Chapter 9

• Potential difference (voltage) is the energy transferred from electrical energy to other forms per unit charge. 


  $V=$$\frac{W}{Q}$
• A voltmeter is used to measure p.d and should be connected to a circuit in parallel.
• Electromotive force is the energy transferred from chemical energy to electrical energy per unit charge


  $\mathcal{E} =$$\frac{W}{Q}$
• Electron guns are used to ionise particles by adding or removing electrons from atoms and can have precise determined kinetic energys
• How does an electron gun work?

  A small metal filament is heated by an electrical current in a vacuum. The electrons in the filament gain kinetic energy and some of them have enough energy to escape the surface of the metal through thermionic emission. Due to a high p.d between the filament and an anode, the filament acts as a cathode and the freed electrons accelerate towards the anode gaining more kinetic energy. The anode has a small hole in it so that only the electrons in line with it are able to pass through creating a thin beam of electrons with a specific kinetic energy.
• In an electron gun, the work one on a single electron is equal to eV where e is the elementary charge and V is the accelerating p.d. therfore:


  $eV =$$\frac{1}{2} mv^2$
• In an electron gun, increasing the accelerating p.d will increase the kinetic energy transferred to the particles, therefore increasing their speed
• A linear particle accelerator uses a series of cylindrical electrodes (drift tubes) to accelerate subatomic particles. The polarity of the drift tubes is alternated with precise timing so that each time the electrons leave one tube, the polarity is changed to attract it to the next. Through this, particles are accelerated to incredibly high speeds.
• Resistance is the opposition to the flow of charge in a component
• Resistance is the opposition of flow of charge


  $V=$$IR$
• Ohm's law states that for a metallic conductor kept at a constant temperature, current is directly proportional to the potential difference
• As temperature increases, resistance increases. because the positive ions have more kinetic energy and vibrate with greater amplitude about their mean positions. Therefore the frequency of the collisions between the ions and the electrons increases so the charge carriers do more work and transfer more energy as they move through the wire
• I-V graph- resistors


  A resistor is an ohmic conductor

  The resistance remains constant

  The resistor behaves the same despite polarity

  
• I-V graph- Filament Lamps


  A filament lamp is a non-ohmic conductor

  Its resistance is not constant

  The filament lamp behaves the same despite its polarity

  
• The gradient of an I-V graph is the resistance
• Filament lamps are non-ohmic conductors as the current through its filament increases, the number of collisions with the positive ions in the wire increases therefore more energy is transferred to the positive ions so they gain kinetic energy causing the ions to vibrate more increasing the temperature of the wire and further increasing the number of collisions and results in an increase in resistance
• Resistance is effected by material, length and cross sectional-area
• Resistivity is the product of the resistance of a material and cross-sectional area divided by the length 


  $R=$$\frac{\rho L}{A}$
• Good conductors have a resistivity of order $10^{-8} \Omega m$whereas insulators have a resistivity of order $10^{16} \Omega m$
• A thermistor has a negative temperature coefficient, as the temperature increases, the resistance decreases. This is because as the temperature increases, the kinetic energy of electrons increases allowing for more electrons to jump to the conducting bands which increases the number density and therefore decreases the resistance
• Thermistors are used in:

  simple thermostats

  thermostats to control heating and air-conditioning

  temperature monitoring temperature in electrical units

  measuring temperature in electrical devices

  monitoring temperature in engines
• To investigate the effect of temperature on the resistance of a thermistor, the thermistor is placed in a water bath and an ohmmeter. The temperature of the water and the resistance is measured. This can then be plotted on a graph to show their relationship
• I-V graph: Thermistor 


  A thermistor is a non-ohmic conductor

  The resistance is not constant

  
• To investigate the effect of light intensity on an LDR, an filament lamp can be used and a tube of black card is placed around the LDR to reduce the effect of background light. The LDR is attached to an Ohmmeter and the filament lamp is moved away from the LDR. The resistance is recorded and then a graph is plotted
• With an LDR, as light intensity increases, resistance decreases. This is because when light shines onto the LDR the number density increases which in turn decreases the resistance.
• Power is the rate of energy transfer, measured in watts.

  $P=$$IV$$P=$$I^2R$$P=$$\frac{V^2}{R}$
• Electricity bills use the unit of kilowatt-hour (kWh) to measure the amount of electricity used.