T = C x R

Example

Resistor = 1K

Capacitor = 200µF

T = 200x10-6 x 1000

T = 0.2 SecondsWhich is the same as 200mS

EJC

Charge in Capacitors

Capacitor ChargeWhen a capacitor is connected to a

supply it develops a charge as

electrons build up on the plates.

To calculate the charge stored in the

capacitor you can use this formula:

Example

Supply voltage = 1000V

Capacitor = 100nF

Q = 100x10-9 x 1000

0.0001 Coulombs

Which is the same as100µC

Charge = Capacitance x Voltage

Q = C x V

EJC

Induction

Mutual InductionThis is where one coil of wire is

energised and causes a voltage to be

created in a second coil of wire.

Self InductionThis is where a voltage is induced

which opposes the initial flow of

current.

Dynamic InductionThis is where a voltage is created in

a length of wire when it is moved

within a magnetic field.

Generators apply this.

Motor EffectA conductor that carries a current will move at right angles to

the magnetic field.

EJC

Heating elementsDevices that are designed to produce heat need a high current

to flow through a length of wire. For a high current to flow the

wire must have a low resistance. This is because I = V ÷ R.

LOW RESITANCE = HIGH CURRENT = LOTS OF HEAT!

Producing Heat

Incandescent lampsIncandescent lamps work by allowing a current to flow

through a tungsten filament.

HIGHER RESITANCE

=

LOWER CURRENT

=

LESS HEAT

EJC

Stepping up & downA transformer is used to step-up or step-down voltages. They

are used in electronic equipment to produced a low voltage

from the mains voltage of 230V.

Transformers

50

100x 230 = 115VSecondary voltage =

Primary

voltage

(input)

Secondary

voltage

(output)

Example

If a transformer has 50 turns on the primary, 100 turns on

secondary and has a primary voltage of 230V, calculate the

secondary voltage:

Secondary turns

Primary turnsx Primary voltageSecondary voltage =

Primary turns

(copper wire)Secondary turns

(copper wire)

EJC

Effect of self inductionWhen an alternating current flows

through a coil an EMF (electromotive

force, measured in volts) is generated, or

induced in the coil.

The EMF induced is in the opposite

direction to the supply voltage and

opposes a change in current.

The EMF induced can be calculated like

this:

Self Induction

EMF = L x Rate of change

Inductance measured in Henrys

Example

If coil has an inductance of 10 Henrys and the current

changes at a rate of 15 Amps / Second:

EMF = 10 x 15 = 150 V

Amps / Second

EJC

Force created by an electric currentIf a wire (conductor) is placed at right angles to a magnetic

field a force acts on the wire and causes it to move at right

angles to the magnetic field.

The amount force depends on:

• The strength of the magnetic field – known as the flux

density which is measured in Tesla

• The amount of current flowing through the wire

• The length of the wire

Magnetism & Force

F = B x I x l

Flux density

Current

Length of wire

Example

A wire has a length of 50 cm and carries a current of 20 Amps,

is placed in a magnetic field. The magnet field strength is

300mT. The force acting on the wire is:

F = 0.3 x 20 x 0.5 = 3 Newtons

EJC

Capacitor theoryCapacitor structureA capacitor consists of two

metal plates separated by an

insulating material called a

dielectric. A capacitor is

capable of storing electric

charge.

The unit of capacitance is the

Farad and depends upon this

formula:

d

Metal plate

Dielectric

Metal plate

εo εr x A

dC =

Surface

area of the

metal

plates

Distance

between the

metal plates

Permittivity of

free space

(8.85x10-12)

Key factsThe capacitance is the permittivity multiplied by the area,

divided by the distance between the metal plates.

Different insulating materials have different permittivity.

Relative

permittivity

EJC

Cells and Batteries

CellsA cell converts chemical energy

into electrical energy.

BatteriesBatteries are made by

connecting a number of cells

together. Connecting cells in

series increases the voltage,

connecting them in parallel

increases the current it can

supply.

Primary and Secondary CellsPrimary cells and batteries can not be recharged

Secondary cells and batteries can be recharged

A car battery consists of six 2 Volt secondary cells connected

in series.

EJC

Inductive & Capacitive

CircuitsInductive CircuitsIn a circuit that includes a coil such as a motor, the current

flowing through the circuit lags behind the voltage.

Capacitive CircuitsIn a circuit that includes a capacitor the voltage lags behind

the current flowing through the circuit.

EJC

Force, Mass and

AccelerationWhat is force?A force acting on an object causes it to accelerate. You can

work out the force needed to make the object accelerate by

using the formula:

Example

A mass of 1500g is accelerated at 200cm/s2

The force required is:

F = 1.5 x 2

3 Newtons

Force = Mass x Acceleration

F = m x a

2m/s2

1.5kg

EJC

Cells and Batteries

CellsA cell converts chemical energy

into electrical energy.

BatteriesBatteries are made by

connecting a number of cells

together. Connecting cells in

series increases the voltage,

connecting them in parallel

increases the current it can

supply.

Primary and Secondary CellsPrimary cells and batteries cannot be recharged

Secondary cells and batteries can be recharged

A car battery consists of six 2 Volt secondary cells connected

in series.

EJC

SI UnitsBasics

Measurement Abbreviation SI Unit SI Unit

SymbolResistance R Ohm Ω

Resistivity 𝜌 ohm-metre Ω m

Current I Ampere AVoltage

(potential difference,

EMF)

V Volts V

Power P Watts W

Charge Q Coulombs C

Energy E Joule J

a.c circuits

True power P Watts WApparent power S Volt-amp VA (or kVA)Reactive power Q Volt-amp

reactive

VAr (or kVAr)

Power factor p.f or COS ϴ none noneInductance L Henry HCapacitance C Farad FCapacitive

ReactanceXL Ohm Ω

Inductive

ReactanceXC Ohm Ω

Impedance Z Ohm ΩFrequency F Hertz HzPeriodic time τ Second S EJC

Formulae

𝑃 =𝑉2

𝑅

𝑃 = 𝐼 × 𝑉

𝑃 = 𝐼2 × 𝑅

Power

P

I V

Ohm’s Law

V

I R

𝑉 = 𝐼 × 𝑅

Force

F

m a

𝐹 = 𝑚 × 𝑎

Series Resistors

𝑅 =𝜌 𝐿

𝐴

Resistors in Parallel

1

𝑅𝑇=

1

𝑅1+

1

𝑅2+

1

𝑅3…

Capacitors in Series

1

𝐶𝑇=

1

𝐶1+

1

𝐶2+

1

𝐶3…

Resistors in Series

𝑅𝑇 = 𝑅1 + 𝑅2 + 𝑅3…

Capacitors in Parallel

𝐶𝑇 = 𝐶1 + 𝐶2 + 𝐶3…

𝑍 = 𝑅2 + 𝑋2

Impedance

AC Power

𝐴𝑝𝑝𝑎𝑟𝑒𝑛𝑡 = 𝑇𝑟𝑢𝑒2 + 𝑅𝑒𝑎𝑐𝑡𝑖𝑣𝑒2

Force on a conductor

𝐹 = 𝐵 𝐼 𝑙

Inductive Reactance

𝑋𝐿 = 2 π 𝑓 𝐿

Inductive Reactance

𝑋𝐶 =1

2 π 𝑓 𝐶

Capacitance

𝐶 =ε𝑜 ε𝑟 𝐴

𝑑

Time Constant

τC R

τ = 𝐶 × 𝑅

Charge

Q

C V

𝑄 = 𝐶 × 𝑉

EMF

𝐸 = 𝐿 Δ𝑡

E

L Δt

EJC