GCSE PHYSICS - Electricity

• Parts of the Atom

  • The nucleus contains:
  • Protons - positively charged particles with a relative atomic mass of one unit
  • Neutrons – no charge, and also with a relative atomic mass of one unit
  • Almost all of the atom is empty space, but moving around the nucleus there are:
  • Electrons – negative charge with almost no mass (1/1860 the mass of a proton or neutron)
• • Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus
• • The nucleus is made up of protons and neutrons
• • The nucleus contains:
  • Protons - positively charged particles with a relative atomic mass of one unit
  • Neutrons – no charge, and also with a relative atomic mass of one unit
• • Almost all of the atom is empty space, but moving around the nucleus there are:
  • Electrons – negative charge with almost no mass (1/2000 the mass of a proton or neutron)
• The terminals of a cell make one end of the circuit positive and the other negative
• Setting up a potential difference across the circuit is sometimes known as voltage
• Potential difference is defined as the amount of energy transferred per unit of charge passing through the terminals
• One volt (V) is equivalent to one joule (J) per coulomb (C): 1 V = 1 J / C
• Potential difference can be calculated using the equation: Energy = charge x potential difference
• Equation variables:

  • V = potential difference (measured in volts, V)
  • E = energy transferred (measured in joules, J)
  • Q = charge moved (measured in coulombs, C)
• Potential difference (or voltage) is measured using a voltmeter
• A voltmeter is always set up in parallel to the component being measured
• Measuring voltage 'across' the component is described as connecting the voltmeter in parallel
• Potential difference (voltage) can be measured by connecting a voltmeter in parallel between two points in a circuit
• Cell / battery:

  • Provides the circuit with a source of potential difference - i.e. energy per unit of charge
  • A battery is two or more cells
• Switch:

  • Turns the circuit on (closed), or off (open)
• Fixed resistor:

  • Limits the flow of current
  • A fixed resistor has a resistance it cannot change
• Variable resistor:

  • A resistor with a slider that can be used to change its resistance
  • Often used in dimmer switches and volume controls
• Thermistor:

  • The resistance of a thermistor depends on its temperature
  • As its temperature increases, its resistance decreases and vice versa
• Light-dependent resistor (LDR):

  • The resistance of an LDR depends on the light intensity
  • As the light intensity increases, its resistance decreases and vice versa
• Motor:

  • Converts electrical energy to mechanical energy
• Diode:

  • Allows current to flow in one direction only
  • Used to convert AC to DC current
• Light-emitting diode (LED):

  • Equivalent to a diode and emits light when a current passes through it
  • Used for aviation lighting and displays (TVs, road signs)
• Ammeter:

  • Used to measure the current in a circuit
  • Connected in series with other components
• Voltmeter:

  • Used to measure the potential difference of an electrical component
  • Connected in parallel with the relevant component
• Each component has an electrical resistance that may impact the current in the circuit
• Resistance of the ammeter and voltmeter are considered negligible in exam questions
• Electric circuit diagrams require:

  • An energy source (cell, battery, or power supply)
  • A closed path or a complete circuit for electrons to flow in a loop
  • Electrical components drawn with the correct circuit symbol
• Key rules:

  • An ammeter is always connected in series
  • A voltmeter is always connected in parallel to the component the voltage is being measured
  • Current flow direction is from the positive to the negative terminal of the power supply
• Electric current is defined as the rate of flow of charge
• The size of an electric current is the amount of charge passing through a component per second
• The equation relating current, charge, and time is: Current = charge/time
• I = current, measured in amperes (or amps, A)
• Q = charge, measured in coulombs (C)
• t = time, measured in seconds (s)
• The wires in an electric circuit are made of metal because metal is a good conductor of electric current
• In the wires, the current is a flow of electrons
• Electrons flow through a lattice of metal ions
• A current will flow in a circuit if:

  • The circuit includes a source of potential difference
  • The circuit is closed, meaning there are no gaps in the circuit