Energy transfer by appliances: power, everyday appliances, static electricity, electric fields

Question 1

what is the energy transfer in blenders and fans

Answer 1

electrical energy to kinetic energy of the electric motors
blender moves to mix food
fan moves to create air currents
small amount of electrical energy is transferred to thermal energy due to friction in the motors

Question 2

what is the energy transfer in the iron and the kettle

Answer 2

both appliances transfer electrical energy into thermal energy

Question 3

some appliances are designed to transfer electrical energy into both thermal energy and kinetic energy

Answer 3

-hairdryer
-washing machine
both of these appliances use electrical energy both of them transfer the electrical energy into thermal energy and kinetic energy of the electric motors
a motor moves air through the hairdryer and heating element heats the air
washing machine- heating element heats the water and a motor turns the drum to wash the clothes

Question 4

what is power definition

Answer 4

power is the rate at which energy is transferred 1 watt is an energy transfer of 1J per second
for example a blender has a power rating of 400w meaning it transfers 400 joules of energy per second

Question 5

A kettle has a power rating of 2200 watts and is used for 80 seconds. calculate the total energy transferred

Answer 5

E= power (watts) x time (seconds)
2200x80= 176000J
176KJ

Question 6

A fan has a power rating of 20 watts and is used for 1 hour. calculate total energy transferred

Answer 6

energy= power (watts) x time (seconds)
20 x 3600=72000
72KJ

Question 7

An iron is used for five minutes and a total of 600,000 joules of energy is transferred. calculate the power of the iron

Answer 7

power (watts)= energy (joules) / time (seconds)

600,000 divided by 300= 2000 watts

Question 8

A blender has a power of 400 watts and a total of 48,000 joules of energy is transferred when the blender is in use. calculate the time the blender is used for.

Answer 8

time ( seconds)= energy (joules) divided by power (watts)

48000/400= 120 seconds

Question 9

power is the rate at which energy is transferred 1 watt (W) is an energy transfer of 1 joule per second
if we know both the potential difference across the component and the current flowing through it then we can calculate the power using:

Answer 9

power (watts)= potential difference (volts) x current (amps)

Question 10

calculate the power of the resistor p.d=50v I=0.2A

Answer 10

power (watts)= current (amps) x potential difference (volts)

50x0.2= 10 watts

Question 11

calculate the power of resistor A
current= 0.5 A
potential difference= 5v

Answer 11

power(watts)= current (ampheres) x voltage (volts)

0.5x5=2.5 watts

Question 12

if we know the current flowing through a component and the resistance then we can use this equation to calculate power:

Answer 12

power (watts)= current (amps)^2 x resistance ( ohms)

Question 13

calculate the power of this resistor resistance= 50 ohms current=0.4 A

Answer 13

power (watts)= current (amps)^2 x resistance (ohms)
0.4^2 x 50
8 watts

Question 14

national grid what is it

Answer 14

the national grid is the electricity distribution network- its a system of transformers and high voltage cables which connect power stations to consumers.

Question 15

whats the problem with the national grid

Answer 15

energy is always lost in the power cables due to the resistance of the wires we can reduce energy lost by building power stations near homes.

Question 16

what are step up transformers

Answer 16

electricity passes through step up transformers which increase the voltage to reduce the size of the current to reduce the amount of energy lost in the wires
so less energy is lost in the cables when the potential difference is very high

Question 17

what are step down transformers

Answer 17

step down transformers reduce the voltage to 230 volts

reduce voltage back to safe levels for households and buildings

Question 18

definition of current

Answer 18

rate of flow of electric charge

Question 19

potential difference definition

Answer 19

energy transferred per coulumb of charge

Question 20

describe action and purpose of a solar panel

Answer 20

• heats water using radiation from the sun

- water used to heat buildings provide hot water

Question 21

In the northern hemisphere, the modules should always face south for the
maximum transfer of energy.
State one other factor that would affect the amount of energy transferred during
daylight hours.

Answer 21

angle of tilt of the cells

Question 22

The temperature of the air inside the freezer compartment is –5 °C.
The air inside the fridge forms a convection current when the fridge door is closed.
Explain why.

Answer 22

air at the top is cold
particles are moving slowly they come together or are closer together so density increases, air is more dense
so cold air falls
and warm air rises from the bottom

Question 23

Give one advantage and one disadvantage of replacing an old fridge with a new fridge

Answer 23

advantage- new fridge is more efficient, wastes less energy

disadvantage- it may be difficult to dispose the old fridge

Question 24

when is work done

Answer 24

when a charge flows in a circuit

Question 25

what are convection currents

Answer 25

currents of moving liquids transferring energy

Question 26

static electricity using the concept of electric fields explain how a build up of static electricity can cause a spark

Answer 26

sparks are caused when high potential difference between charged object and the earth or earthed object. a high potential difference causes a strong electric field between charged object and earthed object a strong electric field causes electrons from air particles to be removed (ionisation) air is normally an insulator but when it is ionised it is much more conductive so a current flows through it. this is the spark.

Question 27

jake removes his jumper in a dark room, as he does so he hears a crackling noise and sees tiny sparks of light between his jumper and his shirt. explain the cause of this

Answer 27

as the jumper rubs against the shirt, a static charge builds up on both the jumper and the shirt, this is due to electrons being removed from one and depositied on another. charge becomes large enough for electrons to jump across a small gap between the jumper and his shirt, causing a spark

Question 28

explain why national grid is efficient at transferring energy refer to potential difference and current during transmission

Answer 28

the national grid is efficient at transferring energy because it has a step up and step down transformer. the step up transformer increases the potential difference and reduces the size of the current so that less energy is lost in the wires due to resistance so less energy is transferred to the thermal energy store of the surroundings. the step down transformer, decreases the potential difference back to save levels for households and buildings.

Question 29

why does the national grid distribute electricity at a high p,d and a low current

Answer 29

because a high potential difference means more power distributed per second. power is the rate at which energy is transferred so a higher power means more energy is transferred.

Question 30

when is work done in appliances

Answer 30

work is done when a charge flows in a circuit