# Power

## Power Revision

**Power and Energy** Transfer

**Energy transfers** occur in electrical appliances. The amount of energy transferred depends on the **power** of the appliance and **how long **it is switched on for. Different appliances have different **power ratings**, which are the maximum power the appliance may be operated at.

**Transferring Energy**

Appliances transfer **electrical energy** to **useful energy**. For example, a fan transfers electrical energy to kinetic energy of the fan and an oven converts electrical energy to thermal energy to heat up your food.

The amount of energy transferred in an appliance can be calculated using the following equation:

\textcolor{aa57ff}{E=Pt}

- E= energy in joules \text{(J)}
- P= power in watts \text{(W)}
- t= time in seconds \text{(s)}

This equation tells us that more energy is transferred if the appliance has a higher **power rating** or if it is switched on for a longer** time**.

**Power rating **is the maximum power an appliance can safely operate at. This means the power rating of an appliance tells us the **maximum energy transferred** per second by an appliance. However, having a higher power rating does not mean that more **useful energy** is transferred. A high power rating appliance may not be as **efficient** as a low power rated appliance.

Energy transferred can also be calculated by working out how much work has been done by the **charge carriers** that have flowed through the circuit across a potential difference:

\textcolor{aa57ff}{E=QV}

- E= energy in joules \text{(J)}
- Q= charge in coulombs \text{(C)}
- V= potential difference in volts \text{(V)}.

**Calculating Power**

Recall that **power** can be calculated using the equation:

P=VI

- P= power in watts \text{(W)}
- V= potential difference in volts \text{(V)}
- I= current in amps \text{(A)}.

If you don’t know the potential difference, you can also calculate power using:

P=I^2R

- P= power in watts \text{(W)}
- I= current in amps \text{(A)}
- R= resistance in ohms (\Omega)

**Example: Calculating Charge Flow**

A lightbulb of power 15\text{ W} is switched on for 2\text{ minutes}. If the potential difference across the lightbulb is 10\text{ V}, how much charge has flowed through the bulb?

**[3 marks]**

Rearrange for Q:

\text{Q}=\dfrac{\text{E}}{\text{V}}=\dfrac{1800\text{ J}}{\textcolor{7cb447}{10\text{ V}}}=\bold{180\text{ C}}

## Power Example Questions

**Question 1:** Describe the energy transfer that heats up the air around an electric heater.

**[2 marks]**

**Electrical energy** is converted to **heat energy** in the heater, this transfers to heat energy in the air.

**Question 2:** A 5\text{ W} bulb is switched on for 1\text{ hour}, how much energy is transferred during this time?

**[2 marks]**

**Question 3:** A battery with a potential difference of 12\text{ V} is connected to a motor. In a period of time, 168\text{ J} of energy is transferred to the motor. Calculate the charge flow through the motor during this time.

**[2 marks]**

So \text{Q}=\dfrac{\text{E}}{\text{V}}=\dfrac{168\text{ J}}{12\text{ V}}=\bold{14\text{ C}}

**Question 4:** What is the power of a 10 \Omega resistor when a current of 2\text{ A} flows across it?

**[2 marks]**