cards - 5.1 - potential difference, current and resistance

8/2/2019 CARDS - 5.1 - Potential Difference, Current and Resistance

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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.

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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

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Remember!!!!


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PotentialDifference

amodel

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Charge


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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

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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?


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Current

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Conventional

Current


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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

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Potential

Difference

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Thedifferencebetween

ElectromotiveForce

andPotentialDifference


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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


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Comparing

circuitsto

waterflow

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Howdo

metals

conduct?


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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


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Factors

affecting

Resistance

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Resistivity

Equation


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Ohms Law

Ohmslawrelatesthecurrent flowingthroughaconductorwiththepotentialdifferenceacrossit.

VI V=IR

Ristheconstantof

proportionalitybetweenIandV

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Ohmic Conductor


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OhmsLaw

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Ohmic

Conductors


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Ohmic Conductor

Thereasonforthisisthatasthelightbulbgetshot therearemorecollisions betweentheatomssotheresistanceincreases

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NonOhmic

Conductors

cards - 5.1 - potential difference, current and resistance

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