Topic 2: Electricity

• Electric current

  The rate of flow of electric charge (flow of electrons through the circuit)
• Conventional current

  Flows from the positive terminal to the negative terminal of the cell
• Electrons from the cell

  Pass energy to the components in the circuit
• Current is the same at all points in a series circuit, because current is never used up in a circuit
• Current

  Measured by an ammeter
• Current of 1 ampere

  1 coulomb of charge flowing per second
• Series circuit

  • Current can only flow in one path
• Parallel circuit

  • Contains branches, so the current splits as some of the current passes through all of the branches
• Cell's chemical energy

  Transferred to electrical energy which is carried by electrons passing out of the cell
• Electrons pass through components

  Electrical energy is transferred to other forms of energy
• Electrons pass through a lamp

  Electrical energy is transferred to light energy and thermal energy (wasted)
• Potential difference of 1 volt

  1 joule of energy transferred for each coulomb of charge that is moving through the circuit
• Potential difference

  Measured by a voltmeter
• The total potential difference across all the components in a series circuit adds up to the potential difference across the cell
• The potential difference is split between components in series circuits
• For components connected in parallel circuits, the potential difference across each component is the same (it is not split)
• Battery

  Two or more cells connected together
• The cells in a battery must be connected in the same direction
• The potential difference of cells adds up to give the potential difference of the battery
• The current in the branches adds up to the total current of the cell in a parallel circuit
• Resistance

  Everything that resists or opposes current to flow
• Resistance

  Tells us the potential difference required to drive a current through a component
• A lot of energy is wasted when moving a current through a component due to resistance
• Higher potential difference in a series circuit

  Means a higher resistance
• Ohmic conductor

  A conductor for which current and potential difference are directly proportional, provided temperature is constant
• The resistance remains constant as the current changes in an ohmic conductor
• Current-potential difference graph for an ohmic conductor

  • A straight line with positive gradient passing through the origin, and we still get a current when the potential difference is reversed
• Filament lamps, diodes, thermistors and light dependent resistors (LDRs)

  • The resistance is not constant as current changes
• Current through a filament lamp

  Not directly proportional to the potential difference, because the filament gets hot, which causes the resistance to increase
• As temperature increases

  Ions in the metal have more energy, so vibrate more, causing more collisions with electrons in the current, so more energy is needed to push current through the filament, so greater resistance
• Current-potential difference graph for a filament lamp

  • Not directly proportional, because when the current increases, the temperature of the lamp increases which causes resistance to increase. The graph is an upward s-shaped curve passing through the origin, and we still get a current when the potential difference is reversed
• Diode

  • The current only flows in one direction, as resistance is very high in the other direction, preventing current flow
• Current-potential difference graph for a diode

  • No current in reverse direction, so current is 0 in the reverse direction, however in the forward direction, as potential difference increases, current increases
• Diodes

  Extremely useful for controlling the flow of current in circuits
• LED

  • The current only flows in one direction, like a normal diode, however an LED gives off light when a current flows through
• LEDs

  An extremely energy-efficient source of light
• Resistors in series add together. The current has to pass through each resistor in turn. It cannot bypass any resistor
• The total resistance of 2 resistors in parallel is less than the resistance of the smallest individual resistor, as with 2 resistors in parallel, there are two pathways for current to take, so more total current will flow through the circuit, and as the current has increased but the potential difference is the same, than the total resistance must have decreased
• As the light intensity increases

  The resistance of an LDR decreases
• In dark conditions

  The LDR has high resistance, and it takes a great deal of energy for the current to pass through the LDR so the potential difference across the LDR is very high