P5: Electricity In The Home (Y11 - Spring 2) Flashcards
๐ข What is Power (+ What is Electrical Power)
Power is how fast energy stores can be transferred from one form to another.
The rate of energy transfer by each electrical component is called electrical power.
๐ข Equation for Power involving Energy and Time
Power (W) = Energy (J) / Time (s)
P = E/t
๐ข Example Question:
A kettle is on for 3 minutes and uses 12000J of energy. What is the kettle power rating?
P = ? E = 12000J T = 3m = 60x3 = 180 s
P = E /t
12000 / 180 = 66.67W
๐ข How can P = E/t be rearranged to get E and t as the subjects of the formula
E = Pt
t = E/P
๐ข Example Question: A 40W light bulb is switched on for 30 mins. Calculate the energy it transfers.
P = 40W t = 30mins 30x60 = 1800
E = Pt
40 x 1800 = 72,000 J
๐ข What are the 2 units for power?
Watts, W or Joules per Second
๐ข Example Question: A light bulb transfers 24kJ in 10 minutes. Calculate the power of the light bulb.
E = 24,000 t = 10 mins 10 x 60 = 600s
P = E/t
24000 / 600 = 40W
๐ข Equation for Power involving Current and Potential Difference
Power Supplied (W) = Current (A) x Potential Difference (V)
P = IV
๐ข Example Question:
a) Calculate the normal current through a 500 W, 230V heater
b) Determine which fuse 1 A, 3 A, 5 A, or 13 A you would use for the appliance.
a)
P = 500
V = 230
P = I x V I = P/V 500/230 = 2.2 A
b)
You would use a 3A fuse because it would not melt when the current is 2.2A, but it would melt if due to a fault, the current exceeded 3A. The 5A and 13A fuses would only melt if the current exceeded 5A and 13A respectively.
๐ข Example Question:
Which fuse out of a 1 A, 3 A, 5 A, or 13 A should be used for a 2.5kW kettle, with a 230V supply?
P = IV I = P/V
2500/230 = 10.9A
So a 13A fuse should be used
๐ข Equation for Power involving Current and Resistance
Power (W) = Current^2 (A) x Resistance (ฮฉ)
P = I^2 x R
๐ข How can the Equation for Power involving Current and Resistance be rearranged to make Current and Resistance the subject.
Current (A) = (square root) Power (W) / Resistance (ฮฉ) (square root)
/---------- I = / P / ---------- \/ R
Resistance (ฮฉ) = Power (W) / Current^2 (A)
R = P / I^2
๐ข Example Question:
Calculate the power of an electrical device using a current of 10 A and having resistance 12 ฮฉ.
P = I^2 x R
10^2 x 12 = 1200W
๐ข Example Question:
Calculate the current flowing through a device with a power of 360 W and 10 ฮฉ resistance
. /---------- I = / P / ---------- V R
/---------- / 360 / ---------- = 6 V 10
๐ข Example Question:
What resistance does a 25 W light bulb have if 12.6 V is applied to it?
P = 25W V = 12.6V
P = I x V I = P / V
25 / 12.6 = 1.984126
V = I x R R = VI
12.6 / 1.984126 = 6.35 ฮฉ
๐ข Example Question:
A computer uses 3.5 A at 110 V. What is its resistance in ohms?
I = 3.5 A V = 110V
V = I x R R = V / I
110 / 3.5 = 31.4 ฮฉ
๐ข Example Question:
A device has a resistance of 11 ฮฉ and a current of 12 A. What is it power and potential difference?
R = 11ฮฉ I = 12A
V = I x R
11 x 12 = 132V (potential difference)
P = I x V
12 x 132 = 1584 W (power)
๐ข What is charge flow?
โ In a circuit, electrons are forced through components by the potential difference of the battery or cell
โ Charge is carried through the circuit by the electrons
โ have a negative charge (-)