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电力运行我们周围的世界。谢谢to Nikola Tesla's discovery of alternating current (AC) power, homes all around the world receiveElectricity. So then what exactly does electrical power have to do with the electricity that powers our appliances? This article will give you a detailed explanation of the definition of power, its equations, and its properties. We will also study the relationship between power, current, voltage, and other quantities. Happy learning!
Electric power is defined as theelectrical energytransferred in a circuit per unit of time.
The unit of electric power is the Watt (
) and it is denoted by the symbol
. It is oftenmeasured in
.
Thepower ratingthat we see in our home appliances defines how much energy is being transferred from the grid to power the device. A mobile phone charger has a power rating in the range of
.This means that the charger draws
or
Joules per second from the mains. An electric kettle on the other hand has apower ratingof
.That is
Joules per second which is
times the power consumed by the charger! This makes it
times more expensive to use than your typical mobile charger. Let us now look at how to calculate the power using the current drawn and the voltage.
The electrical power used by an electrical component depends on two main factors. These factors are:
passing through the component.
across the two ends of the componentincreasing either one of these variables will increase the power proportionally. This can be formulated as an equation for power in terms of these two variables which we demonstrate in the next section of this explanation.
The electric power transferred to an electrical component in a circuit can be calculated using the electric power formula:
Or in words:
where
is the electrical power,
is thepotential differenceacross the component and
is the current passing through the component.
The electric power can also be calculated by knowing the current and resistance using the following equation
where
is the resistance of the electrical component.
Therefore,
of electric power can be defined as the energy transferred when a current of
flows through apotential differenceof
.
Electrical power, like all other forms of power, is a measure of the time rate at which energy is transferred to or from an object or more generally some physical system. We, therefore, expect electrical power to have units of Joules per second
which is the same the Watt
.
.
As an example, imagine we have a lamp that requires a
incandescent lightbulb. Thepower ratingof
indicates that the total energy used by the lightbulb in one second, in the form of bothlightenergy and wasted thermal energy, is equal to
.
The electric power triangle is an easy way to memorize the above equation. This formula can be rearranged with the help of an Electrical power triangle shown below.
Electrical Power Electric Power triangle, StudySmarter Originals
We can derive the second electrical power formula using Ohm's law. The equation for Ohm's law is given by
We can substitute the value for thepotential differenceinto the equation for power.
or in words
Let us look at a few examples where we calculate power:
A
appliance is supplied by a
supply, calculate thepower ratingof the speaker.
Step 1: List out the given quantities
,
Step 2: Choose the right equation for calculating power
We have the values for current and resistance we can use the following equation
So, the power consumed by the appliance is
.
Calculate thepotential differencethrough anelectric motorwith a current of
and an electric power of
.
Step 1: List out the given quantities
Step 2: Choose the right equation for calculating thepotential difference
We have the values for current and resistance, we can use the following equation
So the potential difference across theelectric motoris
.
Calculate the power transferred when a current of
passes through a conductor of resistance
.
Step 1: List out the given quantities
Step 2: Choose the right equation for calculating power
We have the values for current and resistance we can use the following equation
The power is being transmitted is
.
For a given value of current, the power consumed increases with an increase inpotential difference. The magnitude of power consumed depends on both the current and the potential difference. Therefore electric power can be delivered in the same quantity using different combinations of potential difference and current.
Warning signs outside power stations indicate that the voltages present are dangerous to humans and could cause serious harm in the form of an electric shock, StudySmarter
The disadvantage of using a high current with a lowpotential differenceis the heating effect. When large values of current pass through a wire they heat up to high temperatures which reduces the lifetime of the wires. The heating effect is bad as it reduces the efficiency of the electric device. This is because a part of the energy that is being transferred is being converted into heat. For this reason, High powers across the mains are transmitted in high voltages with low currents.
The heating effect is due to the current passing through a resistor. The heat produced is directly proportional to the resistance of the wire or device. When current passes through a conductor it overcomes the resistance of the wire, thework doneagainst the resistance is converted into heat.
Electric power is defined as the electrical energy transferred in a circuit per unit of time.
The electric power dissipated in a circuit by a particular component can be calculated using the electric power formula: P=IV, where P is the power, V is the potential difference drop due to the component and I is the current passing through the component.
Electricity is generated in a power station by the combustion of fossil fuels. Water is heated until it reaches boiling temperature. The steam produced turns a turbine which powers a generator which produces electricity.
The SI unit of electric power is theWattand it is denoted by the symbolW.
Consider any electrical appliance in your home, if it's using some electrical energy then it has a power output associated with it. Lightbulbs emit energy in the form of light and heat over a period of time, meaning they have a power rating.
If you're working on a laptop, you'll notice that the transformer on your laptop charger (the chunky cuboid part way along the cable) will have an input and output voltage and current. See if you can calculate the power input and power output of your laptop transformer!
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