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Resistors Questions

1.  Your are provided with the following apparatus: connecting wires, an ammeter, fixed resistors,

a voltmeter, a variance resistor, a switch and two dry cells in a cell holder. Draw a circuit that

can be used using the apparatus above to verify Ohm’s law  2.  (a) Three resistors A, B and C where A is resistor 200 Ω C of resistance 100 Ω and B is of

unknown resistance are connected in parallel. This arrangement is then placed in a circuit

and current passing through and potential difference across it measured. The table below

shows the result.  

 

p.d.v.

3.0

6.0

9.0

12.0

15.0

Current A

0.15

ecolebooks.com

0.30

0.45

0.60

0.75

(i) Plot a graph of p.d. against current A  

(ii) From the graph calculate the total resistance of the resistors.  

(iii) What is the value of the unknown resistor?

(b) The ammeter in the figure below shows a reading of 2A. What is the reading of the

voltmeter v.?  

 

3.  a) Three resistors of resistance 2.0 and 6.0 are connected together in a circuit. Draw

a circuit diagram to show the arrangement of the resistors which gives

  i) Effective resistance of 3.0

  ii) Minimum resistance

Image From EcoleBooks.com b) In figure 9 the voltmeter reads 2.1V when the switch is closed, the voltmeter reads 1.8v and

the ammeter 0.1 A

 

 

Image From EcoleBooks.comImage From EcoleBooks.com

 

 

 

Image From EcoleBooks.com  Determine:

  i) The e.m.f of the cell

  ii) The internal resistance of the cell  

  iii) The resistance of the of the lamp  

 c) One reason why the earth pin is longer than the neutral and the live pins is to open the

shutters of the socket, state one other reason why it is longer  

 d) Why are lamps in a house always connected in parallel but not in series?

 

4.  State two other factors, which would affect the resistance of a metal conductor other than

the temperature

 b) The ammeter in the circuit below has negligible internal resistance. The cell has internal

Image From EcoleBooks.com resistance of 0.5 and an electromotive force of 3.0V

Image From EcoleBooks.com

 

 

 

 

Image From EcoleBooks.com

 

 

 Determine the value of current the ammeter registers when switch S is closed

5.  a) State Ohms law    

6.  In the configuration of resistors given below, determine the current through the 5 resistor  

 

Image From EcoleBooks.com7.  Figure 5 is a circuit diagram of three resistors connected to a 6V battery

 

 

 

 

 

Image From EcoleBooks.com

 

 

  Determine the potential difference across the 3 resistor

8.  (a) Study the circuit diagram shown below.

 

Image From EcoleBooks.com

 

 (i) Determine the reading of the voltmeter V.  

 (ii) Determine the reading of the ammeter A.  

(iii) Explain the effect on the reading of the voltmeter if the 3 resistor is altered to be 6

 

(b) A transformer is designated to work from a 240V a.c. mains and to give a supply of 8V

to ring house bells. The primary has 4800 turns.

(i) What type of transformer is this? Give a reason.

(ii) Why is the iron core laminated?

(iii) Calculate the secondary turns if the efficiency is 100%.  

Image From EcoleBooks.com9.  The diagram in the figure below shows a wheat stone bridge

 

 

 

 

 

 

 

 

 

 

K, L, M and N are four resistors joined as shown. The value of resistance of resistor K is

unknown. I1, I2, I3 and I4 are the amount of current passing through K, M, L and N respectively.

It is also provided that L is a variable resistor.

 (i) Explain how the set-up is used to determine the value of unknown resistance of K  (ii) State why wheat stone bridge is more accurate in measuring resistance than the voltmeter-

ammeter method  

 (b) In an experiment to determine the resistance of a nichrome wire using the metre bridge, the

  balance point was found to be at 38cm mark. If the value of the resistance in the right hand

  gap needed to balance the bridge was 25, Calculate the value of resistance of nichrome wire

Image From EcoleBooks.com10.  Four capacitors of capacitance, 3µf, 4µf, 5µf and 3µf are arranged as shown below. Find the

 

 

 

 

 

 

 

 

 

 

11.  In the circuit diagram below, X is a fixed resistor while Y can be varied between O


and 100 using a sliding jockey

 

 

 

 

 

 

 

 

 

 

 Calculate:

  (i) The minimum possible current in the circuit (2mks)

  (ii) The maximum possible current in the circuit (2mks)

 (c) The following figure shows an electric circuit in which five resistors are connected

  to a battery of e.m.f 4.0V and negligible internal resistance

 

 

 

Image From EcoleBooks.com

 

 

 

 

 

 

 

 

 

 

  Determine:

  (i) The total resistance of the circuit  

  (ii) The potential difference between Y and Q

 (d) Explain two factors that affect the resistance of a metallic conductor

 

12.  Three identical cells of e.m.f. 2.Ov and of negligible internal resistance are connected as shown

  inImage From EcoleBooks.com figure below. Determine the ammeter reading.

