electricity

• 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.
• Like charges repel and opposite charges attract
• 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 rubbed together

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

  3. Conductors remain neutral when rubbed as electrons flow in and out

  4. Sparking occurs when enough charge builds up and the objects are close but not touching
• Electrostatic force

  • Charged objects experience an attractive or repulsive force
  • 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


  1. Charged objects attract small neutral objects placed near them due to induced charge

  2. e.g. A stream of water will bend towards a charged object due to induced attraction
• 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.)

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


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


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


  • 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.
• If resistance (R) is constant

  The graph of current (I) against voltage (V) is linear for all values of current
• If R changes


  The graph of I against V is nonlinear
• Voltage


  Current × Resistance
• How Resistance Changes 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.
• How Resistance Changes with Temperature

  1. For normal wires, increased atom vibrations when hot increases resistance

  2. For Thermistors, resistance is lower at higher temperatures
• How Resistance Changes 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.
• How Resistance Changes with Cross Sectional Area

  Thinner wires give greater resistance because less overall room for electrons to pass through between atoms.
• How Resistance Changes 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.
• How Resistance Changes 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. Therefore diodes can be used to get direct current (DC) output from alternating current (AC) input.
• Testing Relationships Experimentally

  1. Varied Wire Resistance

  2. Filament Lamps

  3. Diodes

  4. LDR

  5. Thermistor
• 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
• Energy


  Charge × Potential Difference
• Energy = Power × Time