Electricity intro

1. 5.1 - Potential Difference, Current and
Resistance
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2. How can we model electric circuits?
A model can help
us to understand
how current works
in an electric circuit.
In this model, the
moped riders
represent the flow
of charge and the
pizzas represent
the electrical
energy carried
around the circuit.
What do the pizza shop and the
house of party-goers represent?

3. Potential Difference
The easiest way to think
about what batteries do is
to use a water analogy.
Batteries ‘lift’ charges (Q)
to a higher Potential (V).
There is a Potential
Difference (V) between
one end of the battery and
the other.
Batteries store Potential
Energy as Chemical
Energy.
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4. Battery Voltage Simulator on PhET website
This simulator shows
another way of
imaging what
batteries do.
http://phet.colorado.edu/en/simulation/batte
ry-voltage or click the pictures

5. Water model of a circuit
This page has embedded flash which only works in Powerpoint.

6. What are Coulombs?
Because charge is made out of electrons which are very
small, it seems silly to measure charge in electrons because
the numbers of charges that go round a circuit would be
billions and billions.
Instead Charge (Q) is measured in Coulombs (C)
Using this scale 1 electron is only: 1.6x10-19 C
1 Coulomb is:
6,250,000,000,000,000,000 electrons
Remember this number 6

7. Current (I)
Batteries ‘lift’ charges to
a higher potential.
The charges then flow
around the circuit.
The flow of charges per
second is called:
current.
Charge
Current
Time
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8. What is conventional current?
Before the discovery of the electron, scientists assumed that current was due to
positively-charged particles moving from the positive terminal around a circuit to
the negative terminal.
This way of representing the
direction of current is called
Conventional Current.
It is now know that charge is
carried by electrons, flowing
from the negative terminal
to the positive terminal.
This is called electron flow.
Today, both conventional current and electron flow can be
used to represent the direction of current.

9. Potential Difference (V) ..sometimes known as Voltage
Batteries ‘lift’ charges to a
higher potential.
There is a Potential
Difference because each
coulomb of charge has a
different potential energy at
either end of the battery.
Energy
Potential
Difference
Charge
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10. Electromotive Force(EMF) and Potential Difference:
Potential Difference (V) Electromotive Force
The total amount
The total amount of Chemical Energy
of Electrical Energy in the battery
transferred to Heat transferred to
by each Coulomb Electrical Energy
of charge by each Coulomb
of charge 10

11. Website Link or click on the picture 11

12. How do metals conduct electricity?
It is the delocalized electrons involved in metallic bonding
that allow metals to conduct electricity.
The delocalized electrons are
free to flow through the metal
and so carry a current.
Insulating materials do not
contain free electrons and
so current is unable to flow.
Ionic solutions are also able
to conduct electricity
because they have mobile
charge-carrying particles.
delocalized electrons

13. Resistance (R)
Some materials are better than others at allowing current to
flow.
A material that doesn’t let much current flow for a given
Potential Difference is said to have a high Resistance.
This resistance depends on the material and the dimensions of
the conductor
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14. Factors affecting Resistance on PhET website
http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html
or click the picture.
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15. Why does Length affect resistance?
The affect of length of a wire on resistance can be
understood by looking at the atomic structure.
Resistance is caused by electrons colliding with metal ions.
When the length of the wire is increased, the electrons
have to travel further. So the chance of collisions will
increase, causing the resistance to increase.

16. Why does Cross Sectional Area affect resistance?
Increasing the thickness of
a wire increases the cross
sectional surface area that
the electrons can flow
through.
This decreases the
chance of collisions
with metal ions.
In thick materials the
charge carrying
particles are able to
move through the
conductor more easily,
reducing resistance.

17. What is Resistivity?
Resistivity is just a property of the conductor.
Every material has a resistivity.
It is actually the resistance of a 1m long piece of wire with a
cross-sectional area of 1m2.
As you can imagine this is always a very low number.
For Copper ρ = 1.72 x 10-8 Ωm
Units = Ωm

18. Battery Resistance Simulator on PhET website
http://phet.colorado.edu/en/simulation/battery-resistor-circuit 18

19. Ohms Law
Ohms law relates the current flowing through a conductor
with the potential difference across it.
V∝I V=IR
R is the constant of
proportionality between I and V
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20. Both these circuits have 2 Volts per Amp or a resistance of 2Ω
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21. Ohms Law simulator on the PhET website
http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html or click on the picture
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22. Ohmic Conductor
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23. Non-Ohmic Conductor
The reason for this is that as the lightbulb gets hot there are more
collisions between the atoms so the resistance increases
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