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5.1

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  • 1. Electric Currents Topic 5 .1 Electric potential difference, current and resistance
  • 2. 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.
  • 3.
    • When you move a charge in an electric field its potential energy changes.
    • This is like moving a mass in a gravitational field.
  • 4.
    • 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.
  • 5.
    • 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.
  • 6.  
  • 7.
    • 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.
  • 8.
    • 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.
  • 9. 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.
  • 10.
    • 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?
  • 11.  
  • 12.
    • 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:
  • 13. 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)
  • 14. 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 -19 C
    • Then W = 1.6 x 10 -19 C x 1V
    • W = 1.6 x 10 -19 J
    • Therefore 1 eV = 1.6 x 10 -19 J
  • 15. 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.
  • 16.  
  • 17.
    • 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.
  • 18. 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.
  • 19. 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.
  • 20.
    • 1 amp is a flow of about 6 x 10 18 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.
  • 21.
    • In fact:
    Or I = Δ q/ Δ t Current is the rate of flow of charge
  • 22.
    • 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.
  • 23. 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.
  • 24.
    • 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.
  • 25.
    • 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:
  • 26. Resistors
    • Resistors are components that are made to have a certain resistance.
    • They can be made of a length of nichrome wire.
  • 27. 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.
  • 28.  
  • 29.
    • 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: I is 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.
  • 30.  
  • 31.  
  • 32.
    • 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.
  • 33.
    • The graphs for the wire and the lamp are symmetrical.
    • The current‑voltage characteristic looks the same, regardless of the direction of the current.
  • 34. Power Dissipation

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