exam mistakes corrections

• State two advantages of connecting lamps in parallel rather than in series.

  1. If one blows, the circuit stays intact
  2. call all be switched separately
• Energy (E) = Power (P) x Time (t)

  Energy (E) = Voltage (V) x Current (I) x Time (t) Energy (E) = Charge (Q) x Voltage (V)
• Two advantages of using a circuit breaker instead of a fuse:

  1. Circuit breakers are resettable
  2. Circuit breakers work instantly whereas fuses don't wok instantly
• Power (P) = Current (I) x Voltage (V)

  P = IV
• Power = Current2 x Resistance

  P = I2 × R
• At first, the temperature of the water increases

  After a while, the temperature reaches a steady value below the boiling point of water.

  Explain why the temperature reaches a steady value:

  • rate of energy loss= rate of energy supply
• • The advantages of a series circuit are:
  • All of the components can be controlled by a single switch
  • Fewer wires are required
  • The disadvantages of a series circuit are:
  • The components cannot be controlled separately
  • If one component breaks, they will all stop working as well
• • The advantages of a parallel circuit are:
  • The components can be individually controlled, using their own switches
  • If one component breaks, then the others will continue to function
  • The disadvantages of a parallel circuit are:
  • Many more wires involved so much more complicated to set up
  • All components have the same voltage as the supply, so harder to control if components need to have different voltages
• Circuit symbols

  
• A Kitchen has a water supply, an electricity supply and electric lightning. There are several electrical appliances in the kitchen including a toaster, a kettle, a clothes iron, a microwave oven and a dishwasher. Discuss three hazards of using electricity in this kitchen.

  • Water can come in contact with the plugs and this can cause shocks as water conducts electricity
  • one of the electrical devices with a heating element can reach a high temperature which can melt the insulation and start a fire.
  • Overloaded cables can cause a fire because of how much electricity they're taking
• Explain why the hard drives work at a lower temperature when the black one is on top.

  • Heat is transferred (between the two hard drives) by conduction 
  • Thermal energy is emitted in the form of infrared radiation
  • More thermal energy is emitted when the black one is on top because black is a good emitter whereas, white is a poor emitter.
• How does an earth wire act as a safety feature?

  The earth wire is connected to the metal casing so that if the live wire ever becomes loose and comes into contact with the casing, the earth wire provides a low resistance path which means a large surge current into the earth wire which then the fuse blows
• The acceleration changes although the driving force does not change. Explain two possible reasons for this change in acceleration.

  1. Increase in air resistance
  2. Increase in road resistance
• The Photographs show the different cables used for the clothes iron and the lamp.

  Suggest why the wires in the clothes iron cable are thicker than the wires in the lamp cable?

  They carry a larger current
• The student uses a voltmeter to check


  The voltage across the thermistor stays constant throughout the investigation
• A student investigates the current in a thermistor at different temperatures using the circuit shown in the diagram. Give a reason for keeping the voltage across the thermistor constant ?

  Voltage is a controlled Variable
• A student uses a graph and thinks the current reading, measured in mA, gives a direct indication of the temperature measured in C.Give three reasons why the student's electronic thermometer is unlikely to show the correct temp?

  The Graph is curved so its not directly proportional

  Its only right at 10 and 100 degrees

  when the temps are lower, the current value does not match the temperature.
• The resistance of a filament lamp changes as the voltage is increased. Why?

  The lamp heats up meaning there's a greater chance of electron collisions hence, resistance increases.
• The student keeps the temperature of the wire constant during the investigation. Explain how she does this ?

  She keeps it in a water bath
• The metal ball has a mass of 0.52kg and is held at a height of 0.82m above the floor. Calculate the speed of the metal ball just before it hits the ground. How to answer?

  You use the kinetic store value then substitute into the kinetic energy formula then divide the value by the mass then square root to get the speed.
• The speed of the ball just before it hits the floor when moving in oil is less than the speed of the ball before it hits the floor when moving in air. Explain, using ideas about energy, the difference in speeds?

  • Oil is more dense than air so the force of friction is greater than before so now, ball works against friction
• Resistor x is replaced by resistor Y. resistor Y has a higher resistance than resistor x. Describe how the number of charged particles passing through the cell each second changes when resistor x is replaced by resistor Y?

  • Fewer charged particles so current decreases
  • But voltage remains the same
• Describe how the thinking distance is affected by the condition of the car's brakes and the speed of the car. ?

  Condition of brakes: Has no effect on thinking distance

  Speed of car: Greater the speed, the greater the thinking distance.
• Describe what would happen to the current if all components are removed from a junction in a parallel circuit?

  It would be much harder for it to flow as resistance increases as there are now less paths for it to flow
• Correct unit for momentum: kg m/s
• A child drops an egg from a height of 10cm and the egg lands on the floor. Explain why the egg is less likely to break if the floor is covered with a thick carpet than if the floor were covered in hard tiles.?

  • Carpet increases impact time as it reduces rate of change of momentum. Carpet reduces force.
• Callisto has a larger mass than the earth's moon

  Suggest why it has a lower gravitational field strength than the Earth's moon.?

  Callisto has a lower density
• why is the amount of GPE gained by the mass less than the amount of electrical energy transferred to the motor?

  1. Motor is not 100 percent efficient
  2. Some of the Energy was lost as heat
• Describe the energy transfers that take place when they are connected to the power supply


  Its goes from electrical to light energy
• A student feels cold at night and decides to sleep under a thick woolen blanket

  Explain how the woolen blanket helps to keep the student warm

  Heat loss is reduced as air is a good insulator but also the blanket being an insulator helps by trapping the air which means that convection currents are reduced
• Water from a reservoir can be used to generate electricity on a large scale. Describe the energy transfers involved in this process

  The water has an initial GPE then KE of moving water and then electrical power is produced
• Explain how an object becomes charged by friction

  Rubbing two objects together causes a transfer of electrons so then one object becomes negatively charged by gaining electrons and the other becomes positively charged by losing electrons
• Explain why electrical energy is transmitted using high voltages

  • High voltages leads to low current so thinner wires can be used.
  • Less energy is lost from the wires so its more efficient
• Explain why the dc from the generator must be changed to a.c before its transmitted.

  Transformers use alternating current and they change the voltage
• Explain how the electrical energy from a remote wind farm is transmitted to large cities

  At the wind warm : a step up transformer is used

  During transmission: Its transmitted using a high voltage and low current.

  At the city end: the Voltage is reduced to 230V for safety of homes