1. Part of Physics Achievement Standard 2.6 – Demonstrate
Understanding of Electricity and Magnetism
ELECTRICITY

2. Physical Relationships
V 1 2
E F  Eq E p  Eqd Ek  mv
d 2
q E E
I V V  IR P  IV P
t q t
1 1 1
Rt  R1  R2 ...   ...
Rt R1 R2
F  BIL(sin  ) F  Bqv V  BvL

3. Current. Symbol: I unit: A (amps)
 Current is “movement of charge.” The moving
charges could be electrons or sometimes ions.
 1 Amp is 1 Coulomb of charge passing a point in 1
second.
 This concept can be extended to calculate a current:
������
������ =
������
q is the amount of charge,
t is the time.
Question: How many electrons are there in a coulomb?

4. The direction of current?
 Physics is stuffed.
 Before any one had discovered electrons, they
decided that they would be positive.
 Whoops! This means that current is the
direction that positive charges are moving.
 This is known as “conventional current”
 When we mean actual electron flow, we say
“electron current”.
 So current is the movement of positive
charges.

5. A Current rule
 Just like energy, charge cannot be created or
destroyed.
 This means the total current into a point is
equal to the total current out of a point.
 This explains the series and parallel rules
about current that you should have learnt last
year.
 Remember:
Total Current In = Total Current Out

6. Conductors and Insulators
 What is the difference between conductors and
insulators?
 Why do some materials conduct electricity and
some materials not?
 In a material, most of the electrons are attached
to a specific atom.
 However, all materials have a certain number of
free electrons.
 If there are a lot of free electrons, the material is
a good conductor. If there are almost no free
electrons, the material is a good insulator.

7. Voltage. a.k.a. Potential difference
 The symbol for voltage is V, and the unit is
Volts (V).
 The voltage measures the amount of energy
per unit of charge.
 Its kind of like…
 When a charge q has electric potential
energy Ep at a point then the voltage V is
given by
������������
������ = What is a volt?
������
 Voltage is always measured between two
points. Its like a measure of the difference in
the energy.

8. A Rule for voltages
 As a charge travels around a circuit loop, all of
its electric potential energy must be used up.
 When it reaches the other end of the
battery/power pack, its energy is ZERO.
 This is why components will blow if we don’t
think ahead and make sure the component
can handle the amount of energy we are
giving it.

9. Components. (Memorise these symbols)
Switch
Cell
Lamp
Battery Fuse
Resistor Diode
Variable Resistor LED
Ammeter LDR
Voltmeter Thermistor

10. Series and Parallel
 Series:  Parallel
 Components are  Components are
connected one after the connected in such a way
other, so the current has that the current has a
no choice where to go. choice which way to go.
 Ammeters should always  Voltmeters should
be connected in series. always be connected in
 Why? parallel.
 We want to measure how  Why?
much charge is passing
 Remember the term
through.
potential difference!

11. Resistance.
 Symbol: R Unit: Ohms (Ω)
 For many conductors, we find that the
voltage across them is proportional to the
current through them: V ∝ I.
 For example, when we triple the voltage
across them we find that the current through
them also triples.
 Let’s introduce a constant of proportionality,
and call it resistance, R.

12. Ohm’s Law
Ohm’s Law is actually a
graph showing that
V
voltage is proportional
to current.
V=RI
If the V-I graph has a
straight line through
(0,0), it is called an I
Ohmic Resistor.
Any conductor that has a
proportionality that is not a
V V
straight line through (0,0) is
called a non-Ohmic resistor.
I I

13. Combining Resistances
 Series.
 The Current travels through all components.
 The Voltage must add to the correct value for the loop
or the circuit.
 Parallel.
 The Current must go down
either one path or the other.
 The Voltage across components in parallel is equal.
 We want to work towards replacing all our
resistors with 1 resistor that would provide the
equivalent resistance.

14. Some equations…
 For resistances in series:
 For resistances in parallel:
 Note: For resistances in parallel, the total
resistance will be less than the smallest
individual resistance. Why is this?

15. Some Common Resistances…
 Internal resistance:  Skin Resistance.
Anything that supplies a voltage One way to think of insulators
uses some form of conducting is that they have resistances
material to get the energy so high that the electric field
from inside it to the outside. cannot cause electrons to
flow.
For example, a power pack
would use wires, a battery uses The resistance of skin is quite
a small conductive ribbon. high, which means that any
shocks we get will be a
This material has a certain significant voltage and or
amount of resistance, which is current.
known as the Internal Wet skin is much more
Resistance of the component. conductive than dry skin.
The internal resistance will also Sweaty skin is even worse,
limit the maximum current due to the ions in the sweat.
that can be supplied, which is a This is why you should never
good safety aspect. put a fork in a toaster.

16. The Voltage Divider
 A voltage divider uses
two resistors to
produce an output
voltage lower than the
input voltage.
 Problems:
The output voltage decreases when
current is drawn from the output.
Some of the current that is
supplied will be wasted.

