Electricity 1

• Some particles have an electric charge like an electron has a negative charge. In SI units, electric charge is measured in Coulombs (C) and the amount of charge on a single electron is $e =$$1.6 multiply 10^-19C$
• The charges on fundamental particles like electrons are fixed properties of these particles. It is impossible to create or destroy charge, so the total charge must be conserved
• Electric current is the rate of flow of charge. It is measured in amperes
• Electric current occurs when a charged particle, which is free to move, experiences an electric force. If it can move, it will be accelerated by the force. The movement of charge forms the electric current.
• Current can be calculated from the equation: current = charge passing a point / time for that charge to pass or $I =$$Q/t$
• Drift Velocity is when charged particles in a conductor are constantly colliding with other particles in the conductor and so do not travel straight through a conductor, so the average speed at which they move along the conductor is drift velocity and the collisions cause an equal and opposite force to be exerted on the electrons, which is Newton's third law.
• $I =$$nqvA$n is the charge carrier density, q is the charge of a charge carrier, v is the drift velocity and A is the cross-sectional area of the wire
• The potential difference between two points in a circuit is the electrical energy per unit charge converted into other forms of energy
• potential difference = electrical energy converted into other forms/charge passing or, as energy is the result of doing work, W: it is potential difference = work done / charge passing (V = W/Q). From V = W/Q it follows the unit of p.d is joules per coulomb (J/C) this unit is called volts
• A volt is a joule per coulomb, so 1V is 1 J/C
• The potential difference between two points in a circuit is measured by connecting a voltmeter between the points. Voltmeters should have a high resistance and this is so the current is not affected by the component
• The electromotive force (e.m.f) of an electrical source is defined as the energy per unit charge converted into electrical energy by the source
• e.m.f = energy converted into electrical energy/charge passing. ϵ = W/Q or W = ϵQ. Since W/Q has units of J/C the units of e.m.f are the same as p.d, so that is volts
• e.m.f and p.d are both measured in volts, or J/C (1V = 1J/C). e.m.f is the creation of electrical energy from other forms of energy. P.d is the conversion of electrical energy into other forms of energy
• P = IV. The electrical power converted in a device is given by the product of the current in and the voltage across it. The units are Watts
• Change in work done(W) = IVt. This is measured in joules(J)
• The gradient of an ohmic conductor graph is m = i/v. To find the resistance is through the reciprocal of the gradient R = 1/m
• A rheostat can be used to either control the current in a circuit or give a continuously variable potential difference. It is said to be a potential divider and is manufactured specifically to become potentiometers
• Ohm's law states that for metals at a constant temperature, the current in the metal is proportional to the potential difference across it
• The resistance of an electrical component is the opposition to an electric current flowing in it. The resistance is caused by collisions of the electrons with the vibrating lattice ions as the electrons drift through the material of the conductor. So, the electrical energy is dissipated as thermal energy, and the component heats up
• Resistance = Potential difference / Current and resistance is measured in Ohms. The ohm is the name for a volt per ampere(V/A)
• There are Current-potential difference graphs for components including ohmic conductors, filament bulbs, thermistors, and diodes
• P = I^2R and P = V^2 / R are equations that can be derived for power through the equation P = IV
• The power is dissipated in a resistor (scattered) as the electrical energy transferred in the resistor increases the potential energy and the random kinetic energy of the atoms of the material of the resistor. This is the internal energy of the atoms
• The internal energy of a body is the sum of the potential energy and the kinetic energy of its molecules
• Resistivity is a property of a material and is defined by the equation R = pl/A. It is measured in Ohms meters
• Tungsten filament lamp does not obey Ohm's law because the temperature of the filament rises as the current increases. The resistance of the filament increases as its temperature increases
• For the drift velocity of the tungsten filament (I = nAvq), A and q are constant for a given wire. For a metallic conductor, n does not depend on the temperature, so n is constant. As the temperature rises, the increased vibrations of the lattice will reduce the drift velocity,v, of the electrons and so current will decrease that is the resistance increasing with temperature
• A material has a positive temperature coefficient (PTC) if its resistivity increases when its temperature increases
• A semiconducting device called a thermistor is used to show the effect of temperature on the resistance of a semiconductor. There are two types of thermistors (a disc thermistor and a rod thermistor)
• A material has a negative temperature coefficient (NTC) if its resistivity decreases when its temperature increases
• A negative temperature coefficient can be explained through I = nAvq. In a semiconductor, an increase in temperature can provide extra energy to release more charge carriers. This means that n increases with temperature
• Semiconductors with a PTC can also be constructed as their resistance increases with temperature
• Doping - It is where a pure semiconductor has atoms of an impurity added to it. The properties of the semiconductor created are very sensitive to the type and quantity of the impurity atoms
• Drift velocity is when electrons in a wire over with random thermal motion within the crystal lattice. When a potential difference is applied an electric field is created and exerts a force on the free electron, which causes them to drift in the direction of the force. This causes the electrons to accelerate until they collide with lattice ions. These collisions cause an equal and opposite force to be exerted on the electrons. This is Newton's third law
• Resistivity is a property of a material measured in ohm metre (Ωm)