Unit 4

Question 1

What is a refrigerant?

Answer 1

A liquid that evaporates at a very low temperature.

Question 2

How is the temperature of the refrigerant in a fridge lowered?

Answer 2

Heat dissipates from the warm refrigerant in the coils on the outside of the refrigerator and the gas becomes a liquid. The liquid is forced through an expansion valve which vaporizes the gas and lowers the pressure and temperature of the refrigerant.

Question 3

How is the temperature of the refrigerant in a fridge increased?

Answer 3

Gaseous refrigerant absorbs heat as it moves through the coils on the inside of the refrigerator. Warm gas passes through the compressor, so its pressure and temperature increase.

Question 4

What is ‘work’?

Answer 4

The force to cause an object to move.

Question 5

What is the equation for work?

Answer 5

W = ⃗ F x ∆ ⃗d

Question 6

If all vector quantities in an equation are going in the same direction, what can be

Answer 6

The arrows above the symbols.

Question 7

What is the unit of work?

Answer 7

Joule (J) [(kg x m^2)/s^2]

Question 8

What is another name for a Joule?

Answer 8

Newton-metre

Question 9

What are the units of force?

Answer 9

(kg x m) / s^2

Question 10

Doing 2 _ of work involves exerting a force of 1 _ across a displacement of 2, or exerting a force of 2 _ across a displacement of 1.

Answer 10

Joule
Newton
Metre

Question 11

If a force is perpendicular to the motion, is work being done?

Answer 11

No

Question 12

Can energy disappear or get used up? If not, what happens?

Answer 12

No, it is transferred

Question 13

What is the equation of change in energy of an object on which work is done?

Answer 13

W=∆E

Question 14

If a force is acting in the opposite direction to the displacement of an object, is it positive or negative in the work equation?

Answer 14

Negative

Question 15

Which of the following results in work and which results in no work being done.

a) a crane lifts up a load of cement
b) Ali holds a 10kg bag of sand for 15 minutes
c) Jenny pushes a snow shovel for 10s
d) an archer pulls back on a bow, causing it to bend
e) George pushes with a force of 500N on a car stuck in a snowbank, but it doesn’t move.

Answer 15

a) work
b) no work (perpendicular)
c) work
d) work
e) no work (perpendicular)

Question 16

Calculate the vertical displacement of an elevator that does 1.05 10^5J of work with an upward force of 7.0 x 10^3N

Answer 16

W = F x ∆d
1.05 x 10^5J = 7.0 x 10^3N x ∆d
∆d = 7.0 x 10^3 (kg x m x s) / 1.05 x 10^5 (kg x m^2 x s)
∆d = 15m

Question 17

Calculate the work done on a car that moves 300m at constant velocity when there is a frictional force of 400N

Answer 17

W = F x ∆d
W = 400N [backwards] x 300m [forwards]
W = -400(kg x m x s) x 300m 
W = -1.2 x 10^5 J

Question 18

Calculate the work done when a 12kg pail of water is lifted up 3.2m from the bottom of a well.

Answer 18

W = F x ∆d
F = m x acceleration
gravity = acceleration
W = m x g x ∆h
W = 12kg x (9.81m/s^2) x 3.2m
W = 380J

Question 19

What is ‘power’?

Answer 19

The rate at which work can be done.

Question 20

What are the units for power?

Answer 20

watts (W) [The symbol is the same as work, but work’s symbol is italicised]

Question 21

What is the symbol for work?

Answer 21

W [the symbol is the same as the symbol for watts, but italicised]

Question 22

What is a power output of 1 watt?

Answer 22

1 joule of work can be done every second.

Question 23

What is 1kW

Answer 23

1000 watts

Question 24

What is the equation for power?

Answer 24

P = W/∆t

Question 25

Power is always considered to be a __________ quantity.

Answer 25

positive

Question 26

What is a kilowatt hour?

Answer 26

1000 joules of work every second, over a time period of 3600 seconds 1kW/1h

Question 27

Calculate the work done by a car motor when it develops 6.5 x 10^4 W of power in 7.0s, climbing a hill.

Answer 27

``` W = P x ∆t W = 6.5 x10^4W x 7.0s W = 4.6 x 10^5 J ```

Question 28

Calculate the power developed when a crane causes the energy of a steel beam to change from 1.4 x 10^4 J to 3.7 x 10^5 J in 2.5 min

Answer 28

``` P = W/∆t P = (3.7 x10^5J - 1.4x10^4J)/(2.5min x 60s/min) P = 2.4 x10^3 W ```

Question 29

How long does it take a 1.5kW motor to do 4.0 x10^4J of work?

Answer 29

``` P = W/∆t ∆t = W/P ∆t = 4.0 x10^4J/1.5 x10^3W ∆t = 27s ```

Question 30

How much energy is transferred (from electrical energy to light and heat energy) by a 13W CFL bulb in 1.0 hours?

Answer 30

``` W = P x ∆t W = 13W x 1.0h x 60min/h x 60s/min W = 4.7 x10^4 J ```

Question 31

Andrew can develop 9.84 x10^2W of power as he runs up the stairs. He has a mass of 104kg and it took him 3.78s to run up the stairs. How high was the flight of stairs?

Answer 31

``` W = F x ∆d W = (m x g) x ∆h ∆h = W/ m x g ∆h = (9.84 x10^2W x 3.78s) / 104kg x 9.81 m/s^2 ∆h = 3.65m ```

Question 32

A curling stone with a mass of 18kg accelerates from a position of rest to 8.2m/s in 4.0s. a) stone acceleration b) stone displacement in time c) work done on the stone d) power curler develops to cause the motion of the stone

Answer 32

a) ⃗a = ⃗∆v / ∆t ⃗a = (8.2m/s [forward] - 0m/s)/4.0s ⃗a = 2.0m/s^2 [forward] b) ∆d = 1/2 a x ∆t ∆d = 0.5 x 2.0m/s^2 [forward] x (4.0s)^2 ∆d = 16m [forward] c) W = ⃗f x ⃗∆d W = (m x ⃗a) x ⃗∆d W = 18kg x 2.0m/s^2 [forward] x 16m [forward] W = 580J d) P = W/∆t P = 580J / 4.0s P = 145W

Question 33

What is an EnerGuide label?

Answer 33

Label attached to an appliance to indicate facts about the appliance's energy performance and to confirm that the appliance meets Canada's minimum energy-efficiency standards.

Question 34

How is the yearly running cost of an appliance calculated?

Answer 34

Y = ∆E x c | Yearly running cost = EnerGuide energy per year rating x cost of electricity per kWh

Question 35

What is the equation for acceleration

Answer 35

⃗a = ⃗∆v / ∆t