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1
Electric Circuitsand Resistance
M.D.
2
Electric Circuits
V
Voltage pushes the current
-
switch
resistance restricts the current
R
I is the current
A
lamp
actual current
conventional current
3
Resistance
Resistance slows down the movement of electrons
and therefore reduces the current
The resistance of an object is the ratio of the
voltage applied across its ends to the current
which flows through it.
4
Resistance and Temperature
metal
Semiconductor
?
?
Heat source
Heat source
R / ?
R / ?
T / oC
T / oC
5
Resistors in Series and Parallel
Series
R1
R2
R3
Total Resistance
Parallel
R1
R2
R3
Total Resistance
6
Resistance and Resistivity
Which has higher resistance?
l
A ( p r 2)
plastic
Resistivity, ?
7
2005, Q.9
8
In the circuit diagram, the resistance of the
thermistor (a semiconductor) at room temperature
is 500 ?. At room temperature, calculate
(i) the total resistance of the circuit
(ii) the current flowing through the 750
? resistor. (20)
6V
9
In the circuit diagram, the resistance of the
thermistor (a semiconductor) at room temperature
is 500 ?. At room temperature, calculate
(i) the total resistance of the circuit
(ii) the current flowing through the 750
? resistor. (20)
6V
Rp
10
In the circuit diagram, the resistance of the
thermistor (a semiconductor) at room temperature
is 500 ?. At room temperature, calculate
(i) the total resistance of the circuit
(ii) the current flowing through the 750
? resistor. (20)
6V
500
Resistances 500750
23
0.01
currents 32
750
11
As the temperature of the room increases, the
resistance of the thermistor decreases. Explain
why the potential at A increases (7)
6V
Marking Scheme The resistance (R) of
thermistor (and 750 O combination) decreases
(3)
the potential drop (VIR) across the
thermistor and 750 O combination also
therefore decreases (2)
and the potential at A increases
(2)