P5: Electricity In The Home (Y11 - Spring 2)

Question 1

🟢 What is Power (+ What is Electrical Power)

Answer 1

Power is how fast energy stores can be transferred from one form to another.

The rate of energy transfer by each electrical component is called electrical power.

Question 2

🟢 Equation for Power involving Energy and Time

Answer 2

Power (W) = Energy (J) / Time (s)

P = E/t

Question 3

🟢 Example Question:

A kettle is on for 3 minutes and uses 12000J of energy. What is the kettle power rating?

Answer 3

P = ?
E = 12000J
T = 3m = 60x3 = 180 s

P = E /t

12000 / 180 = 66.67W

Question 4

🟢 How can P = E/t be rearranged to get E and t as the subjects of the formula

Answer 4

E = Pt

t = E/P

Question 5

🟢 Example Question: A 40W light bulb is switched on for 30 mins. Calculate the energy it transfers.

Answer 5

P = 40W
t = 30mins 30x60 = 1800

E = Pt
40 x 1800 = 72,000 J

Question 6

🟢 What are the 2 units for power?

Answer 6

Watts, W or Joules per Second

Question 7

🟢 Example Question: A light bulb transfers 24kJ in 10 minutes. Calculate the power of the light bulb.

Answer 7

E = 24,000
t = 10 mins  10 x 60 = 600s

P = E/t
24000 / 600 = 40W

Question 8

🟢 Equation for Power involving Current and Potential Difference

Answer 8

Power Supplied (W) = Current (A) x Potential Difference (V)

P = IV

Question 9

🟢 Example Question:

a) Calculate the normal current through a 500 W, 230V heater
b) Determine which fuse 1 A, 3 A, 5 A, or 13 A you would use for the appliance.

Answer 9

a)
P = 500
V = 230

P = I x V
I = P/V
500/230 = 2.2 A

b)
You would use a 3A fuse because it would not melt when the current is 2.2A, but it would melt if due to a fault, the current exceeded 3A. The 5A and 13A fuses would only melt if the current exceeded 5A and 13A respectively.

Question 10

🟢 Example Question:

Which fuse out of a 1 A, 3 A, 5 A, or 13 A should be used for a 2.5kW kettle, with a 230V supply?

Answer 10

P = IV
I = P/V

2500/230 = 10.9A

So a 13A fuse should be used

Question 11

🟢 Equation for Power involving Current and Resistance

Answer 11

Power (W) = Current^2 (A) x Resistance (Ω)

P = I^2 x R

Question 12

🟢 How can the Equation for Power involving Current and Resistance be rearranged to make Current and Resistance the subject.

Answer 12

Current (A) = (square root) Power (W) / Resistance (Ω) (square root)

        /---------- I =       /      P
     /   ----------
  \/         R

Resistance (Ω) = Power (W) / Current^2 (A)

R = P / I^2

Question 13

🟢 Example Question:

Calculate the power of an electrical device using a current of 10 A and having resistance 12 Ω.

Answer 13

P = I^2 x R

10^2 x 12 = 1200W

Question 14

🟢 Example Question:

Calculate the current flowing through a device with a power of 360 W and 10 Ω resistance

Answer 14

.
             /----------
I =       /      P
         /   ----------
      V         R

         /----------
       /    360
     /   ----------     =    6
  V         10

Question 15

🟢 Example Question:

What resistance does a 25 W light bulb have if 12.6 V is applied to it?

Answer 15

P = 25W V = 12.6V

P = I x V     
I = P / V

25 / 12.6 = 1.984126

V = I x R
R = VI

12.6 / 1.984126 = 6.35 Ω

Question 16

🟢 Example Question:

A computer uses 3.5 A at 110 V. What is its resistance in ohms?

Answer 16

I = 3.5 A V = 110V

V = I x R
R = V / I

110 / 3.5 = 31.4 Ω

Question 17

🟢 Example Question:

A device has a resistance of 11 Ω and a current of 12 A. What is it power and potential difference?

