Current Electricity

negatively charged electrons

Electricity is the flow of charges through a conductor. When charges flow from one point to another, a
current flows.
A conductor is made up of metal atoms
which have negatively charged electrons
that can move when attracted or repelled.
In a simple circuit, electrons are repelled
by the negative terminal of a battery and
attracted to the positive terminal of that
battery if there is a ‘path’ that runs
between those terminals. This results in
the current in the path or circuit.
Conventional current:
Rather than tracking electron flow, in electricity, we will use conventional current – which as the name
suggests, is a convention or a way of thinking that has been used for so long that no-one will change it!
Read the following extract from Conventional Current vs Electron Flow: swimming against the tide?
By Exploriments (reference at end of notes)
“The idea that electricity flows from the positive terminal to the negative goes all the way back to one of
America's Founding Fathers, Benjamin Franklin. In Ben Franklin's time, there were no batteries, and no
learn knowledge of the existence of electrons. His experiments led him to believe that electricity (which he
called electric fluid) flowed from positive to negative.

J.J. Thomson discovered electrons in 1897 to great fanfare. Right after that, it became obvious that
rather than positively charged particles going from positive to negative, the reverse was true! It was the
negatively charged electrons moving from negative to positive, that we experienced as "electricity". By
1897 however, Franklin's convention had been in use for nearly 150 years, so appeared in thousands of
books, tens of thousands of electrical circuit diagrams, and millions of minds by then.

As a concession to tradition, we refer to the direction of current that Franklin imagined as "conventional
flow". Scientists are now in agreement that in metals, electrons flow from negative to positive and
produce electricity (nothing actually flows from positive to negative).” DEFINITION
Thus conventional current is defined as the flow of positive charge from the positive terminal of a
battery, through the circuit, to the negative terminal of the battery.
A path for charges to flow:
If there is a path for charges to flow, it is called a circuit. Current can flow in a closed circuit, but
charges are unable to flow in an open circuit as the path is incomplete. Finally, if there is a direct path
from the positive to the negative terminal, this is called a short circuit. Short circuits are bad because
they can cause batteries to drain very quickly and overheat.

= conventional
current

Current Electricity: Form II 1

Exercise 1:

For each circuit shown below, use a to indicate circuits in which the light bulb WILL glow,
and a for circuits in which the light bulb WILL NOT glow. Can you explain why the bulb does or
doesn’t glow?
conventional
X X
opencircuit
wire
not.fi
on
in n
Turren

7
attached to coil
nothingiscoponnected to an opencircuit

that
ggtedfim.me
in L'iftedto nothing to

an open circuit

Iep
in
Charge L
Measuring current in a circuit:
DEFINITION
Current (I) is defined as the rate of flow of charge, and is measured in
Amperes (A) using an ammeter connected in series.
Ammeters allow the charges to flow through them in order to measure
the quantity of charge that passes a point each second.
An ammeter doesn’t use any energy and
doesn’t obstruct the flow of charge.
og
It’s similar to a vehicle counting system that
detects the number of cars that pass over it
in a period of time. But in no way, interferes
with the motion of the cars as shown in the
picture on the left.

Current Electricity: Form II 2

An Electric Circuit Analogy

Phoebe

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i ii
II

gases ftp.foffd a
mudpit mudpit

KEI
E can

ngeYI
Patent.in other

9
hentiiatries
threats gr eKinetic.hr iianenergy
and heatenergy
up

the white t.inta ier

Wood here's.it iIeier'it

formsof energy
in time.fi

wires no potential energy is lost here

charges
EE anperamifferthtefeitsse.hu
 Important Principles

but es are not usedup

energy is used up inthe circuit changes

current is when changes move ie they have kineticenergy movement

energy

charges keep the same

kinetic
energyin a
circuit but theirpotential energy is converted into othertypes of
energy e.g heatlight

Thus charges flow at the same rate at all points in the circuit they do not slow down
in or after going through a resisterdevice

All the potential energy that is given to the charges by the battery will be used up by the
resistors in the circuit

