Current Electricity Questions and Answers - Physics Form 3 Topical Revision

Questions

1. A student learnt that a battery of eight dry cells each 1.5v has a total e.m.f of 12V the same as a car battery. He connected in series eight new dry batteries to his car but found that they could not start the engine. Give a reason for this observation
2.  
   1. You are required to determine the resistance per unit length of a nichrome wire x, you are provided with A.D.C. power supply an ammeter and voltmeter.
      1. Draw a circuit diagram to show how you would connect the circuit.
      2. Describe how you would use the circuit in (a) (i) above to determine the resistance per unit length of x.
   2.  
      1. State Ohm’s Law.
      2. A filament lamp and a thermostat are ohmic devices to a certain extent. Explain.
   3. Explain why moving coil meters are unstable for the use of alternating voltages.
   4. Four 5Ω resistors are connected to a 10V d. c. supply as shown in the diagram below.
      
      Calculate;-
      1. The effective resistance in the circuit.
      2. The current I following in the circuit.
3. Study the circuit diagram. Determine the potential drop across the 3 resistor.
   
4. State two conditions that are necessary for a conductor to obey Ohm’s law.
5.  
   1. State Ohm’s law.
   2. Describe with aid of a diagram and experiment to verify Ohm’s law
   3. Two resistors R₁ and R₂ are connected in series to a 10V battery. The current flowing then is 0.5A. When R₁ only is connected to the battery the current flowing is 0.8A.
      1. Value of R₂
      2. Current flowing when R₁ and R₂ are connected in parallel with the same batter.
   4. Recharging is one of the practices of maintenance of accumulators. State two measurements, which need to be taken to help you decide when an accumulator is due for charging.
6. A current of 0.08A passes in circuit for 2.5 minutes. How much charge passes through a point in the circuit?
7. An ammeter, a voltmeter and a bulb are connected in a circuit so as to measure the current flowing and the potential difference across both. Sketch a suitable circuit diagram for the arrangement.
8.  
   1. In the circuit diagram shown, calculate the effective resistance between Y and Z.
      
   2. Determine the current through the 3Ω resistor.
   3. One of the 6Ω resistors has a length of 1m and cross-sectional area of 5.0 x 10^-5m². Calculate the resistivity of the material.
9. In the circuit diagram five resistors are connected to a battery of e.m.f. 4V, and negligible internal resistance. Determine:
   
   
   1. The total resistance of the circuit.
   2. The current flowing through the 5.5Ω resistor.
   3. The potentials at points Y and Q.
   4. The potential difference between Y and Q
10. An electric bulb with a filament of resistance 480Ω is connected to a 240V mains supply. Determine the energy dissipated in 2 minutes.
11. A student wishes to investigate the relationship between current and voltage for a certain device X. In the space provide, draw a circuit diagram including two cells, rheostat, ammeter, voltmeter and the device X that would be suitable in obtaining the desired results.
12. In the circuit diagram shown in the figure below, the ammeter has negligible resistance. When the switch S is closed, the ammeter reads 0.13A.
    
    Determine the internal resistance of the cell.
13.  
    1. State Ohm’s law.
    2. The graph in the figure below shows the current voltage characteristics of a device, X.
       
       
       1. State with a reason whether the device obeys Ohm’s laws.
       2. Determine the resistance of the device, X, when the current through it is 60m A.
       3. When the device, X, is connected in the circuit below, the voltage across it is 0.70V.
          
          Calculate the value of the resistance R.
    3. The cell in the figure below has an emf of 2.1V and negligible internal resistance.
       
       Determine the 
       1. Total resistance in the circuit.
       2. Current in the circuit
       3. Reading on the voltmeter
14. State two advantages of an alkaline battery over a lead acid battery
15. The diagram below shows an electrc circuit. When the switch is close the ammeter reading is 0.3A
    
    Determine the voltmeter reading.
16.  
    1. In the circuit diagram shown, calculate the effective resistance between Y and Z
       
    2. Determine the current through the 3Ω resistor.
17. A battery of e.m.f. 3v drives a current through a 20Ω resistor. The p.d across the resistor is 2.8v as measured by a voltmeter. Calculate the internal resistance of the battery.
18. A torch uses two identical dry cells connected in series. When a bulb of resistance 2.0 ohm’s is connected across the cells the pd across the bulb is 2.0 v. When a bulb of resistance 1.5 ohms is used, the p.d is 1.8v, calculate the e.mf and internal resistance of each cell.
19. Suppose a high–resistance voltmeter reads 1.5v connected across a dry battery on open circuit and 1.2v. when the same battery is in a closed circuit when it is supplying a current of 0.3A through a lamp of resistance R.
    1. Draw a circuit diagram to show the above experiment when in;
       1. Open circuit
       2. Closed circuit.
    2. What is
       1. The emf of the battery.
       2. The internal resistance of the battery
       3. The value of R?
20. When a resistor is connected across the terminals of a battery a current of 0.20 A flows. 
    1. What is the time taken for 2.0 coulombs of charge to pass a given point in the circuit?
    2. If e.m.f of the battery is 4.0v and its internal resistance is 0.20hm determine the rate at which heat is produced in the resistor.
21.  
    1. State Ohm’s law. 
    2. In an experiment to determine the resistance of a resistor x, it is connected in parallel with a 100 Ω resistor. The current through the combination and the p.d across the combination is tabulated as shown below.
       

