Home Teachers' Resources CURRENT ELECTRICITY PHYSICS NOTES, REVISION QUESTIONS & ANSWERS

# CURRENT ELECTRICITY PHYSICS NOTES, REVISION QUESTIONS & ANSWERS

1. Define the term “e.m.f” of a cell. (1mk)

1. Distinguish between electromotive force and potential difference (2mks)

1. Distinguish between terminal voltage and e.m.f of a battery. 2mks

1. Define the volt.                                   (1mk)

1. Distinguish between open and closed circuit. (2mk)

1. What is meant by the term “lost volts”? (1mk)

1. State Ohms Law. (1mk)

1. State the condition necessary for Ohms Law to be obeyed (1mk)

1. State two conditions that are necessary for a conductor to obey Ohm’s law.

1. Other than temperature state the other factors that affect the resistance of an ohmic conductor.                   (1 mk)

1. What causes electrical resistance in conductors? ( 1mk)

1. Define a non-ohmic conductor (1mk)

1. Define electric resistance. (1mk)

1. Eight dry cells can be arranged to produce a total e.m.f of 12v just like a car battery.

(a)  What is the e.m.f of an individual cell?                                         (2mk)

(b)        Why is it possible to start a car with the lead acid accumulator, but not with the eight dry cells in series?

1. Give a reason why fluorescent tubes are preferred to filament bulbs for domestic lighting

1. A boy who has been driving a car in rain discovers that the movement he alights from it while touching its body he gets an electrical shock. Offer him a scientific explanation why he was not getting the shock while inside even if he touches the metallic parts of the car.

1. A circuit consists of a battery, a metal wire, ammeter and a switch connected in series. The switch is closed and the ammeter reading noted. The metal wire is now heated.

(ii)      Give one observation for the above observation made.      (1mk)

1. Describe an experiment to verify Ohm’s law given a cell, ammeter, voltmeter, switch, nichrome wire and connecting wires.

1. A battery is connected to an external resistor, R. State any two factors that determine the magnitude of the current produced in the circuit. (2mk)
2. Figure below shows an ammeter used to measure current through the conductor.

State the reading from the meter when the student is using the;

(i) Lower scale.                                                                                 (1mk)

(ii) Upper scale                                                                                 (1mk)

1. Figure below shows a voltmeter used to measure voltage through the conductor. The student used the upper scale.

State the reading from the meter.                                             (1mk)

1. Figure below shows an ammeter used to measure current through the conductor. The student used the lower scale.

1. Figure below shows a voltmeter used to measure voltage through the conductor. The student used the upper scale.

State the reading from the meter.                                             (1mk)

1. State why the filament of a lamp is made of tungsten (1mk)

1. Three identical bulbs are connected in series with a battery. At first, the bulbs shine brightly but gradually become dimmer.  Using the same cells, explain how you would increase the brightness of the bulbs.                  (1mk)

1. In large circuits, large resistors in parallel are preferred to low resistors in series. Explain.

1. Describe a laboratory experiment to verify ohm’s law, by use of appropriate diagrams where necessary.                                     (5mk)

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 voltmete
2. i) Draw a circuit diagram to show how you would connect the circuit.

1. Describe with aid of a diagram and experiment to verify Ohm’s law

1. A student wishes to investigate the relationship between current and voltage for a certain resistor X. In the space provide, draw a circuit diagram including two cells, rheostat, ammeter, voltmeter and the resistor X that would be suitable in obtaining the desired results.  Describe how you would use the circuit in (a) (i) above to determine the resistance of x.          (5mk)

1. The figure below shows a circuit that can be used to verify Ohm’s law

Explain briefly how the setup can be used to verify ohm’s law          (4mks)

1. 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.

1. Three identical bulbs are connected in series with a battery. At first, the bulbs shine brightly but gradually become dimmer. Using the same cells, explain how you would increase the brightness of the bulbs. (2mk)

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.

V = IR

1. A current of 0.08A passes in a circuit for 2.5 minutes. How much charge passes through a point in the circuit?

1. What current will a 500Ω resistor connected to a source of 240V draw?
2. Two resistors of 4W and 2W in parallel are connected in series to a 3W resistor and a cell of 2.0V.