Titan Erm

Ifthereisone s itwilluseallthe
energy

there
aerawiiie.itisei
resistors.me
tmem
 Energy and charge:
Energy is supplied to charges by the battery. The battery provides electrical potential energy.
Charges carry energy through the circuit and this energy is changed into other forms of energy by
components connected in the circuit such as a lightbulb (electrical energy changes to light and heat
energy) or a motor (electrical energy changes to kinetic energy).
Energy is ‘used up’ in the circuit, but charges are not ‘used up’. Charges simply carry the energy around
the circuit.
Potential difference (V) is defined as the work done (energy transferred) per unit positive charge,
is measured in volts (V) using a voltmeter connected in parallel. nn hd
h and
ethisdefiniz

me
(Note: Instead of talking about individual charges, we rather talk about a unit of charge (Q)
definition which is measured in Coulombs (C) – which will only be discussed in detail in Form III.)
westilluse
Thus, a battery has a potential difference across its ends because it supplies energy (ability to do work)
to each unit of charge that passes through it. This potential difference provides a push or force on the
charges allowing them to move through the circuit. The potential difference across the ends of a battery
is sometimes called the voltage of the battery. bulb
a resistoron light
A potential difference can also be measured across a component in the circuit that uses up energy
(where work is done per unit charge).
Potential difference is a comparison of the energy carried per unit charge at two different points in a
circuit (hence a difference).

Note that no
charges flow Lots of potential energy per unit of The voltmeter
through the charge measured at this end of the measures the
voltmeter. battery…. and here at the bulb potential difference
over the ends
ftp.h.fiesistance
of the bulb.
A voltmeter This is the
compares the two potential energy
points and finds the used up per unit
potential difference charge in the circuit.
between them.

This is the potential

energy supplied per No potential energy per unit of charge
unit charge in the measured at the other end of the bulb,
circuit. and hence, none at the other side of the
battery.

Current Electricity: Form II 3

I light bulb
2 light bulb
bulb
3 light
Resistance:
Definition
Anything in a circuit that slows down the flow of charges is called a resistor and has a resistance.
Resistance is defined as a material's opposition to the flow of electric current, and is measured in
Ohms (Ω).
Resistors can be light bulbs, heaters and motors, and any electrical
appliance you can think of.
Resistors can also be simple electronic devices that serve only to change
the rate of flow of charge, and hence decrease the current in the circuit.
This is very important in electronics. The picture alongside shows a number
of these resistors. The coloured stripes give a measure of how much
resistance each one offers.
Read the following extract from “Physical Science: Grade 10 – Theory, exercises and practical
investigations” by JM Lucas and D Wright:

RESISTANCE

Resistance may be defined as that property of a material which causes it to oppose the flow of charges through
it. Resistance has both desirable and non-desirable effects. For example, if the resistance in high voltage
transmission lines is too high, they heat up, and much energy (in the form of heat) is lost to the surroundings.
Conversely, if the resistance in the element of a geyser or electric kettle is too low, the element would not heat
up, and cold showers and cold coffee would be the order of the day! In electronics – computers, television sets,
“play stations”, radios and cell phones etc. – components called resistors are used to protect other components
by restricting the amount of current flowing through them. They are also used to regulate the potential
difference across components, thus allowing them to function efficiently.

When charges move through a resistor, they collide with other charges and with the nuclei of the substance of
which the resistor is made. During these collisions, kinetic energy is transferred to the particles which are in the
way, and the resistor heats up. Thus, in an incandescent light bulb for example, whose filament has a very high
resistance, there is a substantial transfer of kinetic energy and the filament heats up to such an extent that it
glows white.
ME
1.
ILEARN
The resistance of a conductor depends on four factors. They are:

Length – the longer the conductor, the greater its resistance.

L
The L and the3 T's

2. Thickness or cross-sectional area – the greater the cross-sectional area of the conductor, the smaller
its resistance. in niaierii.si
resistance
e
3. Temperature – the hotter the conductor, the greater its resistance.

4. Type of material of which the conductor is made – some materials offer greater resistance to an
electric current than others. For example, copper, silver and gold are excellent conductors of electricity,
while tungsten and nichrome are less effective.

Mathematically, the resistance of a conductor in a circuit is the quotient of the potential difference
across its ends to the current flowing in it.
in
voltage
𝑅=
resifance
can
current

This relationship can be used to understand that the resistance of a circuit affects the size of the current
that flows in the circuit. The current in the circuit is inversely proportional to the resistance in the
circuit (as long as the potential difference supplied to the circuit remains the same).

