**1.1 Important
definitions, descriptions, formulae and units used in electrical science**
**Note: **You may or may
not (but don't worry!), have come across all of these terms or formulae, it depends
on how far your studies have got.

In your course, you might not need
to know every formula or have to be able to do all the calculations
explained and follow-up questions - that's up to you to find out.

If a UK GCSE student you will find your science course help links via

Find your GCSE/IGCSE
science course for more help links to all science revision notes

Most of the practice questions and worked out answers
based on the equations explained on this page are found in Part 1.6

**1.6 **
Power, energy transfer
&
electricity cost calculation questions

**
V****
the potential difference**** **

(**p.d**., commonly called
'**voltage**') is the driving potential that moves the electrical charge,
hence the electrical energy transfer, around
a circuit - **electrical charge usually carried by electrons**.

**Potential difference is the work done in
moving a unit of charge.**

It indicates how much energy is transferred
per unit charge when a charge moves between two points in a circuit
e.g. between the terminals of a battery.

(See energy transferred further down the page)

The p.d. across any part of a circuit is measured in volts,
(unit symbol **V**).

**
I****
the electrical current**** **

The **rate of flow of electrical charge** in
coulombs/second (**C/s**), measured in amperes (amps, unit symbol **A**).

The quantity of electric charge transferred in
a give time = current flow in amps x time elapsed in seconds

Formula connection:
**
Q = It
I = Q/t
t = Q/I**

**I** = current (**A**),
**Q** = electrical charge moved in
coulombs (**C**), time **t** (**s**)

It is
better to be able to rearrange a formula rather than using a triangle

**
R****
the electrical resistance****
**

Electrical resistance in a circuit, measured in** ohms** (unit
symbol **Ω**).

A resistance slows down the flow of electrical charge
- it **opposes the flow of electrical charge**.

Formula connection:
**
V = IR
I = V/R
R = V/I **

(This is the formula often quoted as
Ohm's Law)

**I** = current (**A**), **V** =
potential difference in volts (**V**), R = resistance in ohms (**Ω**)

It is
better to be able to rearrange a formula rather than using a triangle

**
P****
is
the electrical power of a device**

The power delivered by a circuit = the
**rate of energy
transfer** (symbol units **J/s**) and is measured in** watts** (symbol
unit** W**).

Formula connection:
**
P = IV
I = P/V
V = P/I**

also
**P = I**^{2}R**
R = P/I**^{2}
I = √(P/R)
(see also **P = E/t** below)

**P **= power in watts (**W**),
** I** = current (**A**),

**V** = potential difference in volts (**V**),
**R** = resistance in ohms (**Ω**)

It is
better to be able to rearrange a formula rather than using triangle like
those illustrated below

**
**
**
**

**
E is
the energy transferred**

Energy transferred by the quantity of electric charge by a potential
difference of V volts.

energy transferred (joules) =
quantity of electric charge (coulombs) x potential difference
(volts)

Formula connections
**E = QV****
Q = E/V
V = E/Q**

**E** = energy transfer in joules (unit
symbol J)

**Q** = electrical charge moved
(unit symbol **C**)

**V** = p.d. in volts (unit symbol **V**)

Formula connections **
E = Pt
P = E/t
t = E/P**

where **P** = power (**W**), **E**
= energy transferred (**J)**, **t** = time taken (**s**)

Energy transferred in joules = power in watts
x time in seconds

Formula connection: Since E = Pt and P = IV,
energy transferred **
E =
IVt**

It is
better to be able to rearrange a formula rather than using a triangle

**Q is the
quantity of electric charge**

Electric charge transferred is measured in **coulombs** (unit **C**).

Formula connection:
**Q = It****
****
I = Q/t
t = Q/I**

**Q **= quantity of electric charge transferred (**C**),
** I** = current (**A**), **t** =time (**s**)

It is
better to be able to rearrange a formula rather than using a triangle

INDEX of ELECTRICITY Notes 1. Electricity in the home

**
Keywords, phrases and learning objectives for calculations in electricity**

Know the electrical unit for potential difference
(p.d.) in volts (equation symbol V in V units)

Know the electrical unit for current in amps (equation symbol
I in A units)

Know the electrical unit for electrical resistance
in ohms (equation symbol R in Ω units)

Know the unit for power in joules/second or watts
(equation symbols P in J/s or W units)

Know the unit for energy transferred in joules
(equation symbol E in J units)

Know the electrical unit for charge in coulombs
(equation symbol Q in C units)

In problem solving. be able to use and rearrange the
following five equations

Q = It V = IR
P=IV E = QV E = Pt

BUT, not all courses require all these equations,

and it is better to be able to rearrange equations
than use a formula triangle, but that's up to you!

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