4.2 Energy power and resistance

• What is the difference between a cell and a battery?

  A battery is 2 or more cells connected in series
• Positive terminal of a cell / battery is the longer side
• Current flows from positive to negative in a circuit - conventional current
• Potential difference is the energy transerred from electrical to other forms of energy
• One volt is the potential difference across a component when 1J of energy is transferred per unit charge
• Potential difference = work done (J) / charge (C)
• Voltmeters are connected in parallel and they should have an infinite resistance
• emf (electromotive force) - the energy transferred from chemical to electrical energy per unit charge
• An electron gun is used to produce a narrow beam of electrons used to ionise particles by adding or removing electrons from atoms
• Electron guns have very precisely determined kinetic energies
• Electron guns are used in electron microscopes, mass spectrometers and oscilloscopes
• Electrons in an electron gun are emitted from the cathode through thermionic emission - a filament is heated so electrons gain kinetic energy and escape from the surface of the metal
• The beam of high speed electrons is produced after emission from the cathode

  The heated filament in a vacuum has a high pd applied between the filament and an anode so the filament acts as a cathode. The free electrons accelerate towards the anode, gaining kinetic energy. The electrons that align witht the hole in the anode pass through it, creating a beam of electrons with a specific kinetic energy
• Work done on an electron:

  e x accelerating pd (V)= $1/2 *$$mv^2$where v is the velocity of the electron and m is the mass of the electron
• resistance is the ratio between voltage and current
• The ohm is defined as the resistance of a component when a pd of 1V is produced per ampere of current
• Ohm's law : the current in a conductor at a constant temperature is directly proportional to the pd across its ends
• Resistance of a wire increases with temperature as the metal ions vibrate more causing more collisions with the electrons so the charge carriers do more work (transfer more energy) as they travel through the wire
• a diode is a component that only allows current to flow in one direction - a very high resistance in the other direction
• a thermistor has a negative temperature coefficient meaning as temperature increases, resistance decreases
• LDRs are used in phone screens and streetlights
• LDR - resistance is dependent on light intensity - as light intensity increases, resistance decreases
• Thermistors and LDRs are made of semiconductors
• Material, length and cross-sectional area affect the resistance of a wire
• Resistance is directly proportional to the length
• $R$Resistance = ( $\rho$resistivity(Ωm) x$l$ length (m)) / $A$ cross-sectional area (m^2)
• resistivity is measured in Ohm metres
• Resistance only refers to a specific component whereas resistivity is used to describe the electrical property of a material
• As temperature increases, resistance increases, therefore resistivity also increases
• Electrical power is the rate of energy transfer by each electrical component
• Power = voltage x current
• energy transferred = power of a device x time
• kWh : the energy transferred by a device with a power of 1kW per hour