Do you know why your mum or dad is constantly reminding you to turn the lights off when you leave your room? It might be to reduce your electricity bill, but have you wondered how the electricity department decides the amount you pay every month? The electrical appliances that we use in our everyday lives draw a current from the national grid when they are operated that depends on their power rating. This is an energy transfer taking place from the grid to power the devices at home. For example, an electric kettle converts electrical energy into heat energy that heats up the water for our morning coffee or tea.
In this article, we will discuss how electrical energy is transferred between appliances and the factors it depends on. We will also look into how the power consumed by an electrical appliance is calculated and work on a few examples. You will also be able to make informed decisions when you're out shopping for any new electrical device. So keep reading till the end to find out - oh, and don't forget to turn your lights off!
Power ratings definition
Power ratings are what we see on our home appliances, defining how much energy is being transferred from the mains to power the device. The power rating helps consumers choose between different appliances based on their power consumption. It also highlights the maximum power at which the appliance can safely operate, which the cable and plug also need to be able to handle.
A mobile phone charger typically has a power rating in the range of 5-25 watts. This means that the charger draws a maximum of 25 watts or 25 joules-per-second from the mains electricity supply. An electric kettle on the other hand has a power rating of 3 kilowatts. That is 3000 joules per second, which is 120 times the power consumed by the charger! This means that with the same amount of energy, you could boil a kettle for 1 minute or charge your phone for 2 hours (120 minutes)! Let us now look at how to calculate the power using the current drawn and the voltage. Now let us look at different equations to calculate the amount of power consumed by appliances using the current drawn and the voltage.
Not all the electrical energy transferred into an appliance is converted into useful work. A part of the energy is almost always converted into heat energy or some other form of waste in electrical devices. The efficiency of the appliance tells us how much of the input energy is converted into useful work. We'll speak more about the term efficiency and what it means in a later section.
Power rating formula
The Electric power rating formula in a circuit can be calculated using the formula:
or in words:
Whereis the power in watts,is the potential difference in voltsbetween the points of energy transfer andis the current in amperes. Therefore, 1 watt of electric power can be defined as the energy transferred when a current offlows through a potential difference of.
There is also another method to calculate the power rating of an appliance. It can also be calculated using the work done or energy transferred by an appliance in a given amount of time.
or in words,
Hereis the work done orthe energy transferred in joules, andis the time in seconds. The power rating unit is watts. For appliances that consume higher values of power, we use kilowattsor megawatts.
From the above equations, we can see that the power consumed by an appliance depends on the total energy transferred and the time the appliance is on for. The above equation can be re-arranged to obtain the energy consumed by an appliance. Another way to calculate the rate of energy transfer to an appliance is by measuring how many coulombs of charge flow through a given potential difference. This is given by:
or in words,
whereis the charge in coulombsandis the potential difference in volts. Now lets look at the efficiency; this will not only help you in your GCSEs but also when you're out shopping for any new electrical appliance.
Power Rating of appliances
This is an efficiency label that you'll find on devices. The different coloured bands allow you to compare which device is more efficient in operating. Wikimedia.
When you turn on an electric device it is designed to convert the electricity into whatever useful work it was intended to perform. During this conversion, some energy is always lost, typically as heat or noise. An efficient device is one that minimizes this energy loss. So if we have two devices with the same power consumption rating, examining their efficiencies tells you how much of the power consumed is being converted into useful work. Efficiency can be calculated as follows.
It can also be calculated as
Calculating efficiency will give you a decimal value less than or equal to one - a useful way to represent this is by using a percentage. When you multiply the efficiency with, we get what is called a percentage efficiency. A theoretical device withefficiency converts all the supplied power into useful power. A percentage efficiency ofmeans that the device is only convertingof the supplied power into useful power or work.
A well-known example of an inefficient device is a filament lightbulb - these are designed to produce light, but over 95% of the input energy is converted into waste heat instead!
Now let's work on a few examples to practice what we just learned.
Power rating examples
Akettle can boils a litre of water in; how long would it take aphone charger to transfer the same amount of energy as the kettle?
Step 1: List out the given values
Step 2: Convert quantities
Step 3: Calculate the energy transferred to the kettle by rearranging the equation for power.
Step 4: Calculate the time taken for a phone charger to transfer the same energy as the Kettle.
,
rearrange this equation for time.
Finally, converting the time in seconds to minutes:
The charger will taketo transfer the same amount of energy transferred by the kettle in
Calculate the amount of energy transferred if an electric bulb connected to a supply ofhasof charge pass through it?
Step 1: List out the given values
Step 2: Calculate energy transferred using the right equation
The energy transferred by moving a charge across a potential difference ofis.
Power rating of the resistor
The power rating of a resistor gives the maximum amount of power it can dissipate before it will fail and break the circuit.
Different resistors have different power ratings, which define the maximum power that can pass through the resistor without damaging it.
Every resistor comes with a specific power rating. A resistor gets heated up as it obstructs the flow of current through it. Thus if a resistor has a maximum power rating, it is to prevent it from heating beyond the limit of how much heat it can dissipate. The power rating of the resistor is usually measured in watts.
Power Rating - Key takeaways
- The power rating that we see in our home appliances defines how much electrical energy is being transferred from the national to power the device.
- Not all the energy transferred into an appliance is converted into useful work.
- Electric power or the electric energy transferred in a circuit can be calculated using the electric power formula
- It can also be calculated using the work done or energy transferred by an appliance in a given amount of time
- The power rating symbol is represented by the same symbol for power, W.
- The power rating of a resistor gives the maximum amount of power it can dissipate without failing and breaking the circuit.
Explanations 
Textbooks 
Exams 
Magazine 
