Electricity Flashcards
(1) State the difference between the currents which makes one alternating and the
other direct.
alternating current can take positive
and negative values RA (1)
(6) power station, transformer R, transmission lines, transformer S, houses
Transformers R and S are not 100% efficient.
By using transformers, energy losses in the transmission lines are reduced.
Explain how this reduction is achieved, even though some energy is wasted in the
transformers themselves.
Basic ideas
• transmission lines have resistance
• the current warms the transmission wires
• energy is wasted as heat
• transformers change voltage and/or current
• R increase the voltage / RA for S
• R decrease the current / RA for S
Linked ideas
• the long transmission lines have high resistance
• power depends on both current and voltage
• power = current × voltage (P = I × V)
• at high voltage, the same power needs less current
• correct mention of turns ratio related to voltage change
• a smaller current in a wire produces less heat
• high voltage transmission saves more energy than is lost in the
transformers
(3) The students use the electric motor to lift a weight.
The current in the motor is 0.5 A.
The potential difference (voltage) across the motor is 6 V. Calculate the input power to the motor.
State the unit
substitution (1)
0.5 × 6.0
evaluation (1)
3
unit (1)
W / watts
(2) Describe the difference between direct current and alternating current.
direct current (the flow of
charge) is only in one
direction (1)
• alternating current (the
flow of charge periodically)
{changes / reverses}
{direction / eq} (1)(1) The output from the solar panel is 60 V.
State why a transformer cannot be used to increase this voltage
transformers only change
alternating {voltages /
currents}
• transformers will not work
with direct current(2) explain why using solar panels to generate electricity for the National Grid benifits the environment
reduction of fossil fuels burnt (1) • less reliance on fossil fuels (1) • reduction of greenhouse gases / pollution/global warming (1) • increased use of renewable energy source (1) • less use of non-renewable energy source (1) • reduce need for additional power station building (1) • reduction of negative impact of specified type of power station (1)
(1) State one disadvantage of using the wind to generate electrical power.
unreliability (1) e.g. wind does not always blow / wind speed may be too high/too low • pollution (1) e.g. noise from wind turbines / wind turbines spoil the view
(6) The new transmission line will cross more than 200 km of mountains.
The cables will hang 50 m above the ground from 600 new, taller pylons.
Eventually, about 1000 of the old, shorter pylons will be removed.
Discuss the advantages and disadvantages of this plan.
Social factors / economic factors
• people may not like it (NIMBY) / pressure groups
• cost arguments
Environmental factors
• spoiled view / risk of birdstrike
• space for extra infrastructure eg. access roads / substations
Associated hazards
• danger from higher voltage
• dangers from construction work in mountainous area
• danger to maintenance crew from working at greater height
Energy efficiency arguments
• higher voltage leads to lower current
• lower current means reduced heat losses
• higher voltage means / lower current / can transmit energy
further
• reduced heat loss means improved efficiency
Logical use of data
• taller pylons can be seen from further away
• net reduction in number of pylons / need to remove old ones
• stronger materials needed for pylons / cables
• need for new transformers
Appropriate calculations
• 1000 - 600 = 400 fewer pylons (approx)
• current reduced by a factor of 132/400 (0.33)
(1) Sketch an alternating current on the axes shown. y= current, x=time
Any continuous line which has a
section above and below the time
axis without going (deliberately)
back in time
(3) Explain how transformers are used to improve the efficiency of power
transmission in the National Grid.
step-up transformer(s) (1)
• increase voltages (1)
• (this) reduces the current
(1)
• (which) reduces the {heat /
thermal} {energy /
power} losses (1)(2) Explain why flying a kite near power lines could be a danger to the person
flying the kite.
{kite / string} touching the
power line (1)
• {movement of charge /
current} (1)
• (electricity) {to earth /
through the kite-flyer} (1)
• giving (the kite-flyer) an
electric shock (1)
