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# Electric Currents - PowerPoint PPT Presentation

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

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.

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

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

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

• 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.

• 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.

• 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.

• 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

• 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.

• 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?

• 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

• 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

• 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

• 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.

• 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?

• 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

• 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.

• 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.

• In fact:

Or I = Δq/ Δt

Current is the rate of flow of charge

• 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

• 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.

• 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.

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

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

### 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

• What do the current‑voltage graphs tell us?

• 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.

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

• 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.

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

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