Electrical Energy And Power

CONTENT

• Electrical Energy
• Electrical Power
• Buying Electrical Power

ELECTRICAL ENERGY

Energy = Work (Joules)

W = QV_______________________(1)

But Q = It

W = VIt ______________________(2)

From ohm’s law V = IR

W = I²Rt______________________(3)

From ohm’s law I = V/R

W = V²t/R_____________________(4)

W = QV = VIt = I²Rt = V²t/ R

ELECTRICAL POWER

Power = work done/time taken (watt) __ (5)

P =QV/t = VI = I²R = V²/R…………….(6)

EVALUATION

1. The headlamp of a car takes a current of 0.4A from a 12v supply.Calculate the energy produced in 5 minutes.
   1. State four factors that affect the resistance of a conducting wire.

BUYING OF ELECTRICAL ENERGY

Commercial power is consumed in kilowatt- hour (kWh)

NB   1kwh = 3.6 х106 J = 3.6MJ

Cost = Pct/ 1000………………………….(7)

P – Power (w)                     NB :  P(W)/ 1000 = P (KW)

c – Cost per kWh (unit) it means that it is in kWh

T – Time (hr)

EVALUATION

1. Find the cost of running two 100w amplifier, ten 50w television and twenty 60w lamp for 24hrs, if the electrical energy cost 60k/unit
2. A landlord has eight 40w electric lighting bulb, four 60w bulb and two 100w bulb. If he has all the point on for 5hrs, what is the bill for 30 days?

READING ASSIGNMENT

Read more on electrical energy & power – New school Physics (pg 81-82

GENARAL EVALUATION

1. Define frictional force
2. Give three ways of reducing friction.

WEEKEND ASSIGNMENT

1. Electrical energy is measured in (a) watt (b) kilowatt-hour (c) kilowatt (d) none of the above
2. If a p.d of 240v is applied across a lamp that supplies energy at the rate of 60w. What is the value of current? (a) 0.2A         (b)0.25A (c) 0.3A (d) 3A
3. The resistance of the filament in question 2 is ____ (a) 96Ω (b) 48Ω (c) 960Ω (d) 900Ω
4. Calculate the conductance of a conductor having a resistance of 10Ω (a) 0.1 (b) 1

(c) 10Ω (d) 0.01Ω

5. The SI unit of resistivity is ____  (a) Ω/M (b) Ω²/M (c) ΩM (d) none of the above.

THEORY

1. Find the cost of running two 100w amplifier, ten 50w television and twenty 60w lamp for 24hrs, if the electrical energy cost 60k/unit
2. A landlord has eight 40w electric lighting bulb, four 60w bulb and two 100w bulb. If he has all the point on for 5hrs, what is the bill for 30 days If electric energy cost 80k per KWhr.