P3

• Charge


  A property of all matter that can be positive or negative. If a body has the same amount of positive and negative charge they cancel out, forming a neutral body.
• Insulators


  • Don't conduct charge as their charged particles cannot flow throughout the material, and are fixed
• Conductors


  • Can conduct charge as their charged particles are delocalised and can flow instead of being fixed
• Static Electricity


  1. Electrons are transferred from one insulator to another when they are rubbed together

  2. This forms a positive charge on the object losing electrons and a negative charge on the object gaining electrons

  3. The object that loses or gains electrons depends on the materials involved
• If conductors are rubbed, electrons will flow in and out of them cancelling any overall effect, so they remain neutral
• Sparking


  Occurs when enough charge builds up, and the objects are close but not touching. The "spark" is when the charge jumps through the air from the highly negative object to the highly positive object to balance out the charges.
• Electrostatic force


  • Charged objects experience this force which can be attractive or repulsive
  • Greater charge and smaller separations results in greater force as the force is proportional to the inverse square of the distance
  • Electrostatic force is a non-contact force, as it is experienced even when the objects are not touching
• Testing the Force


  Charged objects attract small neutral objects placed near them because the positive/negative charge on the object will attract/repel the electrons within the small objects, inducing a charge inside them causing them to be attracted
• Electric Fields


  Like magnetic fields for magnets, electric fields are for charges. They point in the direction a positive charge would go (i.e. away from positive charges, and towards negative charges). The fields point charges at right angles to the surface.
• Current


  For a current to flow there needs to be a closed circuit and a source of potential difference (i.e. a battery). This p.d. source is needed to "push" the current through the resistance of the circuit and needed for the "difference in potential" which causes electrons to flow.
• Current has the same value at any point in a single, closed loop.
• Potential Difference (p.d.)


  Measured in Volts and is the energy transferred per unit charge. P.d. is measured across two points using a voltmeter placed in parallel across a component.
• Current


  Measured in Amps and is the rate of flow of charge (electrons). It can be measured at any single point on the circuit using an ammeter placed in series.
• Series Circuit


  • A closed circuit where the current only follows a single path. This current is the same everywhere.
• Parallel Circuit


  • A branched circuit where the current splits into multiple paths. This current may be different in the different branches but voltage is the same across each branch.
• Voltage


  Current × resistance
• How Does Resistance Change

  1. With Current: As current increases, electrons (charge) have more energy. When these electrons flow through a resistor, they collide with the atoms in the resistor. This transfers energy to the atoms, causing them to vibrate more. These increased vibrations make it more difficult for electrons to flow through the resistor, so resistance increases, and the current decreases.

  2. With Temperature: For normal wires the same process occurs as above due to increased atoms vibrations when hot. For Thermistors, resistance is lower at higher temperatures.

  3. With Length: Greater length, the more resistance, and the lower the current. Electrons have to make their way through more resistor atoms, so it is harder than using a shorter wire.

  4. With Cross Sectional Area: Thinner wires give greater resistance because less overall room for electrons to pass through between atoms.

  5. With Light: For light dependant resistors (LDRs) : a greater the intensity of light means there is a lower resistance, so resistance is greatest when dark.

  6. With Voltage: Diodes allow current to flow freely in one direction. In the opposite direction, they have a very high resistance so no current can flow.
• Testing Relationships Experimentally


  1. Varied Wire Resistance: Use wires of resistance 1Ω to 10Ω, connect to DC of 2, 4, 6, ..., 10, 12V, connect in series to an ammeter and in parallel to a voltmeter, measure I for each voltage and for each wire, then plot a graph to show the relationship.

  2. Filament Lamps: Connect to DC of 2, 4, 6, ..., 10, 12V, connect a filament lamp to an ammeter and a voltmeter, measure the current for each voltage, plot an I-V graph to show the relationship.

  3. Diodes: Connect to DC of 1, 1.5, 2, 4, 6, ..., 10, 12V, connect an ammeter and a voltmeter, measure current for each voltage, switch the diode the other way around to record current for -1, -1.5, -2, -4V, plot an I-V graph.

  4. LDR: Use a constant voltage of 12V, connect to an ammeter, shine a lamp immediately onto the thermistor and measure the current, move the lamp ~10cm away and measure current again, repeat until the lamp is 50cm away, calculate resistance at each light intensity, plot graph of resistance against light intensity.

  5. Thermistor: Use a constant voltage of 12V, connect to an ammeter, place in ice water with a thermometer, measure current at 0ºC, add hot water and stir, measuring current at 10, 20, ..., 60ºC, calculate the resistance at each temperature, plot graph of resistance against temperature.
• Resistors in Series


  • Components are connected end to end so all the current flows through all the components. The total resistance is the sum of the resistance in each component.
• Resistors in Parallel


  • Components are connected separately to the power supply so current flows through each one separately. Total resistance is less than the branch with the smallest resistance and is calculated as the sum of the reciprocals of resistance.
• Power


  Current × Voltage
• Power


  I^2 × R
• Energy


  Charge × Potential Difference
• Energy


  Power × Time