T level Core Science PPT 5

1 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

2.5 Electricity principles

PowerPoint 5: Electrical theory

int entation
2. Construction science principles

2 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

An electricity–water analogy

3 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Units of measurement revisited

•The base SI unit of amperes (A) – often shortened to ‘amp’, measures is
the flow of electrons and therefore electrical current (I). For example,
standard lighting circuits typically draw about 5A, whereas typical socket
outlets are rated at 13A.

•Volt (V)is the derived unit for electric potential, electric potential
difference (voltage) and electromotive force (generator or battery
output). Voltage can be thought of as ‘electricalpressure’.

4 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Units of measurement revisited

•An ohm (Ω) is the derived unit of electrical resistance.

•A watt (W) is an SI unit of power used to measure the rate of energy
transfer.

•A coulomb is the derived unit of electrical charge.

•A joule is the SI unit for energy over time (ie the amount of electrical
power used over time).

5 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Resistance

All conductors show some opposition to electrical current. This
opposition to current is called resistance. There are several factors that
affect the resistance of a conductor such as:

•material – copper is a better conductor than steel as it has a lower
resistance, although silver is the best conductor of all

•thickness – wires with smaller diameters have greater resistance

•length – the longer the wire, the greater the resistance

•temperature – heating a wire increases its resistance.

The two main ways of increasing the current in an electrical circuit are by
increasing the voltage or by decreasing the resistance.

6 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Conductivity and resistivity

Electrical conductivity is a measure of a material’s ability to conduct
electricity – current flow.

Resistance is in opposition to current flow.

Resistivity relates to the conductor material, and is a constant value for
a given conductor, measured in ohms/meter (Ω/m).

Think of it in these terms:

Conductors resist very little,

while insulators resist a lot

7 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Formula for resistance

The resistance formula shown here is very important because it
shows that:

•resistance is directly proportional to length

•resistance is indirectly proportional to area.

A
pL
R =

8 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Formula for resistance

Increasing the length

increases the resistance

Increasing the area (size)

decreases the resistance

A
pL
R =

9 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Calculating circuit resistance

When calculating the resistance of a circuit, it is important that its length,
size and resistivity are in the same unit.

However, although length and resistivity tend to be represented in
metres (m), the sizing of electrical cables tends to come in mm² (cross-
sectional area).

As m² = m x m (with each metre = 1000mm), in total there is a six
decimal place difference. In other words, when we convert, we divide
our cable size in mm² by 1,000,000. In general it is easier to attach a
‘magic number’: 10ˉ⁶.

Therefore, when converting a 1.5mm² cable it becomes 1.5 x 10ˉ⁶m.

10 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Ohm’s law

Ohm’s law states that voltage (V)
= current (I) x resistance (R)

V = IR

The symbol for resistance is R.
Resistance is measured in
ohms (Ω).

The symbol for voltage is V, it is
measured in volts (V).

The symbol for current is I, it is
measured in amperes (A).

Example

A torch uses a 3V battery
whichtakes a current of
0.3A.
Calculate the resistance.

V = IR
3 = 0.3 x R
R = 3/0.3
R = 10 Ω

11 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Ohm’s law triangle

The relationship between voltage (V), resistance (R) and current (I) is
defined by Ohm’s law and can be show as a simple formula. To help
transpose this formula it can be placed into a triangle. To determine a
property of a circuit (eg voltage), cover over the V in the triangle with the
associated ÷, and you are left with I × R.

12 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Voltage, current and resistance: in series

Current – the amount of current is the same through any component in
a series circuit.

•The total resistance of any series circuit is equal to the sum of the
individual resistances.

•The supply voltage in a series circuit is equal to the sum of the
individual voltage drop.

13 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Resistors in series

In the ‘old fashioned’ way of wiring Christmas lights, if one
filiment failed, every light went out.

A series circuit of
lights is shown here.

14 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Resistors in series

In series, current is common to all resistors. Unless the resistors
are equal, the majority voltage will develop across the biggest
resistor.