Calculate

1. Equivalent resistance of the circuit. (3 marks)
2. Current through each resistor. (4 marks)

1. Three identical dry cells each of m.f. 1.6 V are connected in series to a resistor of 11.4Ω. a current of 0.32A flows in the circuit.  Determine.
2. i) The total e.m.f. of the cell                                    1mk
3. ii) The internal resistance of each cell;                     3mk
4. A wire of resistance R connected in series with 1.5V cell is found to be carrying a current of 0.05A. If the wire is now connected in parallel with an identical wire, find the new current in the circuit. (3mk)

1. Two heaters A and B are connected in parallel across a 10volts supply. Heater A produces 1000J of heat in one hour while B produces 200J in half an hour. Calculate the ratio RA/RB. (3mks)

1. A current flows through a coil wire of resistance 50W when it is connected to the terminals of a battery. If the potential difference is 20V, find

(i) The value of current                                                                     2 mks

1. ii) The number of electrons that pass through the coil per second. (Take

charge of electronse = 1.6 x 10-19 c)                                          3 mks

1. Three identical resistors each of resistance 2.0are connected so as to obtain the least effective resistance. If a battery of voltage V is connected in series to the arrangement of the resistors and supplies a current of 3A. Calculate the value of V                                                                (4mks)

1. 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.

(a)  Calculate the

(i)     Value of R2

1. ii) Current flowing when R1 and R2 are connected in parallel with the same batter.

(b)        One of the 6W resistors has a length of 1m and cross-sectional

area of 5.0 x 10-5m2. Calculate the resistivity of the material.

1. 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.

(a)         Draw a circuit diagram to show the above experiment when in;

(ii)     Open circuit

1. ii)     Closed circuit.

(b)       What is

1. i) The e.m.f of the battery.
2. ii) The internal resistance of the battery

iii) The value of R?

1. When a resistor is connected across the terminals of a battery a current of 0.20A flows. What is the time taken for 2.0 coulombs of charge to pass a given point in the circuit? 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.

1. A cell supplies a current of 1.8A through two 2W resistors connected in parallel. When the resistors are connected in series, the current is 0.6A. Calculate the internal resistance of the cell. (3mk)

1. A cell can supply a current of 1.2A through two 2W resistors connected in parallel. When they are connected in series, the value of the current is 0.4A. Calculate the internal resistance of the cel          (3mks)

1. A battery of e.m.f. 3V drives a current through a 20W The p.d across the resistor is 2.8V as measured by a voltmeter.  Calculate the internal resistance of the battery.

1. Resistors of 2W and 3W are connected in series with a cell and voltmeter connected across the 3W resistor reads 1V, but this increases to 1.2V when an extra 2W resistor is connected in parallel with the first 2W resistor, calculate the e.m.f and the internal resistance of the cell.

1. A torch uses two identical dry cells connected in series. When a bulb of resistance 2.0W is connected across the cells the pd across the bulb is 2.0V. When a bulb of resistance 1.5 ohms is used, the p.d is 1.8V, calculate the e.m.f and internal resistance of each cell.

1. A cell drives a current of 3.2A through a 2.8 Ω resistor. When it is connected to 1.6Ω resistor, the current that flows is 5A. Find the:

(i) E.m.f (E) for the cell.        (2mk)

(ii) Internal resistance (r) for the cell.   (2mk)

1. A cell drives a current of 8A through a 1.2resistor when the same cell is connected to a 1.8resistor, the current that flows is 6.0A. Determine
• The internal resistance. (4mks)
• m.f of the cell. (2mks)

1. A cell supplies a current of 0.5A when connected to a 2W resistor and 0.25A when connected to a 5W Find the e.m.f and the internal resistance of the cell.                                                     (4mks)

1. A wire of resistance X connected in series with a 1.5 V battery is found to carry a current of 0.075 A. If the wire is now connected in parallel with an identical wire, find the total current in circuit. (3mk)

BULBS

1. An electrician installed electric wiring in a house and connected the bulbs and the switches as shown in the figure below.

