1. 7.5 Electrical energy and power Electrical fundamentals Physics 2AB

2. Lesson objectives Explain the connection between electrical work, charge and potential difference – this will include applying the relationships of electrical work and power Work = Vq = VIt and Power = VI = I 2 R = V 2 /R

3. Electrical energy  The EMF, or voltage, of a power source is a measure of the number of joules of energy stored per coulomb of charge.  E = Vq  As the current is the rate at which charge is moving  Then the total charge q can be expressed as: q = It  Electrical energy (joules) = potential difference (volts) x current (amps) x time (seconds) E = VIt

4. Electrical power  Power is the rate of energy use (joules per second)  The rate at which electrical energy is converted to another form of energy  The rate at which appliances use energy varies

5. Electrical power  Power is the rate of energy use (joules per second)  Since E = VIt then P = VI  P is the power in watts (W)  V is the potential difference in volts (V)  I is the current in amps (A)

6. Electrical power  How much energy is used in 5 minutes by a 240 V hairdryer drawing 13A?

7. Electrical power  How much energy does a 100 W light bulb use in half an hour?

8. Electrical power  Use Ohm’s Law V = IR and P = VI to prove the following: P = I 2 R

10. Electric power production and transmission  Power is generated at around 20kV then ‘transformed’ up to around 400kV for transmission to the city  It is then ‘transformed’ down to 22kV and finally 240V before entering homes

11. Electric power production and transmission  AC vs DC  Batteries and solar cells are DC (direct current) EMF sources  This means they push the charge in one direction – e.g. From the positive to the negative terminal of a dry cell battery  The mains power supply is AC (alternating current)  An alternating current continually changes direction – e.g. mains 240V power reverses direction 50 times every second  For our purposes this year, we will treat AC and DC the same Heinemann Pg 303 Q 1-5 STAWA Pg 149 Set 15