Answer 17

R = 11Ω I = 12A

V = I x R
11 x 12 = 132V (potential difference)

P = I x V
12 x 132 = 1584 W (power)

Question 18

🟢 What is charge flow?

Answer 18

● In a circuit, electrons are forced through components by the potential difference of the battery or cell
● Charge is carried through the circuit by the electrons
● have a negative charge (-)

Question 19

🟠 The equation that relates Charge, Current and time

Answer 19

Charge Flow (Q) = Current (I) x Time (t)

Question 20

🟢 A wind up radio uses two rechargeable cells, each capable of storing 1800 coulombs of charge.

a) For how long can the radio be used without
recharging the cells if the radio uses 0.5A of
current?

b) If the owner recharges the cells for 5
minutes, how much charge will be stored in the cells if the total current flowing to the cells is 1.5A?

Answer 20

a)
Two cells with 1800C each

2 x 1800 = 3600
t = Q/I 3600/0.5 = 7200s

7200s = 120 mins = 2hrs

b)
5 x 60 = 300
1.5 x 300 = 450C

Question 21

🟢 1 Coulomb of charge is equal to 6.242x10^18 protons and 6.242x10^18 electrons

Answer 21

1 coulomb of charge is equal to 6.242x10^18 protons, and 6.242x10^18 electrons. (Q = It)

Question 22

🟢 How to Resistors get hotter

Answer 22

Electrons flowing through the resistor constantly collide with the vibrating metal ions in the resistor, transferring energy to metal ions.

The metal ions gain energy to their kinetic stores and vibrate more, this cause the resistor to get hotter.

Question 23

🟢 What can affect how hot the resistor gets?

Answer 23

When a resistor is placed under a voltage that approaches the upper limits of its power rating, the resistor generates more heat than normal. This is due to the voltage attempting to force more current (electrons) through the resistor than it is designed to pass.

Question 24

🟢 A circuit with a 9 V battery set-up a 6 V potential difference across the bulb. What is the potential difference across the variable resistor?

Answer 24

3V as potential difference is shared across components in series.

Question 25

🟢 Describe the energy store changes in this circuit?

Answer 25

The chemical potential energy store of the battery decreases as it transfers energy by electric current increasing the thermal energy store of the bulb and variable resistor. Energy is transferred to the surroundings by heating and as light waves.

Question 26

🟠 Equation for Energy Transferred using both Potential Difference and Charge

Answer 26

Energy Transferred (J) = Charge (C) x Potential Difference (V)

E = Q x V

Question 27

🟢 The charge that flows through an electrical device in 180 seconds is 5400 C. The new electric shower has a power of 5 kW.

Calculate the resistance of the heating element in the new shower.

Write down any equations you use.

Answer 27

5400 = I x 180 
I = 5400/180 = 30
p = R x I^2     

5000 = R x 30^2 R = 5000/900 = 5.5

Answer: 5.5 Ω

Question 28

🟢 A 120 W electric blanket is left on for 8 hours?

a) How many joules of electrical energy are transferred?
b) How many kilowatt hours of electrical energy are transferred?
c) If the unit cost of electricity is 8 pence, what is the cost (to the nearest penny) of using the electric blanket each evening?

Answer 28

a)
P = E/t      P = P x t
120 x (8 x 60 x 60) = 3,456,000
3,456,000 = 3456kJ

b)
120W = 0.120 kW
0.120 x 8 = 0.96kW/h

c) 0.96kW/h x 8 = 7.68p

Question 29

🟢 What is efficiency?

Answer 29

Energy efficiency is about getting as much useful energy output as possible from a device.It is the ratio of the useful work performance of a machine

Question 30

🟢 How can you Calculate Efficiency

Answer 30

The energy efficiency of a device can be calculated using this formula:

Energy Efficiency = Useful Output Energy / Total Input Energy

Question 31

🟢 What is Useful Energy, Total Energy, and Energy Efficiency measured in

Answer 31

● Useful energy is measured in joules (J).

