Electric currents
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Electric Currents. Topic 5 .1 Electric potential difference, current and resistance. Electric Potential Energy. If you want to move a charge closer to a charged sphere you have to push against the repulsive force You do work and the charge gains electric potential energy.

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Electric Currents

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Electric currents

Electric Currents

Topic 5.1 Electric potential difference, current and resistance


Electric potential energy

Electric Potential Energy

  • If you want to move a charge closer to a charged sphere you have to push against the repulsive force

  • You do work and the charge gains electric potential energy.

  • If you let go of the charge it will move away from the sphere, losing electric potential energy, but gaining kinetic energy.


Electric currents

  • When you move a charge in an electric field its potential energy changes.

  • This is like moving a mass in a gravitational field.


Electric currents

  • The electric potential V at any point in an electric field is the potential energy that each coulomb of positive charge would have if placed at that point in the field.

  • The unit for electric potential is the joule per coulomb (J C‑1), or the volt (V).

  • Like gravitational potential it is a scalar quantity.


Electric currents

  • In the next figure, a charge +q moves between points A and B through a distance x in a uniform electric field.

  • The positive plate has a high potential and the negative plate a low potential.

  • Positive charges of their own accord, move from a place of high electric potential to a place of low electric potential.

  • Electrons move the other way, from low potential to high potential.


Electric currents

  • In moving from point A to point B in the diagram, the positive charge +q is moving from a low electric potential to a high electric potential.

  • The electric potential is therefore different at both points.


Electric currents

  • In order to move a charge from point A to point B, a force must be applied to the charge equal to qE

  • (F = qE).

  • Since the force is applied through a distance x, then work has to be done to move the charge, and there is an electric potential difference between the two points.

  • Remember that the work done is equivalent to the energy gained or lost in moving the charge through the electric field.


Electric potential difference

Electric Potential Difference

  • Potential difference

  • We often need to know the difference in potential between two points in an electric field

  • The potential difference or p.d. is the energy transferred when one coulomb of charge passes from one point to the other point.


Electric currents

  • The diagram shows some values of the electric potential at points in the electric field of a positively‑charged sphere

  • What is the p.d. between points A and B in the diagram?


Electric currents

  • When one coulomb moves from A to B it gains 15 J of energy.

  • If 2 C move from A to B then 30 J of energy are transferred. In fact:


Change in energy

Change in Energy

  • Energy transferred,

  • This could be equal to the amount of electric potential energy gained or to the amount of kinetic energy gained

  • W =charge, q x p.d.., V

    (joules) (coulombs)(volts)


The electronvolt

The Electronvolt

  • One electron volt (1 eV) is defined as the energy acquired by an electron as a result of moving through a potential difference of one volt.

  • Since W = q x V

  • And the charge on an electron or proton is 1.6 x 10-19C

  • Then W = 1.6 x 10-19C x 1V

  • W = 1.6 x 10-19 J

  • Therefore 1 eV = 1.6 x 10-19 J


Conduction in metals

Conduction in Metals

  • A copper wire consists of millions of copper atoms.

  • Most of the electrons are held tightly to their atoms, but each copper atom has one or two electrons which are loosely held.

  • Since the electrons are negatively charged, an atom that loses an electron is left with a positive charge and is called an ion.


Electric currents

  • The diagram shows that the copper wire is made up of a lattice of positive ions, surrounded by free' electrons:

  • The ions can only vibrate about their fixed positions, but the electrons are free to move randomly from one ion to another through the lattice.

  • All metals have a structure like this.


What happens when a battery is attached to the copper wire

What happens when a battery is attached to the copper wire?

  • The free electrons are repelled by the negative terminal and attracted to the positive one.

  • They still have a random movement, but in addition they all now move slowly in the same direction through the wire with a steady drift velocity.

  • We now have a flow of charge ‑ we have electric current.


Electric current

Electric Current

  • Current is measured in amperes (A) using an ammeter.

  • The ampere is a fundamental unit.

  • The ammeter is placed in the circuit so that the electrons pass through it.

  • Therefore it is placed in series.

  • The more electrons that pass through the ammeter in one second, the higher the current reading in amps.


Electric currents

  • 1 amp is a flow of about 6 x 1018 electrons in each second!

  • The electron is too small to be used as the basic unit of charge, so instead we use a much bigger unit called the coulomb (C).

  • The charge on 1 electron is

    only 1.6 x 10‑19 C.


Electric currents

  • In fact:

Or I = Δq/ Δt

Current is the rate of flow of charge


Electric currents

  • Which way do the electrons move?

    • At first, scientists thought that a current was made up of positive charges moving from positive to negative.

    • We now know that electrons really flow the opposite way, but unfortunately the convention has stuck.

    • Diagrams usually show the direction of `conventional current' going from positive to negative, but you must remember that the electrons are really flowing the opposite way.


Resistance

Resistance

  • A tungsten filament lamp has a high resistance, but connecting wires have a low resistance.

  • What does this mean?

  • The greater the resistance of a component, the more difficult it is for charge to flow through it.


Electric currents

  • The electrons make many collisions with the tungsten ions as they move through the filament.

  • But the electrons move more easily through the copper connecting wires because they make fewer collisions with the copper ions.


Electric currents

  • Resistance is measured in ohms (Ω) and is defined in the following way:

    • The resistance of a conductor is the ratio of the p.d.applied across it, to the current passing through it.

  • In fact:


Resistors

Resistors

  • Resistors are components that are made to have a certain resistance.

  • They can be made of a length of nichrome wire.


Ohm s law

Ohm’s Law

  • The current through a metal wire is directly proportional to the p.d. across it (providing the temperature remains constant).

  • This is Ohm's law.

  • Materials that obey Ohm's law are called ohmic conductors.


Ohmic and non ohmic behaviour

Ohmic and Non-Ohmic Behaviour

  • What do the current‑voltage graphs tell us?


Electric currents

  • When X is a metal resistance wire the graph is a straight line passing through the origin: (if the temperature is constant)

  • This shows that: Iis directly proportional to V.

  • If you double the voltage, the current is doubled and so the value of V/I is always the same.

  • Since resistance R =V/I, the wire has a constant resistance.

  • The gradient is the resistance on a V against I graph, and 1/resistance in a I against V graph.


Electric currents

  • When X is a filament lamp, the graph is a curve, as shown:


Electric currents

  • Doubling the voltage produces less than double the current.

  • This means that the value of V/I rises as the current increases.

  • As the current increases, the metal filament gets hotter and the resistance of the lamp rises.


Electric currents

  • The graphs for the wire and the lamp are symmetrical.

  • The current‑voltage characteristic looks the same, regardless of the direction of the current.


Power dissipation

Power Dissipation


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