• Explain what happens when switch

(i) S1 is closed.                                                                   (1mk)

(ii) S2 is closed.                                                           (1mk)

(iii) S3 is closed.                                                          (1mk)

• (i) Using a redrawn diagram show the best position the bulbs should be installed. (3mks)

1. State the changes in brightness of the bulbs in the circuit diagram as the switches S1, S2 and S3 are switched on one after the other. (2mks)

1. Figure below shows a circuit diagram containing three identical bulbs.

Compare the brightness of the bulbs                                          (2mks)

1. The voltmeter and the ammeter in figure (a) below read 2.1v and 0.35A respectively.

Determine the reading in the voltmeter and ammeter in figure(b) respectively. (The lamps are identical to those in (a)

1. A student set up the circuit shown in Fig. 3. The lamps didn’t light when she closed the switch S.

Suggest a reason for this observation.                                       (2mk)

1. Figure below shows two circuits with identical dry cells and identical bulbs;

(i) In which circuit will the bulbs be brighter                               (1mk)

1. A student was investigating the brightness of bulbs when setup in an electric circuits. She used identical bulbs and cells. The circuits shown in figure 3(a) and (b) were what she setup.

Which setup was the bulb brightest? Give a reason for your answer.      (2mk)

1. Fig (a) and (b) show two possible arrangements of a bulb to a source of power.

In which of the arrangement above would the cells drain faster. Explain your answer.                                                                                     (2mk)

1. The circuit diagram in figure below has two switches P and Q. The brightness of the lamp isobserved when P only is closed and when P and Q are both closed.

Compare the brightness of the lamp in the two cases. State a reason for your answer.                                                                                    (2mks)

CIRCUITS

1. Three resistors of resistance 2 Ω, 3 Ω and 4 Ω, are to be connected to a cell such that they have the least effective resistance.

(i)  Draw a circuit to show how they can be connected to achieve this.(2mk)

(ii)  Determine the least effective resistance of the three resistors. (3mk)

(iii) State two factors that determine the resistance of a metallic conductor.

(2mk)

1. Figure shows a voltmeter connected across the cell. The voltmeter reads 1.5V when the switch S, is open and 1.2V when the switch is closed.

(i) What is the e.m.f. of the cell.                                                (1 mk)

(ii) What is the terminal voltage of the cell.                                (1 mk)

(iii) Calculate the internal resistance of the cell.                 (3 mks)

1. You are provided with three resistors of 3Ω, 2Ωand1Ω. Arrange the resistors in the circuit so as to have minimum resistance. Determine the effective resistance in the above circuit         (3mk)

1. Figure below shows part of an electric circuit. The current through the 18Ω resistor is observed  to be 2A.

State the value of the current through each of the 10Ω resistors.             (1mk)

1. Study the circuit diagram below and answer the questions that follow.

Calculate

• The current flowing through the ammeter.   (3mks)
• The P.d. across AB (2mks)
• The current through the 4Ω resistor (2mks)

1. The figure below shows an electric circuit. When the switch is closed the ammeter reading is 0.3A.

1. In the circuit diagram below, the battery source has a voltage of 12V and an internal resistance of 5Ω.

(i)  Calculate the total resistance in the circuit.                  (3mk)

(ii) Calculate effective current in the circuit.                      2mk)

1. Use the circuit in figure below to answer the questions that follow

(i)     Calculate the total resistance in the circuit                          (3mk)

(ii)     Determine the current flowing through the 5W resistor          (3mk

1. In a balanced condition, the resistance of resistors P,Q and R is 80 ,120and 100 Find the resistance S.                 (3mk)

1. Two resistors K and L are placed in the gaps of the metre bridge as shown

• (i) State the purpose of the resistor M. (1mk)
• (ii) Find the resistance K below if the Jockey balances the galvanometer at the 20cm
1. The figure below shows a 6V battery connected to an arrangement of resistors. Determine the current flowing through the 2 Ω resistor. (3mk)

1. It was noted that for the circuit diagram below, when the switch is open, the voltmeter gives a reading of 12V, but when the switch is closed the voltmeter drops to 10V.

