topic 10

• What is current?

  The flow of electric charge (eg electrons) around the circuit
• What are the units for current?

  Amperes (Amps) - A
• What is potential difference?

  The driving force that pushes the charge around, and energy transferred per coulomb of charge
• What are the units for potentia difference?

  Volts (V)
• What is resistence?

  Anything that slow the flow of charge around a circuit down.
• What is the formula for Charge? Use symbols

  Q = I x t (Charge = current x time)
• What are the units for charge?

  Coulombs (C)
• What is the formula for energy transferred (which involves p.d.)

  Energy Transferred = Charge Moved x Potential Difference

  E = Q x V
• What are the units for energy (transferred)?

  Joules (J)
• What is the formula for potential difference?

  V = I x R or Potential Difference = current x resistence
• What are the units for resistence?

  ohms, (Ω)
• What happens to resistence when temperature increases, and why?

  As charge flows through the resistor, the ions in the resistor gain energy and begin to vibrate. This means that the electrons carrying the charge find it harder to get through, de to there being more collisions. Therefore, usually resistence increases with temperature. The only exception is a thermistor.
• What is an LDR?

  A Light Dependent Resistor - a resistor whose resistence is dependent on light intensity. In less light, the resistance is higher, and it is lowest in brightness
• What are applications of an LDR?

  - automatic night lights

  - outdoor lighting

  - burglar detections
• What is a thermistor?

  A temperature dependent resistor. In hotter conditions, the resistence falls, and in cooler conditions the resistence increases.
• What are some applications of thermistors?

  Car engine temperature sensors and electronic thermostats
• What is a diode?

  A device that only allows current to flow in one direction, meaning that when the p.d. is negative, current cannot flow through.
• What are diodes used for?

  control the direction in which current flows in a circuit e.g. power supplies for D.C. equipment
• What is a series circuit?

  a circuit that only has one path
• What is a parallel circuit?

  A parallel circuit has two or more paths for current to flow through.
• How can you measure current?

  Using an ammeter. It must be used in series.
• How can you measure potential difference?

  Using a voltmeter in parallel
• What happens to the potential difference in parallel circuits?

  It is the same everywhere
• What happens to current in a parallel circuit?

  The current is shared between the components, therefore the total current is found by adding up the currents of all the components
• What happens to the current in a series circuit?

  The current is the same in all components
• What happens to the potential difference in a series circuit?

  It is shared between the components. Therefore the total p.d. can be found by adding up the p.d. at all the individual components.
• What happens to the resistence when you add resistors in series?

  Total resistence increases, because the p.d. is shared between the resistors, meaning there is a lower driving force across each, however the current is the same everywhere. With a lower p.d., and same current, resistence has to increase.
• What happens to the resistence when you add resistors in parallel?

  Total resistence is reduced. The p.d. remains the same, but by adding another loop, the currents has more than one direction to flow in, so increasing total current that can flow around the circuit, meaning that because p.d. is the same, and current increases, total resistence decreases.
• What is the mains supply in the UK?

  A.C. , 50Hz, 230-240V
• What is the formula for Energy Transferred (involving current)

  E = I x V x t (energy = current x p.d. x time)
• Give an example of when a circuit heating up can be beneficial?

  Toasters have a coil of high resistence wire, and this heats up as current passes through. It glows, and gives off infrared radiation which cooks the bread.
• What is power?


  the rate at which energy is transferred
• What is a power rating on an appliance?

  The maximum safe power they can operate at - the maximum amount of energy transferred between stores per second when the appliance is in operation
• What is the formula for power (involving energy)?

  P = E/t or power = energy transferred/ time take
• What are the units for power?

  Watts (W), or J/s
• What is the formula for power (involving p.d.)?

  P = I x V (power = current x p.d.)
• What is the formula for power (involving resistence)?


  P = I^2 x R or Power = current^2 x resistence
• What is the function and colour of the neutral wire?

  It is blue and completes the circuit.
• What is the p.d. of the neutal wire?

  around 0V
• What is the colour and funciton of the Live Wire?

  It is brown, and carries the p.d.