-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
1/14
22/04/2012
1
Potential Difference
The easiest way to thinkabout what batteries do isto use a water analogy.
Batteries lift charges (Q)to a higher Potential (V).
There is a PotentialDifference (V) betweenone end of the battery andthe other.
Batteries store Potential
Energy as ChemicalEnergy.
1
What are Coulombs?
Because charge is made out of electrons which are verysmall, it seems silly to measure charge in electrons becausethe 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
2
Remember!!!!
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
2/14
22/04/2012
2
3
PotentialDifference
amodel
4
Charge
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
3/14
22/04/2012
3
Current (I)Batteries lift charges toa higher potential.
The charges then flow
around the circuit.
The flow of charges persecond is called:current.
1C/s
Charge
Time
Current
5
This way of representing thedirection of current is called
Conventional Current.It is now know that charge is
carried by electrons, flowingfrom the negative terminalto the positive terminal.This is called electron flow.
Before the discovery of the electron, scientists assumed that current was due topositively-charged particles moving from the positive terminal around a circuit tothe negative terminal.
Today, both conventional current and electron flow can beused to represent the direction of current.
What is conventional current?
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
4/14
22/04/2012
4
7
Current
8
Conventional
Current
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
5/14
22/04/2012
5
9
Potential Difference (V) ..sometimes known as Voltage
Batteries lift charges to ahigher potential.
There is a PotentialDifference because each
coulomb of charge has a
different potential energy ateither end of the battery.
Potential
Difference
Energy
Charge
1J/C
Electromotive ForcePotential Difference (V)Thetotalamount
ofChemicalEnergyinthebattery
transferredto
ElectricalEnergybyeachCoulomb
ofcharge
Electromotive Force(EMF) and Potential Difference:
Thetotalamount
ofElectricalEnergytransferredtoHeat
byeachCoulomb
ofcharge
10
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
6/14
22/04/2012
6
11
Potential
Difference
12
Thedifferencebetween
ElectromotiveForce
andPotentialDifference
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
7/14
22/04/2012
7
13
How do metals conduct electricity?
The delocalized electrons arefree to flow through the metaland so carry a current.
Insulating materials do notcontain free electrons andso current is unable to flow.
Ionic solutions are also ableto conduct electricitybecause they have mobilecharge-carrying particles.
It is the delocalized electrons involved in metallic bondingthat allow metals to conduct electricity.
delocalized electrons
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
8/14
22/04/2012
8
15
Comparing
circuitsto
waterflow
16
Howdo
metals
conduct?
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
9/14
22/04/2012
9
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 = 1.72 x 10-8 m
Units = m
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
10/14
22/04/2012
10
19
Factors
affecting
Resistance
20
Resistivity
Equation
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
11/14
22/04/2012
11
21
Ohms Law
Ohmslawrelatesthecurrent flowingthroughaconductorwiththepotentialdifferenceacrossit.
VI V=IR
Ristheconstantof
proportionalitybetweenIandV
22
Ohmic Conductor
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
12/14
22/04/2012
12
23
OhmsLaw
24
Ohmic
Conductors
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
13/14
22/04/2012
13
25
Ohmic Conductor
Thereasonforthisisthatasthelightbulbgetshot therearemorecollisions betweentheatomssotheresistanceincreases
26
-
8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance
14/14
22/04/2012
27
NonOhmic
Conductors