Module 4 - Section 1 - Electrical Energy And Power

Question 1

What is power and what is it measured in ?
Whats the equation for power ?

Answer 1

Power is the rate of energy transfer - or it is defined as the rate of doing work

Power is measured in Watts ( W ) - where 1W is equal to 1J of work per second

P = W / t

Question 2

What’s the equation for power in electrical circuits ?

Answer 2

P = VI

This makes sense because Voltage is the work done per unit charge - and current in the rate of flow of electrons so - pd x current = work done per second = power

You also have different equations for power
P = V2 / R P = I2 x R

Question 3

Why does the W=VIt equation work ?

Answer 3

Because W=Pt and P=VI —> W=VIt or V2 / R x t or W=I2xRxt

Question 4

What do electric companies charge their customers with ?
And how do you calculate how much work and appliance does ?

Answer 4

Electric companies don’t use watts or joules - they use ‘units’ of electricity - another name for a unit is kilowatt-hour - if you know the power of the appliance and the amount of time that its been on then you can find the work it does in kWh

Work done = Power x time
kWh kW h

Question 5

Why do we use kWh instead of joules ?

Answer 5

Well because J is such a small amount of energy compared to the amount of energy a household uses - for example if you use a hairdryer with a power of 1500W for 10mins you get 900000J of energy used and that’s just impractical - in kWh - that’s 0.25kWh

Question 6

Whats 1 kW in W

Answer 6

1kW = 1000W

Question 7

Whats 1 hour in seconds ?

Answer 7

3600s

Question 8

How do you find out the cost of electricity ?

Answer 8

You need to know how much units your have used and the price of each unit

Cost = No. of units x Price per unit

Question 9

What are the environmental impacts of electricity ?

Answer 9

• burning fossil fuels releases carbon dioxide into the atmosphere which adds to the greenhouse house gas effect and contributes to global warming
• Nuclear power stations produce radioactive waste which can be dangerous to the environment and human health if it isn’t disposed of safely
• Even renewable energy sources like biofuels have an impact because lots of lands is need to produce them that means that lots of wildlife habitats are ruined and also this means lots of deforestation

Question 10

What are some energy saving devices ?

Answer 10

• Energy saving light bulbs are more expensive than traditional light bulbs but are about 4x more efficient and also last for about 10 years rather than the traditional filament lamp lasting a year - this reduces the amount of lightbulbs going into landfill and reduces energy wasted by creating new bulbs
• LED light bulbs are even m,ore efficient that energy saving light bulbs - last even longer - but cost more
• Domestic appliances all have an energy rating and this helps the customer choose the appliance that’s most efficient to run - which can reduce their environmental impact

Question 11

What is efficiency ?

Answer 11

Efficiency = useful energy output / total energy output x 100