● Total energy is measured in joules (J).

● Energy efficiency does not have any units.
It is a number between 0 and 1 which can be converted into a percentage by multiplying by 100.

Question 32

🟢 Why can’t a device be more than 100% efficient

Answer 32

No device can be more than 100% efficient because you cannot create energy.

You cannot get more energy out of a machine than you put into it.

Question 33

🟢 Calculate the energy efficiency for a radio with a total input of 300J and a useful output of 130

Answer 33

(130/300) x 100 = 43.3%

Question 34

🟢 Calculate the efficiency of a heater which transfers 500J of sound energy out of a possible 830J

Answer 34

830 - 500 = 330

(330/830) x 100 = 39.76%

Question 35

🟢 What energy transfer occurs in a solar panel? Do you think solar power would be worth investing in globally? Justify your answer.

Answer 35

Energy from the sunlight is transferred to the electrical energy store of the solar panel, however some energy is also transferred to the thermal energy store and some by sound. Overall, solar panels are quite efficient as they are renewable energy resources, but have quite high installation costs, meaning it should not be installed globally.

Question 36

🟢 State 2 drawbacks to using solar power in the UK

Answer 36

• The UK doesn’t usually have clear and sunny skies.

- The UK has shorter days than other countries, or The panels will be less efficient in the UK.

Question 37

🟢 A filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy

a) How much energy is wasted?
b) In what form is the energy wasted?
c) What is the efficiency of the bulb?

Answer 37

a) 55J
b) Through heat into the thermal energy store.
c) 45%

Question 38

🟢 A radio is supplied with 300 J of electrical energy which it converts to 96J of sound energy.

a) How much energy is wasted?
b) In what form is the energy wasted?
c) What is the efficiency of the radio?

Answer 38

a) 204 J
b) Through heating and thermal energy.
c) 32%

Question 39

🟢 This television converts 2 000 J of electrical energy into useful energy at an efficiency of 65%.

a) What useful energy does a television produce?
b) How much useful energy is produced?

Answer 39

a) Sound and light energy

b) 1300J of useful energy.

Question 40

🟢 What are the two Equations for Efficiency

one involving Useful Energy Output and Energy Output, and the other involving Useful Power Output and Power Input

Answer 40

Efficiency = (Useful Energy Output / Energy Output) x 100

The same calculation can be done with the power delivered instead.

Efficiency = (Useful Power Output / Power Input) x 100

Question 41

🟢 What is Efficiency/What does Efficiency usually refer to

Answer 41

Efficiency usually refers to the amount of useful energy that an appliance transfers.

Question 42

🟢 How to Calculate Useful Power (+Equation)

Answer 42

To calculate the useful power just multiply the efficiency by the input power:

Useful power = Efficiency x Input power

Question 43

🟢 Example Question:

A television has an input power of 2000W and an efficiency of 75%. What is the useful power?

Answer 43

2000 /100 = 20

20 x 75 = 1500W

Question 44

🟠 What are Step-up Transformers and what do they do

Answer 44

Step-up transformers are used at power stations to transfer electricity to the National Grid. These transformers are used to make the size or alternating potential difference much bigger (typically from about 25,000V to about 132,009V

Question 45

🟠 What are Step-down Transformers and what do they do

Answer 45

Step-down transformers are used to supply electricity from the National Grid to consumers. Homes and offices in the UK are supplied with mains electricity that provides the same power as a 230V direct-current supply.

Factories use much more power than homes, so they are supplied with a p.d of 100kV or 33kV.

Question 46

🟠 How is the National Grid efficient by making the potential difference very large

Answer 46

By making the grid potential difference very large, much less current is needed to transfer the same amount of power. So the power loss due to the resistance heading in the cables is much reduced. This means that the National Grid is an efficient way to transfer power.