# resistance

Document Sample

```					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
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

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

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 10 posted: 10/4/2011 language: English pages: 25