P7.2 Flashcards

You may prefer our related Brainscape-certified flashcards:
1
Q

what is the equation to calculate efficiency?

A

efficiency = useful output energy transfer (J) / total energy (J) x 100

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q
define power
(and what is the main equation)
A

the rate of energy transfer

p = e/t
energy / time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

what is the relation between 1 watt and 1 joule?

A

1 watt is 1 joule of energy transferred in 1 second

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

what unit do electrical companies measure energy in?

A

kWh
(kilowatt-hour)

as e = p x t
(energy = power x time)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

is kilowatt-hour a measure of energy or power?

A

measure of energy

as e = p x t

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

1kWh is the amount of ……… transferred by a ….. applicance in …… hour

A

energy transferred, 1kW, 1 hour

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

3 advantages of electrical energy?

A

1 - easily transferred into other forms
2- easily transferred over long distances
3 - is portable

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

downside about an electrical current flowing?

A

some energy will always be transferred as heat to the surroundings

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

when will thermal energy transfer always occur?

A

when there exists a temperature difference (or energy difference) between two points/media

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

define a system

A

a group of bodies interacting

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

when would the energy of a system stay constant?

A

when the system is closed (no external forces acting on it)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

two ways to reduce unwanted energy transfer

A

lubrication, thermal insulation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

how does insulation reduce unwanted energy transfer?

A

traps air and stops it moving

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

what are the 4 factors which affect the rate at which heat energy is transferred through a wall?

A
  • temperature difference
  • area of wall
  • thickness of wall
  • material of wall (thermal conductivity)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

how does the thermal conductivity of a wall affect the rate of heat energy transfer?

A

lower the thermal conductivity = lower rate of heat energy transfer

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

define thermal conductivity

A

the heat energy transferred, per second, through a 1m thick wall (of area 1m^2) when a temperature difference of 1C exists across it

17
Q

what is the unit for thermal conductivity?

A

W/mK

18
Q

what most common form of energy waste happens in mechanical devices?

A

through form of heat

(force of friction) which acts between parts/surfaces

19
Q

what do Sankey diagrams do?

A

show energy transfer through a device (by separating useful and wasted energy)

20
Q

for the following methods of wasting energy suggest how to reduce the problem

a) friction in moving parts = heating
b) resistance of wire = wire gets hot
c) air resistance = energy to surroundings
d) sound created by machinery

A

a) lubricate
b) wires with little resistance
c) streamline shape of moving objects
d) cut out noise (tighten loose parts to reduce vibration)

21
Q

how does lubrication work?

A

a layer of fluid between two solids

- surfaces no longer in direct contact, friction reduced