       Potential difference (v)	1.5	3.0	4.5	6.0	7.5
       Current (A)	0.075	0.015	0.225	0.30	0.375

       Draw a diagram of the circuit that could have been used 
    3.  
       1. plot a graph of current against potential difference.
       2. Calculate the gradient of the slope 
       3. Calculate the resistance of resistor x.

Answers

1. Dry cells have a very high internal resistance hence give very little current that start a vehicle.
2.  
   1.  
      1.  
         
      2. - Measure length of wire (L)
         - Take readings of ammeter (I) and voltmeter (V)
         - Calculate its resistance R
         - Then find the value ^R/_L
   2.  
      1. The current flowing through a resistor is directly proportional to the p.d. applied as long as the physical factors remain constant.
      2. If the temperature is controlled to remain constant then they obey ohm’s law. If temp is changed, they don’t obey.
   3. Every time the current is reversed the direction of key pointer also reversed. This would give an average of zero.
   4.  
      1. For the two 3Ω resistors in parallel their total = ^product/_sum=1.5 Ω 
         For series connection i.e. 3.3 and 1.5Ω, total 7.5 Ω
         

         ∴ RT = 7.5 Ω

      2. I =  E  = 10v = 1.33A
              RT     7.5

3. RT = ^product/_sum = 9 x 18= 4.5 Ω
   I = ¹²/_4.5 = ⁸/₃ Amps
   I through the 3 Ω is equal to I through the Ω since total resistance in each route are equal = ⁸/₃ A x ½ 
   p.d = ⁴/₃ x 3 = 3V
4. Constant temperature, magnetic field, tension, compression, kinks etc.
5.  
   1. The current flowing through a resistor is directly proportional to the p.d applied as long as the physical constants are held constant.
   2.  
      
      - By varying R obtain a set of corresponding values for I and p.d readings
      - Tabulate them
      - Plot V against I
      - If it is a straight line the law is obeyed, over wise not.
   3.  
      1. When R1 only is connected I = 0.8a
         R₁ = ^E/_I = ¹⁰/_0.8 = 12.5 Ω
         When both are connected, then 
         (R₁ + R₂) 0.5 = 10
         (R₂ + 12.5) = 20
         R₂ = 7.5 Ω
      2. When in parallel RT = ^products/_sum = 4.6875 Ω
         I= ¹⁰/_4.6875 = 2.133A
   4. - Rod of the acid
      - Voltage emf of the battery
6. Q= It = 0.08 x 2.5 x 60 = 12C
7.  
   
8.  
   1. ¹/R_T = ¹/₆ + ¹/₃ + ¹/₆ = ⁴/₆ = ²/₃ 
      ∴ RT = 1.5 Ω
   2. For the whole circuit Rt = 1.5 + 2.5 = 4 Ω
      Main current = ^E/R_T = ²/₄ = 0.5A
      p.d across YZ = IR = 0.5 x 1.5 = 0.75V
      p.d across any of the resistors in parallel
      3 x I = 0.75A
      I = 0.25 A
9.  
   1. For parallel connection RT = Products/ sum
      = (5 x 5) = 2.5 Ω
         (5 + 5)
      ∴ Total resistance = 2.5 + 5.5 = 8.0 Ω
   2. Current= main current = ^R/R_T = ⁴/₈ = 0.5 Ω
   3. Note currents through Y and Q are equal since the resistance values are equal – through the two routes.
      Let potential be rep. by P
      P_y – P_p = 0 (earthed)
      ∴ P_y = IV
      P_Q = P_P = P.dQP = 0.25 x 2 = 0.5V
      P_Q – 0= 0.5
      P_Q = 0.5V
   4. PD_YQ = P_y – P_Q = 0.5V
10. E= Pt = V² x 2 x 60 = 240 x 240 x 120
                R                           480
    = 14400J
    = 14.4KJ
11.  
    
12. I =  1.5  
         R + r
    0.13 =  1.5  
             10 + r
    R= 1.5Ω
13.  
    1. The ratio of the pd across the ends of a metal conductor to the current passing through it is a constant.
    2.  
       1. It does not obey Ohm’s law; because the current – voltage graph is not linear throughout
       2. Resistance = ^V/_I = inverse of slope
          (0.74 – 0.70)V
          (80- 50)mA
          = 0.40V 
          30 x 10^-3 A
          = 1.33 Ω
       3. From the graph current flowing when pd is 0.70 is 60mA
          Pd across R= 6.0 – 0.7 = 5.3V
          R= 5.3V 
               60mA
          = 88.3 Ω
    3.  
       1. Parallel circuit ¹/₃₀ + ¹/₂₀ = ⁵/₆₀
          R= 12 Ω
          Total resistance = 10 + 12 = 22 Ω
       2. I = ^V/_R = ^2.1/₂₂ = 0.095 A
       3. V= IR = 10 x 2.1
                              22
          = 0.95V
14. - Alkaline cell last longer than lead acid cell
    - Alkaline cell is more rugged than lead acid cell
    - Alkaline cell is lighter than lead acid cell
15. 2V
16.  
    1. 4 Ω
    2. 0.5A
17. 1.43 Ω
18. 1.5v, 0.5 Ω
19.  
    2.  
       1. 1.5 V,
       2. 1Ω
       3. 4 Ω
20.  
    1. 10s
    2. 0.792w