 

 

 

 

 

 

 

 

 

 

Resistors Answers

1.   – The ammeter should be in series

  – Voltmeter in parallel

  – Variable resistor in series

  – The apparatus must be workable

2.  a) ii) gradient = 10v1 mk

0.5A

= 20
1 mk

 

  iii) 1 + 1 + 1 = 1
1 mk

   200 100 R 20

 1 = 1 – 1 – 1

R 20 200 100

 1 = 9 1 mk

9 200

= 20
1 mk

b) Current through 10  resistor = 2A

p.d across 10  resistor = 2 x 10

= 20v

 p.d. across 5  = 20v

current = 20 = 4A 1 mk  

  5

Total current 4 + 2 = 6A

 Current through 2  = 6A

p.d = 6 x 2 = 12V

 Total voltage = 12 + 20

= 32V

Image From EcoleBooks.comImage From EcoleBooks.com

3.  (i)   (ii)

 

 

 

b)  i) 2.1 V

ii) 2.1 v – 1.8V = Ir = 0.1V

r = 0.3 = 3

0.1

iii) 0.1 X R = 1.8 V

R = 18R

 

4.  – Length of conductor

– Type /nature of material

– Diameter/thickness of material

b)  E = IR + Ir

3.0 = I(3.5 + 0.5) = I (4.0)

I = 0.75A

5.   a) – The current passing through a conductor is directly proportional to the

Image From EcoleBooks.com  potential difference across its ends provided temperature and other

physical conditions are kept constant

 

6.   I = 1 + 1 = 7

  R 5 2 10

  R = 10/7

RT = 10 + 3 = 31 (Effective resistance)

7 1 7

but I = 12.4 x 7 = 2.8A

Image From EcoleBooks.com 31

p.d across the 3 resistor = 2.8 x 3 = 8.4V (p.d across the 3 resistor)

Image From EcoleBooks.comp.d across the 5 and 2 = 12.4 – 8.4 = 6.0V

Current across the 5 resistor = 6/5 = 1.2A  (answer 3mks)

7.  Rs = 3 + 4

 Rp = 7 x 5 = 35

Image From EcoleBooks.com 7 + 5 12

 II = 6 x 12 = 2.057A

  35

I through 3 resistor = I1

I1 x 5 = I2

Image From EcoleBooks.com5(2.05 –I2).25 – 5I2

Image From EcoleBooks.com10.25 -5I2 = I2

6I2 = 10.25 – UI2= 10.25 = 1.708A

  6

8.  a) (ii)

 (ii) Effective resistance:

 RE = 4 + 6 x 3

    3 + 6

    = 4 + 18

  9

    = 6

V = IR

    I = 12 = 2A

    6

 V = 2 x 4 = 8V

 

Image From EcoleBooks.com(ii) V = 12 – 8 = 4V

   V = IR

Image From EcoleBooks.com  I = 4 = 2 = 0.667A

    6 3

 

 (iii) Effective resistance in parallel

Image From EcoleBooks.com RE = 6 x 6 = 36 = 3

Image From EcoleBooks.com 6 + 6 12

 The potential drop will increase; hence the reading of V will decrease

Image From EcoleBooks.com

 (c) (i) Step-down- The voltage is reduced from 240V to 8V

Image From EcoleBooks.com   (ii) To reduce loss of energy due to eddy currents

 

(iii) VP = Np

Vs Vs


740 = 4800

Image From EcoleBooks.com 8 Vs

Vs = 4800 x 8

Image From EcoleBooks.com 240

= 20 x8 = 160turns

 

9  (i) – Set Galvanometer to zero balance by adjusting the variable resistor L.

Image From EcoleBooks.com – P.d across BD is therefore zero

Image From EcoleBooks.com – P.d across AB = P.d across AD

Image From EcoleBooks.comP.d across BC = p.d across DC

Image From EcoleBooks.comI1 flows through K & L (I1 = I3)

I2 flows through M and N (I2 = I4)

Image From EcoleBooks.comI1K = I2M

I3L = I4N

I1K = I2M

I1L I2N

Image From EcoleBooks.comK = M

Image From EcoleBooks.comL N

 

  (ii) The method does not depend on the accuracy of the current measuring instrument

Image From EcoleBooks.com (b) R = 0.38

25 0.62

  R = 25 x 0.38

Image From EcoleBooks.com  0.62

= 15.32

 

 

 

Image From EcoleBooks.com10.  C11 = 4f + 5 f = 9 f

I = 1 + 1 +

CT 3 9 3

3 + 1 + 3 = 7

Image From EcoleBooks.com   9 9

  CT = 9 = 1.29F

 

11.   (i) Minimum current is when y is at max resistance , i.e 100 (x and Y parallel)

  current I = 220V

100

= 2.2A    

 

 (ii) Maximum current is when R = 500 at y (when X and Y are parallel)

 I = 220V

50 = 4.4A  

 (c) (i) For the upper resistors in series

R = 1 + 4= 5

for the lower resistors in series

R = 2 + 3 =5

For the combined resistance of the parallel sets

I = 1 + 1 = 2

R 5 5 5

R = 2.50

  Total resistance = 2.5 + 5.50 = 8.00

 

 (ii) Current 1y = 0.5A

  Vy = 40 x 0.25A = 1.0V

 V2 = 2 x 0.25A = 0.5V

 Vy2 = 0.5V   *

 

(d) – Thickness/x-sectional area – Resistance is inversely proportional to the thickness of a

conductor

  – Length : Resistance is directly proportional to length of a conductor

 


 




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