Current Electricity: Form II 4

Circuit components in symbol form:

Name of component Symbol Use/Application

Cell Is an electrical power supply

morethen
makes battery
Battery Many cells together
is
longerline andshorter
lineis
Allow charges to flow easily through
Connecting wires
them

Creates an open circuit in which current

Open switch
does not flow

Creates an closed circuit in which

Closed switch
current does flow

A device that opposes the flow of charge

Resistor or Mn in the circuit

A type of resistor that produces light

Light bulb we
or (and sometimes heat)

Variable resistor A type of resistor that can be adjusted

(rheostat) or mm to give a range of resistances

in
connected miseries

Ammeter Measures current in a circuit

a
corner battery
Measures potential difference between
Voltmeter
two points in a circuit

Drawing circuit diagrams:

Firstly draw your cell or battery. Then work your way round from one end of the cell or battery to the
other end, drawing the symbols for all wires, devices and components as you go. You must remember to
draw the wires as straight lines using a ruler. The direction of conventional current is also shown by
placing an arrow head in the correct direction on a connecting wire.

< I resistor
amiable

currentdirection
drawin the
1

0
Exercise 2:
Use symbols to create a circuit diagram containing: a battery of 2 cells, an ammeter, 3 light bulbs, a
resistor and an open switch. Do not forget to include an arrow for conventional current.

Current Electricity: Form II 5

Exercise 2

A
Practical electricity:
Meters: Ammeters and voltmeters are meters that measure electrical quantities in a circuit.
Ammeters have an extremely low resistance (we ignore it) and are connected in series in a circuit.
In the school laboratory, you will work with ammeters much like the one shown below.
One wire connects to the black (negative) terminal,
and another wire connects to one of the red (positive)
terminals.
Always connect the positive terminal of the ammeter
to the positive terminal of the power supply, either
directly or after one of the circuit components.
Similarly, always connect the negative terminal of the
ampat
ammeter to the negative terminal of the power supply
via a resistor of some sort.
The ammeter shown alongside has 3 different levels
of sensitivity depending on which one of the red
connections is used.
Always start with the 5 A connection which is the
least sensitive. If the current is less than 0,5 A, then
move the connection to the 500 mA terminal which is
more sensitive for currents under 0,5 A. If the current
is less than 50 mA then use the most sensitive 50 mA
connection.
Note that each positive connection has a different scale to be used on the face for measuring the
current.
For example, look at the needle in the position indicated by the arrow on the picture above.
If the 5 A connection was in use, the current would be 2,5 A,
if the 500 mA connection was in use, the current would be 250 mA, and
if the 50 mA connection was in use, the current would be 25 mA.

Voltmeters:
Voltmeters have an extremely high resistance so much so that
charges cannot flow through them. For this reason, we have to
connect them in parallel in their own path in a circuit.
In the school laboratory, you will work with voltmeters much
like the one shown alongside.
Polarity connections (connect + to +, and – to -) still apply as
for ammeters.
There are also 3 sensitivities: 15 V which is the least sensitive,
10 V more sensitive and 3 V which is the most sensitive. Always
start by connecting to the 15 V connection.
Exercise 3:
What potential difference is shown by the needle at when
the:
3.1 15 V connection is used: _____________________
3.2 10 V connection is used: _____________________
3.3 3 V connection is used: ______________________

Current Electricity: Form II 6

Circuit connection types:
Series connection: the components are connected one after the other in a circuit. The current only has
one path along which it can flow.
Parallel connection: the components are connected in separate paths that all start at the same point
and end at the same point. These paths offer different routes along which current can flow.

form3work
Batteries in series and parallel compared:
Some circuits have more than one battery. You have probably used many devices that require two or
more batteries, like torches or remote controls.
Multiple batteries can be connected two different ways: in series or in parallel.

When multiple batteries are connected

in series, the positive terminal of one battery is
connected to the negative terminal of the next
battery (and this repeats if there are more than
providesmore
two batteries). Power greater
In this type of connection, all the charges pass 464k bulb
through both batteries and so they acquire
double the potential energy compared to
passing through only one battery.
If each battery has a potential difference of
1,5 V, then the battery connection will have a
potential difference of 3 V in total.

When batteries are connected in parallel, all

of the positive battery terminals are
connected together, and all of the negative
battery terminals are connected together.
batteries
Notice in the picture on the left, that the
lastlonger arrows moving on each path to and from
each battery are smaller. That is to show
A B that the current through each path is half of
the full current in the main circuit. Each
charge in the circuit, either passes through
battery A or battery B on each cycle through
the circuit.

