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5.1 - Potential Difference, Current and
              Resistance




                                          1
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?
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.
                               3
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
Water model of a circuit




This page has embedded flash which only works in Powerpoint.
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
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
                                       7
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.
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
                                                                 9
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
Website Link or click on the picture   11
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
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




                                                                13
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.
                                                                                   14
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.
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.
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
Battery Resistance Simulator on PhET website




  http://phet.colorado.edu/en/simulation/battery-resistor-circuit   18
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
                                                                   19
Both these circuits have 2 Volts per Amp or a resistance of 2Ω
                                                                 20
Ohms Law simulator on the PhET website




http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html or click on the picture
                                                                              21
Ohmic Conductor




                  22
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
                                                                  23

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5.1 - Potential Difference, Current & Resistance

  • 1. 5.1 - Potential Difference, Current and Resistance 1
  • 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. 3
  • 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 7
  • 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 9
  • 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 13
  • 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. 14
  • 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 19
  • 20. Both these circuits have 2 Volts per Amp or a resistance of 2Ω 20
  • 21. Ohms Law simulator on the PhET website http://phet.colorado.edu/sims/ohms-law/ohms-law_en.html or click on the picture 21
  • 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 23