Why is energy wasted when surfaces rub together in a device?
The surfaces wear down and have to be replaced
The device is not supposed to rub
Internal (thermal) energy of the surfaces increases, and this is then transferred to the surroundings
What makes a device inefficient?
It wastes most of the input energy
It usefully transfers most of the input energy
The total energy input is less than the usefully transferred energy
A motor transfers 40% of the input energy usefully. How much of every 100 J does it waste?
60 J
40 J
0 J
In a hairdryer, what energy is not wasted?
The energy that heats the water on the hair
The energy that dissipates out of the hairdryer into the surroundings
The sound heard as the air blows
What is the efficiency of a kettle that dissipates 90 J of energy to the surroundings when supplied with 630 J?
0.14
0.86
0.90
How might a toy car be made more efficient? - Higher
Push it with a greater force
Press down on it harder
Add a little oil to the moving parts
What is a good thermal conductor?
A material that easily allows thermal energy through
A material that will not allow thermal energy through
A material with a low conductivity
Which is the best insulator?
Copper
Brick
Air
Why is loft insulation so effective?
It traps air, which is a good insulator and reduces the heat lost from the home
It is itchy but reduces noise in the loft
The fibres do not burn easily if there is a fire
What would make the best material for wrapping around a mug to keep a hot drink warm for as long as possible?
Aluminium foil
Bubble wrap
Copper foil