Topic 10: Electricity and circuits

• Circuit symbols and names

  symbols (motor not included):

  
• What are the differences between series and parallel circuits?

  Comparison:

  
• How is a voltmeter attached to a circuit to measure the voltage of a component?

  In parallel, for example:

  
• How is a ammeter attached to a circuit to measure the current of a component?


  In series with a component and measures the current in amps (A)
• What is current (in metals)?

  The flow of electrons per second
• What is the equation for charge?

  Charge = current x time

  Q = I x t
• What is potential difference (voltage)?

  The energy carried by the electrons - energy transferred to every coulomb of charge (electron)
• What is one volt equal to?

  One joule / coulomb
• What is the equation for potential difference?

  energy transferred (J) = charge (C) x potential difference (V)
• What happens to potential difference/voltage in a series circuit?

  It splits
• What happens to potential difference/voltage in a parallel circuit?

  It stays the same
• What happens to current in a series circuit?

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

  It splits
• p.d = current x resistance

  v = I x R
• How do you find the total resistance in a series circuit? How is the p.d shared?

  1. By adding up the individual resistances.
  2. Shared based on the resistance of each individual resistance e.g the higher the resistance the higher the p.d. If 15 ohms was being shared between 2 resistors and one had a resistance of 0.5 v and the other 1v, 5 ohms would go to the first resistor and 10 through the second
• Why do circuits heat up?

  1. as electrons move along the wire they collide with the metal ions in the lattice
  2. the collisions make the ions vibrate more
  3. this makes the metal hotter causing wasted energy to be transferred as heat
• What is the relationship between I (current) and V (p.d) when a fixed resistor is in the circuit? Are they proportional?What happens to the resistance?


  1. As I increases, V increases
  2. directly proportional
  3. Stays same
• What is the relationship between I (current) and V (p.d) when a bulb is in the circuit? Are they proportional? What happens to the resistance?Why does this happen?


  1. As I increases, V increases
  2. Increases
  3. Bulbs heat as the current flows through them which makes it harder for it to flow = more resistance
• What does the IV graph look like for a filament lamp (bulb) ? How does it relate to resistance and why?

  2. the lamp gets hotter which gives the electrons more energy to collide with each other so the resistance increases

  
• What does the IV graph look like for a fixed resistor? How does it relate to resistance and why?

  2. Resistance stays the same because or the resistor so as current increases, pd increases (directly proportional) - V = I x R

  
• What does the IV graph look like for a diode? How does it relate to resistance and why?



  2. Resistance is very high when the current is negative (on the left/flowing the opposite way) as diodes are designed to make current only flow in one direction.

  
• What is the equation for energy transferred?

  e = V x I x t
• What is the equation for power involving energy?

  P = e / t
• What is the equation for power involving pd?

  P = I x V
• What is the equation for power involving resistance?

  P = I^2 x R
• what is power measured in?

  Watts (w)
• What is the average Mains voltage in the UK?


  230v
• What is the average Mains frequency in the UK?

  50Hz
• What is alternating current? (a.c) What supplies alternating current?

  1. Electrons keep changing direction (either positive or negative) and so we get an alternating voltage (increases and decreases)
  2. Current supplied by Mains (plug sockets)
• What is direct current? (d.c) What supplies direct current? (2)

  1. Electrons travel in one direction only
  2. Current supplied by batteries and solar panels
• What is the function of the neutral wire in a plug?

  The connection back to the power station which completes the circuit (carries the current away again)
• What is the function of the live wire in a plug?

  Connects the ac voltage from the (Mains) supply to the appliance
• What colour is the neutral wire? What about the live wire?

  1. blue
  2. brown
• What is the function of the earth wire in a plug? What colour is it?

  1. normally carries no current, purpose is to stop the appliance from becoming 'live' if there's a fault - it redirects the current away from appliance if this happens
  2. Yellow and green
• What is the danger of any connection between the live and earth wire?

  • Because of the large potential difference between the live (230 V) and the earth (0 V), if the two are connected together, a very large current can be created
  • If a person provides the connection between live and earth then a large current can pass through them, providing a potentially lethal shock
• What is the function of a fuse/switch?

  if a faulty appliance draws too much current it can cause the wire to heat up, if the current in the circuit gets too high, the wire in the fuse melts and breaks the circuit to stop current from flowing
• What is the graph of a resistance graph of an LDR? Can you describe the relationship?

  1. (image) below
  2. as light intensity increases, resistance decreases, meaning the current increases (because less resistance), so the voltage stays the same. --> current and resistance are inversely proportional when voltage is constant

  
• What is the graph of a resistance graph of a thermistor? Can you describe the relationship?

  1. (image) below
  2. as temperature increases, resistance decreases, meaning the current increases (because less resistance), so the voltage stays the same. --> current and resistance are inversely proportional when voltage is constant

  
• What happens to a circuit when there's an energy transfer? Why?

  1. heats up
  2. due to work done against resistance
• What are the advantages of circuit heating?(3)

  • come appliances use circuit heating for their function e.g. a toaster
  • filament bulbs have to use circuit heating
  • fuses use circuit heating to keep circuits safe