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					Resistance
Warm-up
 1 Margaret adds light bulbs in a circuit
   gradually until bulbs are very dim. Why are
   the bulbs very dim?
     Resistance of each bulb becomes large.


    Current becomes too small.


    Voltage across each bulb becomes too small.


     Others:_______________________
Warm-up
2 How does wetness affect the resistance of our
  body?
        Increase.


       Decrease.


        No change.
  Introduction
Current passes through some conductors more
easily than others.

The opposition of a conductor to current is called resistance.




Good conductor                        low resistance
Bad conductor                         high resistance
1 Resistance
 Resistance of a conductor is...

                  voltage across conductor
 resistance =
                  current through conductor

                           V
                    R=
                           I


 Unit: ohms ()       (1  = 1 V A–1)
1 Resistance
  If 1 A flows through a resistor and
  the voltage across the resistor is 1 V,
  the resistor has 1 .



                            1V
        1A

                          resistor   =1
    Example 3
     Resistance
When an iron is plugged into the 220 V mains supply, a current of
5 A flows through the heating element. What is the resistance of
the heating element?




                          V
   Resistance      R=
                          I
                           220
                      =           = 44 
                             5
 Experiment 14a
Ohm’s law

  Use this circuit to
  find out the relation
  between the current
  passing through the
  wire and the voltage
  across it.
Ohm’s law
In Expt 14a, the V-I graph obtained is...

voltage / V
                                            straight line
                                           passing origin
 4
                           
 3                                              VI
                                               (Ohm’s law)
2
               
 1      
                                        current / A
 0     0.2    0.4   0.6   0.8   1.0
2 Ohm’s law
Ohm’s law:

Voltage across a conductor  current passing it, if the
temperature and other physical conditions are unchanged.




Note:

Ohm’s law applies to some conductors only.

If a conductor does not obey Ohm’s law , its resistance is still V/I
    but it changes.
3 Measuring resistance
 Measure the current passing a      12 V power supply
 bulb and the voltage across it.         +        –


                                                   12 V 24 W
 Then by R = V/I, the resistance
                                       A            •          •
 of the bulb can be found.
                                                         V

 This method is called voltmeter-ammeter method.
4 Factors affecting resistance
  a Temperature
 For most metals,

  temperature                 resistance


  But for carbon and some semiconductors,

  temperature                 resistance
    Experiment 14b
Effect of length and thickness on resistance




                                         How does the resistance of a
                                         wire change with the length
                                         and the thickness?
4 Factors affecting resistance
  b Length and thickness of wire
  Wires of the same thickness and material,
  resistance R  length l                (l   R )


  Wires of the same length and material,

  resistance R  (cross-sectional area A)–1

  (A   R )
4 Factors affecting resistance
b Length and thickness of wire
lR      and       AR


                 l
           R
                 A
       5 Resistors
a Fixed resistors
Resistor = a device with known resistance

E.g.




Fixed resistors as their resistance is fixed.
   5 Resistors
b Variable resistors
Their resistance can be changed.


                                     movable
E.g.
                                     contact


                                    resistance
                                    track

Rotate the slider to vary the length of the
resistance track and hence the resistance.
   5 Resistors
c Rheostats
They are large-current
                                                             A
variable resistors used in
school laboratory.
                                    B
Slid the contact
to vary the length of wire through which the current flows
                           change its resistance

Connect it at A and B  fixed resistor
  6 The multimeter
Multimeter can measure current, voltage and
resistance.
                           multi-range for
                           measurements




                           red and black probes
                           touch different points of
                           the circuit
Experiment 14c
 Using the multimeter


 Use a multimeter to measure the
 current through, the voltage across and
 the resistance of a lamp.
 Q1        When a resistor is...
An iron nail replaces a resistor. The            0.4 A 
voltmeter reading is 1.4 V before                0.2 A
the replacement. What is the
voltmeter reading after replacement?

                                                 1.4 V
                                                 ?V


 A Halved to 0.7 V.                B Remains at 1.4 V.
 C Slightly larger                 D Doubled to 2.8 V.
   than 1.4 V.
Q2      A V-I graph of a bulb...
A V-I graph of a bulb is            voltage / V
obtained.                           4
                                    3
   1st statement:                   2
   At V = 0, the resistance of
   the bulb is 1 .                 1

                                    0       1   2   3     4
                                                       current / A

  2nd statement:
  The slope of the V-I graph is 1
  .                                               A
 Q3      Voltage across a...
 Voltage across a resistor  current passing
 through it. What would happen to the current
 passing through a resistor if both voltage across
 it and its resistance are
(a)   doubled,
                    (increases/decreases/unchanged)
(b) halved?
                    (increases/decreases/unchanged)
  Q4        2 metal wires, P and...
2 metal wires, P and Q, of the same metal have the same length
but different radii. The dimension of P is known. Can the radius of
Q be found with voltmeter, ammeter and cell?




                       can
  The radius of Q __________ be found.

  Set up a circuit, with metal wire connected in _______
    series                             ammeter
  with the cell and the __________ and in ________ with
  voltmeter.
    parallel
 Q4       2 metal wires, P and...
2 metal wires, P and Q, of the same metal have the same length
but different radii. The dimension of P is known. Can the radius of
Q be found with voltmeter, ammeter and cell?



            V = IR
Then by _________ , the resistance of P and Q, RP
and RQ, can be found.
Since R  lA–1, where l is _______ and A is
                              length
cross-sectional area
_________________ of the wire, compare RP and RQ
and use A = ___ tor2 the radius of Q.
                    find

				
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posted:10/4/2011
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