If each battery has a potential difference of 1,5 V, that means that the potential difference of the
battery connection is also 1,5 V. The advantage of this connection is that the batteries last longer and
don’t need to be replaced as often.

Current Electricity: Form II 7

Exercise 4:
4.1 Between circuits A and B below, which light bulb will glow brighter and why?

Circuit A Circuit B
Modcells morevoltage
areitems sina.int

Circuit B a circuit with more cells has higher voltage which increases
_______________________________________________________________________________

the current flowing through More current makes thelightbulb glow brighter
_______________________________________________________________________________

_______________________________________________________________________________

Resistors in series:

In
Riotal
I s.sn
l.sn

21

The more resistors connected in series, the more

resistance in the circuit.

Mathematically:
𝑅 = 𝑅 + 𝑅 +⋯

2Ω 4Ω

𝑅 = 2+ 4=6Ω

Practical exploration:
In each of these investigations, use PhET simulations: Circuit construction kit: DC. You can search for this
in your Safari browser on your iPad. The press the play button and select the “Intro” option by
double tapping on it.
Once in the simulation, construct the required circuits by dragging components into the “prac space”
and then make appropriate observations.

Current Electricity: Form II 8

(The following practicals are adapted from “Physical Science: Grade 10 – Theory, exercises and practical investigations” by JM Lucas and D
Wright)

Practical A: Current in a series circuit:

1. Construct the following circuit:

X Y

2. Close the switch and use the ammeter to take readings of the current at the following positions:
a. Between the battery and X: _______________________________
1,35 A
b. Between X and Y: __________________________________
1,35A
c. Between Y and the battery: _______________________________
1,35A
Also take a screenshot of your circuit so you can compare the brightness of the bulbs to
bulbs in circuits later on.

are equal
3. What do you notice about the readings (a – c)? __________________________________________
they
__________________________________________________________________________________
4. Add an additional bulb into the circuit, connected in series, and record the new values of current
in various places in the circuit.

0,9A
New current: ________________________________
5. What relationship can you determine between the current in circuit and the total resistance in
the series circuit?
resistance the smaller the current
___________________________________________________________________________________
the greater
___________________________________________________________________________________
Inversely proportional relationship
Practical B: Potential difference in a series circuit:
1. Construct the following circuit:

2. Close the switch and connect a voltmeter across the points indicated below.
3. Record the reading on the voltmeter for each position:
a. V1: Points 1 and 2: ___________
b. V2: Points 2 and 3: ___________

9V
c. V3: Points 3 and 4: ___________

27V
d. V4: Points 1 and 4: ___________

270
e. VT: Points 5 and 6: ___________
4. Using these results, calculate V1 + V2 + V3 = __________________________________________
5. Compare your answer (in 4 above), to V4 and VT. What do you notice?

The voltage is thesame

________________________________________________________________________________

Thepotential Energy given to the electrons by the battery is the

Explain: ________________________________________________________________________
same amount thatis lost or givento theresistors in thecircuit
________________________________________________________________________________

Current Electricity: Form II 9

 Summary of main points

Series circuit

Resistors added in series INCREASE the total resistance of

the circuit and DECREASE the total current in the circuit.
in
Current in a series circuit is constant throughout the toeach other
circuit.
Ammeters in a series circuit record the same current at all
points in the circuit.

2A 2A
A Az
2Ω 4Ω

Voltage
Potential difference splits between resistors in the
connection in proportion to their resistance.

VT
Total i9f.tt

VT measures between
yellow and green
2Ω 4Ω

VitV2 also measures

V1 V2 between yellow and green
E9 4 g
𝑉 = 𝑉 + 𝑉

Vt 4 8
Exercise 5:
124
5.1 Between circuits E and F below, which light bulb/s will glow brighter and why?

Circuit E Circuit F
Circuit F has more
lightbulbs and thus
more resistance This
slows thecurrent
1 2 1 2 3 down andso they
then
IEirki
5.2 In circuit F above, what would be observed if the filament of light bulb 2 were to suddenly break?
There would be a break in the circuit and no current
would flow All light bulbs would be
Current Electricity: Form II off 10
Calculations within a circuit:

Once you understand how current and potential difference do or do not divide up within a circuit, you
can use this information to solve for unknown values in the circuit.
pohms voltsv
In order to do this, you will need to apply the equation that defines resistance: 𝑅 =
78Me.es
It is ESSENTIAL to make sure that the three variables that you use in the equation are LINKED together.