17. The Potentiometer
 The potentiometer is
a variable resistance
device that overcomes
some of the above
difficulties with
voltage dividers.
 To obtain a particular
voltage from the
divider, we can simply
vary the resistance
ratio till the desired
output voltage is
obtained.

18. Power Power =
Change in Energy
TIme
 When current flows ������������
������ = ������������ = ������������
through a resistor, there is ������
a potential difference ������ ������
������ = × ������ ������ =
across the resistor. ������ ������
 This means that some of
the energy is being used
up in the resistor. ������ = ������������
 We measure this
difference in energy as a
voltage.
What is the unit
 The amount of energy that
is lost per second is a for power?
measure of the power.

19. You have 30 minutes…
You choose which order you do the
following 3 activities. Each must be done.
• You have been given three 10Ω resistors. Draw
and calculate all of the different resistances you
can achieve using different combinations of
these.
• Household mains supply voltage is 230 V, yet
transmission voltages in power lines can be 1000
times as high or more. Write a paragraph
discussing this difference.
• Discuss whether a light bulb is an ohmic or a non-
ohmic resistor.

20. Diodes
 A Diode is a electronic component that does
not obey Ohm’s Law.
 The Voltage-Current graph of a Diode looks
like this:

21. Bias
 A Diode is constructed so that it has a Bias:
 If it is connected the right way around, current will
flow once the voltage is higher than the cut-off
voltage.
 If it is connected the incorrect way, then it will act
as an insulator until the point of Zener
Breakdown occurs. This will normally wreck the
diode.
 Note that diodes indicate the direction of
connection with reference to conventional
current.

22. Light Emitting Diodes (LEDs)
 LEDs are diodes which
emit light when a current
passes through it.
 They have a number of advantages over
filament lamps.
 Typically they are cheaper to make,
 require less electrical power to run and…
 less easily damaged.
 Plus you can make an awesome TV ->

23. Sensors
 In physics, we frequently need to measure
temperature, light intensity, sound intensity,
force, position and so on.
 Often this is done by turning the input into a
voltage.
 This is because voltage can easily be measured.
 Example: In almost all cars, the petrol tank level is
measured by having sliding contact of a
potentiometer floating at the level. The output
voltage can then be calibrated to give a reading in
litres rather than volts.

24. More types of common sensors:
• Light Dependent Resistors
(LDRs)
• The circuit shows a voltage
divider in which the top
component is an LDR.
• At a high light level, the
LDR’s resistance is low.
• At a low light level, the
resistance is high.
• What could these be used for?

25. More types of common sensors:
• Thermistors
• This circuit shows a voltage
divider in which the top
component is a thermistor.
• At a high temperature, the
thermistor’s resistance is
low.
• At a low temperature, the
resistance is high.
• What could these be used for?

26. Fields
 What is a Field?
 A region of space where every point can be
assigned a vector which indicates the motion of an
appropriate test particle.
 Types of fields that we have already seen…
 Gravitational fields
 These are attractive fields that act upon mass.
 We are going to focus on electric fields.
 Electric fields are regions of space where electrically
charged particles feel a force.

27. Fields of the earth:
The Earth’s Gravitational Field
Why is a gravitational field different to all other types of fields?

28. Electrostatic Forces
 Electrostatic forces exist between all charged
particles.
 These forces are actually the reason I can’t put my
hand through a table. Why?
 This is why we say that gravity is the weakest
of all forces.
 To show the electrostatic forces, we draw
field lines.
 Field lines are drawn to indicate the direction that
a small positive charge would move.

29. Examples:
Unlike charges
attract.
Like charges
repel.

30. A spherical (point) charge.
 The field lines around a point charge are radial:
 The strength of the electric field is the amount
of force, in Newtons, that would act on a +1
Coulomb test charge.
 (In other words, the amount of force per unit
charge.)

31. The equation:
������
������ =
������
 Electric Field Strength (E) equals Force (F)
divided by charge (q)
 Force is measured in…
 Newtons (N)
 Charge is measured in…
 Coulombs (C)
 Therefore Electric Field Strength must be
measured in…
 Newtons per Coulomb (NC-1)

32. Your turn…
 Draw the Electric Field that exists between the
following two charged plates.
+ + + + + + + + +
- - - - - - - - -

33. Electric Potential Energy
 Electric potential energy is gained by a
charge when it is moved against an electric
force.
 The work done to move a charge against an
electric force is found by:
Work = Force × Distance
������ = ������������
But we know in an electric field: ������ = ������������
Therefore ������ = ������������������

34. So the amount of work done to move each charge
in an electric field is
������ ������������������
=
������ ������
The work done to move a charge is commonly
called Voltage. This gives
������ = ������������
or rearrange to find the Electric Field Strength:
������
������ =
������
This equation gives the Electric Field Strength
between two charged plates a distance d apart at a
voltage V.
Therefore another unit for E is…
Vm-1