Topic 10 Electricity and Circuits

• The atom has a central nucleus of positively charged protons and uncharged neutrons both with a mass of 1
• Electrons are negatively charged particles found in the outer shells of an atom with a mass of 1/1835
• Components in a circuit can be connected in parallel or series
• In series circuits there are just one route the current can take around the circuit. In parallel circuits there are junctions that allow the current to take different routes
• In series lamps cannot be switched on and off individually and if one lamp fails they all switch off
• In the parallel circuit each lamp can be switched off separately
• A voltmeter is connected in parallel with a component to measure the potential difference (voltage) in volt across it
• Potential Difference is the energy transferred per unit charge passed and hence the volt is a joule per coulomb
• Energy transferred (J) = charge moved (C) * Potential Difference (V)
• An Ammeter is connected in series with a component to measure the current, in amps, in the component
• The current in metals is a flow of electrons
• Current is the rate of flow of charge.
• Charge(c) = current (A) * time (s)
• When a closed circuit includes a source of potential difference there will be a current in the circuit
• Current Is conserved at a junction in a circuit
• you need voltage in a circuit to 'push' current around an electric circuit
• Potential difference (V) = current (A) * Resistance (ohm)
• The cell transfers energy to the charge, and so the charge then has the potential to transfer energy to other components in the circuit (potential energy)
• The potential difference of a cell is the amount of potential energy the cell transfers to each coulomb of charge flowing through it
• Some wires and components need a larger potential difference to produce a current through them than others, this is because of their large electrical resistance
• When resistors are connected in series the total resistance of the circuit is increased because the pathway becomes harder for current to flow through.
• There will be a greater potential difference between circuits with higher resistances
• The greater the resistance of a component, the smaller the current that flows
• Variable resistors can change the resistance by changing the length of wire that makes up the circuit A longer length of wire has more resistance than a shorter length of wire
• When resistors are connected in parallel the total resistance of the circuit is less than the resistance of the individual resistors. This is because there are more paths for the current
• in a series circuit the different components are connected in a line, end to end, between the positive and negative ends of the power supply
• series circuits are often designed to build and measure components because the p.d, current and resistance are quite easy to change, measure and calculate.
• For a filament lamp, current and potential difference are not directly proportional. The resistance of the filament lamp increases as the temperature of the filament increases
• A diode allows current to flow in one direction only
• In fixed resistors the current is directly proportional to the voltage and inversely proportional to the resistance
• In a diode more voltage leads to more current.
• As voltage increases across the filament, the current also increases
• thermistors have high resistances at low temperatures but as the temperature increases the resistance decreases
• An LDR has a high resistance in the dark but the resistance gets smaller when the light intensity increases
• A circuit used to explore variation in resistance of a lamp. It can be adapted to explore resistance in a diode, LDR or thermistor

  
• when there is an electric current in a resistor there is an energy transfer which heats the resistor
• electrical energy is dissipated as thermal energy in the surroundings when an electrical current does work against electrical resistance
• The energy transfer as a result of electric current in a resistor, is the result of collisions between electrons and ions in the lattice
• Resistance in circuits can be reduced by using wires made from metals with low resistance. thicker wires have lower resistance, resistance can also be decreased by cooling metals so that the lattice ions are not vibrating as much
• The heating effect of an electric circuit is useful in an electric heater or kettle