(a)         Give an explanation for the difference in reading on the voltmeter when

the switch is open and when it is closed.                     (2mk)

(b)    If the ammeter gives a reading of 0.8A when the switch is closed,

determine the value of R.                                         (1 mk)

(c)     Determine the internal resistance of the accumulator     (2mk)

1. The figure below shows a series – parallel circuit.

Calculate:

(i) Total resistance of the circuit.                                       (3mks)

(ii) Total current flowing in the circuit.                                (2mks)

(iii) Voltage drop across R1                                                               (2mks)

(iv) Current through the 3W resistor.                                 (3mks)

1. Use the circuit in fig below to answer the questions that follow.

(i)     Calculate the total resistance in the circuit.                                        (2mk)

(ii)    Calculate effective current in the circuit.                                        (2mk)

(iii)    Calculate current through the 2Ω resistor.                                  (2mk)

1. A large battery is connected as shown in figure 5 to a resistor of resistance 1000W. The voltmeter across the resistor reads 50V.

(i)     What is the reading of the ammeter (A)?          (3mk)

(ii)     Determine the electrical energy dissipated by the resistor in one

minute.                                                                           (3mk)

1. Determine the effective resistance in figure below. (3mk)

1. Figure shows three resistors connected as shown.

If the voltmeter reads 4v, find the

(i)     Effective resistance.                                                         (2mks)

(ii)     Current through the 3W resistor.                              (2mks)

(iii)    Potential difference across the 8W resistor.                         (2mks)

1. In the circuit diagram shown in figure below, determine the reading of the meters if:

(i)    The switch S1 is open and S2 closed.                                  (3mks)

(ii)    Switch S1 closed, S2 open.                                                (1mk)

(iii)   Both switches are closed.                                                  (6mks)

1. Study the circuit shown in fig below.

• Determine the effective resistance when both switches S1 and S2 are open.                                                                                   (3mk)
• Determine the effective resistance when both switches S1 and S2 are closed.                                                                                   (3mk)
• Determine the ammeter reading when both switches are closed. (2mk

1. Figure represents a 4.8V battery connected to two conductors XY and YZ of the same Material and same length welded together at Y. The radius of XY is half that of YZ. The resistance of YZ is 1.6.

1. a) Calculate;
2. i) The resistance of the conductor XZ                                    (5mks)
3. ii) The current which flows in the circuit when the switch S is closed(3mks)

iii)     The potential drop across the conductor XY                        (2mks)

1. The circuit diagram in figure shows three identical resistors connected to a cell of e.m.f. 12V.

(i)     Determine the reading of the voltmeter.                                    (2mk)

(ii)     If another identical resistor R is connected parallel to PT, determine the

potential difference across QS

1. In the circuit below figure, the e.m.f of the cell is 1.5V. Its internal resistance is r ohms. Resistors P and Q are each 3Ω. If 0.3A current flows through P. find the value of r.

1. Fig shows three resistors connected to a 12V battery of internal resistance 2Ω

Calculate

1. i) The current drawn from the cell.
2. ii) The “Lost voltage”, in the cell

iii) The potential different across the 7Ω resistor.

1. The cells shown in the figure below have each an e.m.f of 2.1V and negligible internal resistance

Determine the reading of the voltmeter                                              (3mk)

1. The cell in figure has an e.m.f of 1V and negligible internal resistance.

Determine the

(i)     Total resistance in the circuit                   (2mk)

(ii)    Current in the circuit                                       (1mk)

(iii)   Reading on the voltmeter                                (2mk)

1. Four 5W resistors are connected to a 10V d. c. supply as shown in the diagram below.

Calculate;-

1. The effective resistance in the circuit.
2. ii) The current I following in the circuit.
3. Sketch a graph current against voltage for an electrolyte solution such as dilute sulphuric acid.

1. For a particular bulb, a series of readings of the current through the bulb for different potential differences across it is taken and plotted as shown.

1. Draw the circuit diagram you would use for the experiment.(2mks)
2. Explain how the resistance of the bulb changes        (3mks)
3. How would the resistance of the bulb change if

(I) The length were doubled                                                   (2mks)

(II) The diameter were doubled                                             (2mks)

1. The graph below shows results obtained in an experiment to determine the m.f.(E) and the internal resistance, r, of a cell.

• (i) Determine the slopeof the graph

Given that the equation of the graph is Determine

• (ii) The e.m.f E of the cell.
• (iii) The internal r resistance of the cell.

1. The graph below shows the Voltage current relationship for a certain battery.

Determine:

(i)     The e.m.f of the cell.                                                        (1mk)

(ii)     The internal resistance of the cell.                    (4mks)

1. Two resistors 4Ω and the other unknown resistor x are connected in a circuit as shown in the fig below

The current I passing through the combination is measured for various potential differences, A graph of p.d. against current is shown in the grid below.

