Week 2 and 3 – SS1 Third Term Physics Notes

 
 
 
 
 
WEEK 2
TOPIC: RESISTORS & CELLS CONNECTED IN SERIES & PARALLEL
CONTENT: Resistors in series and parallel
     Cells in series and parallels
     Calculations
Resistors in series: These are end to end connection.

 
 Characteristics

• Same current flow through each resistor
• Potential difference across each resistor is different
• P.d are additive
• Power are additive
• Applied voltage equals the sum of different p.d
• Resistance are additive.

R_T = R₁ + R₂ + R₃ _______ for 3 resistors
R_T = R₁ + R₂ + R₃….. + Rn ___ for n numbers or resistors

 Resistors in parallel: These are side by side connection.

 
 
 
 
 Characteristics

• Different resistors have their individual current
• P.d across each resistor is the same
• Branch current are additive
• Conductance are additive
• Power are additive

1/R_T = 1/R₁ + 1/R₂ + 1/R₃ ____ for 3 resistors
1/R_T = 1/R₁ + 1/R₂ + 1/R₃…..+ 1/Rn ____ for n numbers of resistors
EVALUATION

1. Differentiate between resistors connected in series and parallel
2. State the relationship between resistance and conductance

 Cells in series: end to end connections

 
 E_T = E + E + E = 3E
For n cells; E_T = n E

 
 E_T = E₁ + E₂ + E₃
Cells in parallel: side by side connections

 
 
 
 E_T = E+E+E/3 = 3E/3 = E

 Calculations

• if 2Ω, 3Ω, and 5Ω resistors are connected in (a) series (b) parallel, calculate the equivalent resistance

(a) R_T = 2+3+5 = 10Ω
(b) 1/R_T = 1/2 +1/3+1/5 = 15+10+6/30 = 31/30
     R_T = 30/31Ω
EVALUATION

1. If 2Ω, 2Ω, and 1Ω resistors are connected in (a) series (b) parallel, calculate the equivalent resistance
2. Calculate the potential difference across a resistor of resistance 2KΩ that allows a current of 1mA to pass through it

Reading Assignment : Read more on series & parallel connections of resistors & cells – New school physics (pg 79)

WEEKEND ASSIGNMENT

1. If four identical cells each 2volts are connected in series, the equivalent pd is ……… (a) 2v (b) 4v (c) 6v (d) 8v
2. If four identical cells each 2volts are connected in parallel, the equivalent pd is ……… (a) 2v (b) 4v (c) 6v (d) 8v
3. If 2Ω, 2Ω, and 1Ω resistors are connected in series the equivalent resistance (a) 5 Ω (b) ½ Ω (c) 4 Ω (d) 2 Ω
4. If 2Ω, 2Ω, and 1Ω resistors are connected in parallel the equivalent resistance (a) 5 Ω (b) ½ Ω (c) 4 Ω (d) 2 Ω
5. Two resistors of resistance 1 Ω are connected in parallel to a 2 Ω resistor, the equivalent resistance is …….. (a) 4 Ω (b) 3 Ω (c) 2 Ω (d) 1 Ω

THEORY

1. A battery of 15v and internal resistance 5Ω is connected to a resistor 0f 20Ω.

   Calculate the value of (a) electric current    (b) terminal voltage    (c) lost voltage.

2. If 8c of charge passes through a wire at a steady rate in 2s. calculate the

   Current.

WEEK 3
TOPIC : 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.
2. 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)

 
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?