7.5 Electrical energy and power Electrical fundamentals Physics 2AB
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
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
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
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)
Electrical power How much energy is used in 5 minutes by a 240 V hairdryer drawing 13A?
Electrical power How much energy does a 100 W light bulb use in half an hour?
Electrical power Use Ohm’s Law V = IR and P = VI to prove the following: P = I 2 R
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
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