For example:

If you are working with the entire circuit, 𝑅 =

If you are working with a single resistor in series, 𝑅 =

In this case, you can use 𝐼 because all resistors in series have all the current in the circuit flowing
through them.

Make sure that you use subscripts to indicate which variables you are linking.

Exercise 6:
(Some of the questions below are taken from “Physical Science: Grade 10 – Theory, exercises and practical investigations” by JM Lucas and D
Wright)

6.1 (a) Calculate the resistance of a lightbulb that has 1,8 A flowing through it and a potential
difference of 9 V across its ends.
EE4fAR E E sn
(b) What is the potential difference across a 3 Ω resistor when 0,4 A of current flow through
it?
324 31 i r lar
(c) Determine the amount of current that flows through a resistor marked: 150 Ω; 21 V.

6.2
IT 1501 i I Eo i 0,14A
hadtoredothisthingintothisorder
remind
Study the circuit diagram below and then answer the questions that follow. The battery, R
ammeter and wires have negligible resistance (extremely low – so we can ignore it).
RI V
I

6.2.1 What is the total resistance in the circuit? RT 40 20

6.2.2 What is the reading on the ammeter? RT i It 0,1A
6.2.3 What is the reading on voltmeter V2?

R 40 59 V 4V
REIKA
Current Electricity: Form II 11
6.3 In the part of a circuit shown alongside, V
shows a reading of 220 V, while V2 and V3
show readings of 50 V and 60 V respectively.
What is the reading on V1?

6.4 Which of the following statements is true regarding a number of resistors connected in series?

A An increase in the number of resistors would decrease the resistance in the circuit.

B The smallest current flows through the resistor with the highest resistance.

C The sum of the potential difference across each resistor is equal to the potential
difference across all the resistors.

6.5 Draw the circuit consisting of the following components: two cells connected in series; an
ammeter, a closed switch, two resistors, R1 and R2, connected in series, two voltmeters, one
connected across the cells and the other across R1. The ammeter reads 0,1 A, and the voltmeter
connected across R1, reads 2 V while the voltmeter connected across the cells reads 3V.

6.5.1 What is the potential difference across R2?

6.5.2 Try use proportion to determine the values of R1 and R2 respectively if the total resistance
in the circuit is 30 Ω. Confirm your answers by calculating the values using 𝑅 =

6.6 In which of the following conductors would the resistance be the greatest?
(D = diameter; ℓ = length).

A ℓ = 2m; D = 0,5mm

B ℓ = 1m; D = 0,5mm

C ℓ = 2m; D = 0,3mm

D ℓ = 1m; D = 0,3mm

Current Electricity: Form II 12

 229,5

create
or

VT VitV2 3 2 V2
Vz IV

The potential difference across Rz is IV

6 512 RT 301
R zone R 0,1A
R andV are A
Rz on Proportional to
each other
t 3V

Rt Rit R2
Rt 10 20.2 101
20 R R2
301
Vil
ar
 Maths
Using
RT 7 30 IT IT 0,1A

i Ri 201
R
 L and 3T's

or
Acknowledgements of sources of images:
https://learn.sparkfun.com/tutorials/what-is-electricity/flowing-charges

http://exploriments.blogspot.com/2012/05/conventional-current-swimming-against.html

https://www.shutterstock.com/search/electric%20current%20diagram?image_type=illustration

https://www.sciencebuddies.org/science-fair-projects/project-ideas/Energy_p010/energy-power/potato-
battery?isb=cmlkOjE1ODk1MjQ0LHNpZDowLHA6MQ&amp%3Bfrom=TSW#background

https://www.thinglink.com/scene/644230834512986114

https://store.schoolspecialty.com/OA_HTML/ibeCCtpItmDspRte.jsp?minisite=10224&item=35807

https://www.shutterstock.com/search/ammeter

http://www.bmseducation.co.za/index.php/ammeter-dc-triple-range.html

https://store.schoolspecialty.com/OA_HTML/ibeCCtpItmDspRte.jsp?minisite=10224&item=3251458

https://www.shutterstock.com/search/voltmeter?image_type=vector&page=2

https://commons.wikimedia.org/wiki/File:Metrocount_vehicle_classifier_system_on_B3033_-_geograph.org.uk_-_1033728.jpg

https://en.wikipedia.org/wiki/Resistor

Current Electricity: Form II 13