15 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Resistors in series: total resistance

R1 4Ω

R2 6Ω

240V

Total resistance (RT) = 4 + 6

= 10 Ω

The total resistance for the circuit shown here is
calculated as:

16 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Resistors in series: volt drop

R1 4Ω

R2 6Ω

230 V

RT = 10Ω

VD R2 = 23 x 6
= 138V

The drawing shows how to calculate the volt drop across each resistor the
circuit.

VD R1 = 23 x 4
= 92V

I = 23 A

VD = I x R1
VD = I x R2

17 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Resistors in parallel

In a parallel circuit the voltage is equal across all components.

•The total circuit current is equal to the sum of all the individual
branch currents.

•Individual resistances diminish to equal a smaller total resistance
rather than adding together to make the total.

•In other words, the total resistance is always less than the
smallest value.

•This has implications when measuring a given point in a circuit.

18 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

In a parallel circuit:

•Voltage is the same and common to all resistors.

•Current divides through separate branches.

•The total is always less than the smallest value.

•Unless resistors are all equal, the majority of the current will
flow throught the branch with least resistance.

Resistors in parallel

19 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Calculating resistances in parallel

40V

You cannot just add the resistor values in a parallel circuit to
calculate resistance, and the required equations are complex.
Therefore the easiest method is to use a calculator with
mathematical functions.

R1 12Ω

R2 6Ω

20 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Look for the Xˉ¹ or 1/x function

40V

R1 12Ω

R2 6Ω

Enter the value of each resistor followed by Xˉ¹:

RT = 12 Xˉ¹ + 6 Xˉ¹
RT = 0.25, then press Xˉ¹ one more time: RT = 4Ω

21 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Given RT = 4Ω

40V

I = V
R

I = 40
4

I = 10A

Given RT = 4Ω, then,

R1 12Ω

R2 6Ω

22 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

To calculate the current through each branch

IR1 = 40 = 10A
6

IR2 = 40 = 6.67A
6

R1 12Ω

R2 6Ω

40V

23 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

The power triangle

Like the Ohm’s law triangle,
the power triangle helps you
to remember how to calculate
I (current).

24 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Using the power triangle

I = P
V

25 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Calculating energy

How much energy is produced over
a period of 5 minutes?

Formula for energy:

E = V x I x time in seconds

E = 24 x 2 x 5 (min) x 60 (secs)

E = 14400J or 14.4kJ

26 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Calculating efficiency

100
power

input
power

output

Efficiency

%

×

=

27 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Efficiency: calculation example

To calculate the efficiency of a motor if its accepts 3kW of input
power but produces 2kW:

Eff = 2000 x 100
3000

Eff = 66.6%

100
power

input
power

output

Efficiency%

×

=

28 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Ammeters

Ammeters measure current (amps). In a circuit it is critically important
that the ammeter is connected in series.

In other words, the ammeter is connected within the circuit, and the
circuit current flows through it, very much like water flowing through a
water meter.

If an ammeter is connected in parallel then the circuit current could
take an alternative route through the ammeter and could possibly
destroy it.

Multiple Choice

When it comes to electrical flow, What is the equivalent to water pressure?

• Current

• Voltage

• Amperes

• Resistance

Open Ended

The Voltage is a derived unit of?

Multiple Choice

When it comes to measureing power. What do we measure power in?

• Amps

• Ohms

• Watts

• Volts

Open Ended

Why is understanding the formula R = PL/A important?

Open Ended

If you see 1.5 x 10ˉ⁶m. What size cable would you have converted.ˉ⁶

Open Ended

With resistors in series what is common to all resistors?

Open Ended

What is equal across acroos all components in a parallel circuit?

29 of 29

T Level Technical Qualification in
Building Services Engineering for Construction (Level 3)
350 Building Services Engineering Core

© 2021 City and Guilds of London Institute. All rights reserved.

Any questions?