§12.5 Electrical Power

• Science
• Upper Secondary (Key Stage 4)

§12.5 Electrical Power

Science - TTO2

Learning Objects

• Recall how electrical power depends on current and pd. 
  
• Calculate electrical power from current and pd: P = I * V. 
  
• Recall how electrical energy supplied to a device depends on its electrical power and the time in use. 
  
• Calculate electrical energy from current, pd and time. E = I * V * t or E = P * t 
  

Have you ever wondered...

How much energy it actually costs to toast a slice of bread?

Well... 

Watch the video on the next slide to find out!

Toaster challenge

I think you noticed that Robert is a pretty strong guy. I hope you now realise how lucky we are to not have to get on a bike every time we have to use our electrical devices!

In stead of that, we simply get our energy from the socket on the wall. 

Electrical Power

When you use an appliance, electrical energy gets transformed into other forms of energy. 

• Power is the amount of energy transformed in joules per second. 
  
• Power (P) is measured in Watts (W)
  
• 1 W = 1 J / s

Electrical Power

There are two formulas to calculate Power.

The first one derives the definition of Power: "Power is the amount of energy transformed in joules per second". So to calculate power, you have to devide the energy transformed by the time taken.

In words:

Power = Energy / time

[watts] = [joules] / [second] 

In symbols:

P = E / t

[W] = [J] / [s]

Electrical Power

For the second formula, we have to go back to what we have learned already about Electricity. We know that:

"Current is the amount of electrons passing through a point each second"

"Potential difference is the amount of energy each coulomb transfers to the device"

"Power is the amount of energy transformed in joules per second."

I = Q / t

V = E / Q

P = E / t

Electrical Power

Looking at these three definitions, you can see that both current and potential difference have something to do with charge. If you multiply the Current with the Potential Difference, Charge gets cancelled out and you are left with Power! 

Let's see how this works on the next slide!

I = Q / t

V = E / Q

P = E / t

x

=

Electrical Power

By cross multiplication of Q / t times E / Q, Q gets cancelled out:

We know that Q/t=I , E/Q=V and E/t=P so...

Q / t

 E / Q

E / t

x

=

I

x

V

P

=

Electrical Power

The second formula to calculate power therefore is:

In words:

Power = current * potential difference

[watts] = [amperes] * [volts] 

In symbols:

P = I * V

[W] = [A] * [V]

Electrical Power

If you did not get the math behind the formula P = I * V, 

don't worry, we will get back to this in year 3. 

For now the most important thing is to understand the two formulas to calculate Electrical Power:

and 

Now, watch the video on the next slide to get a nice overview of Power, Current and Potential difference!

P = E / t

P = I * V 

Time for some practise!

• Read p. 172-173 (TB) 
  
• Do Summary Questions p. 173 (TB) 
  
• Do workbook exercises p. 88 (WB)