(i) Use the graph to determine the total resistance of the combination. (3mk)

(ii) Determine the value of the unknown resistance x            (2mk)

1. Figure 12 shows the variation of voltage against current.

Given the equation; E = V + Ir

Find from the graph.

(i) the e.m.f, E of the cell.                                                             (1mk)

(ii) the internal resistance, r, of the cell.                                          (3mks)

1. On the axes provided, sketch a graph to show how current, I varies with potential difference, V, across a metallic conductor that is being heated at the same time. Explain your answer                 (2mks)

1. The graph below shows how the voltage, V, varies with the current, I for a filament lamp.

(i) From the graph, determine the resistance of the lamp when a current of 0.5A  flows.                                                                               (3mks)

(ii) State with a reason whether the device is ohmic or non-ohmic.    (2mks)

1. The graph in fig. shows the current – voltage characteristics of a certain device Q.

(i) State with a reason whether the device obeys ohm’s law.                 (2mk

• Determine the resistance of the device, Q when current through it is

30mA.                                                                    (3mk)

iii)     When the device, X, is connected in the circuit below, the voltage across it is 0.70V.

Calculate the value of the resistance R.

1. The current I through a given diode for various values of voltage V between anode and cathode is given in the table below.
 V (volts) 0 25 50 75 100 125 150 175 225 I(mA) 0 1 3 4 6 10 16 28 80

• Draw the characteristic graph of the diode using the data. (5mks)
• Explain the nature of graph. (2mks)
• Determine the resistance of the diode when the voltage is 150V.

(3mks)

1. A battery is connected in series with an ammeter and a variable resistor R. The resistor is varied and the corresponding readings of the ammeter recorded in the table below.
 Resistance R (Ω) 1 2 3 4 5 10 Current I (A) 2 1.5 1.2 1 0.75 0.5
• Draw a circuit diagram of the circuit that was used to perform this experiment. (1mk)
• On the grip provided, plot a graph of R (y-axis) against I (x-axis). (5mk)
• Use your graph to determine the internal resistance of the battery. (2mk)
• Determine the e.m.f of the cell. (2mk)

1. A student carried out an experiment to investigate how current varies with potential difference

applied across a filament lamp. The following readings were obtained.

 P.d.(V) 0 0.2 0.4 0.6 0.8 1.2 1.6 2.4 I (A) 0 0.11 0.2 0.28 0.34 0.43 0.5 0.58

(a) Draw a diagram for the circuit used to obtain the values.       (2mks)

(b) Describe briefly how the experiment was carried out.  (2mks)

(c) Plot a graph of V against I for the values presented in the table.   (5mks)

(d) Determine the resistance of the lamp when a current of 0.4A flows through it.

(3mks)

(e) Explain why a filament lamp does not obey Ohm’s law.      (2mks)

SCHEEM

1. Define the volt. (1mk)

ANS   It is the work done in moving a unit charge in a circuit from one point to anotherü1

1. Three identical bulbs are connected in series with a battery. At first, the bulbs shine brightly but gradually become dimmer.  Using the same cells, explain how you would increase the brightness of the bulbs.                  (1mk)

Connect the three bulbs in parallel so that the internal resistance is

reduced. This set up increases the current making the bulbs more bright. 1mk

1. Three identical bulbs are connected in series with a battery. At first, the bulbs shine brightly but gradually become dimmer. Using the same cells, explain how you would increase the brightness of the bulbs. (2mk)

Connect the three bulbs in parallel so that the internal resistance is reduced. This set up increases the current making the bulbs more bright.

1. Figure below shows two circuits with identical dry cells and identical bulbs;

(i) In which circuit will the bulbs be brighter                               (1mk)

ANS  (i)   B      √1mk

(ii)   In both A and B the voltage is the same while current in B is twice the current in A.

B cells are in parallel hence lower effective internal resistance

1. The voltmeter and the ammeter in figure (a) below read 2.1v and 0.35A respectively.

Determine the reading in the voltmeter and ammeter in figure(b) respectively. (The lamps are identical to those in (a)

ANS          Voltmeter would read 2x 2.1V = 4.2 V ü1

Ammeter would read 0.35 A ü1