Paper two

• A current can only flow through a circuit if there is a potential difference across that component and if the circuit is complete
• Potential difference is the drivibg force that pushes a charge around and measured in volts

  resistance is anything that slows down and measured in ohm

  • generally the higher the potential difference across a component the higher its current
• Potential difference is energy transferred per unit charge flowing through a circuit.
• Resistance increases with temperature as the electrical charge does work against Resistance electrons collide with ions in the resistor transferring energy causing them to vibrate and increase in temperature

  • the only exception is in thermistors where resistance decreases with temperature
• Investigating components

  1. Use a set circuit

  2. Connect a variable d.c source, ammeter, and component in series

  3. Connect a voltmeter in parallel to the component

  4. Change the output p.d of the power supply

  5. Take several readings of the voltmeter and ammeter

  6. Plot a graph of current against voltage

  7. Calculate resistance

  8. Ensure the circuit doesn't get too hot

  9. Repeat the practical and calculate a mean
• Purpose of the investigation

  • To investigate the relationship between voltage and current for a range of components
• The graph of current against voltage can be used to calculate resistance
• Repeating the practical and calculating a mean is important
• Ensuring the circuit doesn't get too hot is important to avoid tampering with the results
• when investigating diodes on a range of currents you would find that swapping the diodes direction stops the flow of a current as there is a high resistance in said direction
• When investigating thermistors keep the p.d supply constant and gradually heat the thermistor with a beaker filled with hot water and as resistance decreases the thermistors temperature increases
• When use LDR do it in a dim room whilst keeping the p.d supply constant and adjust light level with lamp with a dimmer switch as the light grows brighter the current through an ldr increase as ohm decrease
• In series component are connected end to end between the ve+ and ve-

  • voltmeters are always in parallel
• If one component is disconnected or removed the circuit is broken and nothing will work
• theres A bigger supply of voltage when more cells are in series

  • two batteries with a p.d of 2V share 4 V between them
  • the total voltage is shared between components in the series
  • no matter where in the circuit the current is the same
  • the total resistance increases as you add resistors because of this
• In parallel circuits

  • components are connected separately to the -ve and+ve of the power supply excluding ammeters which are always in series
• Removing or disconnecting a component from the system will not cause any affect

  • when looking at components in the same branch the rules for a circuit In series apply
  • the voltage is the same in every branch v1= v2= v3
  • current is shared between branches so adding up the current in each branch gives the total current
• Adding resistors in a series increases total resistance as they share the same voltage

  • where as in parallel circuits adding resistors decreases total because when adding more loops the current has more space to travel meaning it can increase with more length and because the voltage is equal in every branch the resistance will have little effect
• In a fixed resistor increasing current can decrease resistance as the current is the only thing that is changing
• The power of an appliance is the energy transferred per second

  • the more energy transferred in a given time the higher its power
• Appliances are given power rating which indicate the maximum safe power that they can operate at

  • it’s the highest amount of energy that can be transferred between stores per second when an appliance is in use
  • power rating ms help people know which appliance is more useful as microwave with a rating of 700w is better than a microwave with a rating of 500w
  • however just because an object has a high rating doesn’t mean it is used usefully and may be wasted into other stores
• power transferred by an appliance depends on the potential difference across it and the current flowing through it
• Alternating currents are constantly changing direction and are caused by alternating voltages

  a direct current is one that only flows in a certain direction and the voltage is only either negative or positive
• Electrical appliance’s are connected to a mains supply via a three core cable with a copper core and plastic coating

  • neutral wire , blue completes a circuit electricity flows through the live wire and out the neutral and always has a voltage of 0
  • the live wire is brown and carries the voltage and alternates between a high -ve and ve + at 230V
  • the earth wire is green and yellow and is for safety it carries the current away if something wrong and stops the casing becoming live and has 0V
• the human body is like an earth wire and has 0V touching the live wire causes an electrical shock as a large potential difference flows through you
• Surges are sudden increases in a current and cause your appliance to melt or catch fire and faulty appliances can cause shock
• A fuse prevent a surge causing harm if a surge is higher than a fuse rating (which is a little higher than normal operating current ) the fuse breaks and so does the live wire as they are connected isolating the whole device the larger the current the thicker a cable should be so as fuse ratings increase so do cable thickness
• an alternative to fuses are circuit breakers

  • a surge trips the breaker turning it off quicker than a fuse needs to melt
  • they can be reset rather than replaced like a fuse
  • but are more expensive
• Earthing is attaching an earth wire to the casing as earthed conductors can’t become live
• Apoliances with no visible metal and a plastic casing is double insulated

  double insulated appliances don’t need a live wire and consist of two core cables a live wire ( brown ) and neutral ( blue )