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### Transcript

• 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