Download Electric Circuits

Electric Circuits
and
Resistance
M.D.
The Professional Development Service for Teachers is funded by the
Department of Education and Skills under the National Development Plan
Electric Circuits
V
+
Voltage ‘pushes’ the current
-
switch
R
resistance restricts the current
V  IR
I is the current
A
actual current
lamp
conventional current
PDST Resources for Leaving Certificate Physics
2
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.
V
R
I
PDST Resources for Leaving Certificate Physics
3
Resistance and Temperature
metal

Semiconductor
Heat source
Heat source
R/

R/
T / oC
T / oC
PDST Resources for Leaving Certificate Physics
4
Resistors in Series and Parallel
Series
R1
Total Resistance:
Parallel
R2
RT  R1  R2  R3
R3
R1
R2
R3
Total Resistance:
1
1
1
1
 

RT R1 R2 R3
PDST Resources for Leaving Certificate Physics
5
Resistance and Resistivity
Which has higher resistance?
Rl
l
A (= π r 2)
plastic
1
R
A
R
l
A
Resistivity, ρ
R
PDST Resources for Leaving Certificate Physics
6
2005, Q.9
PDST Resources for Leaving Certificate Physics
7
In the circuit diagram, the resistance of the thermistor (a
semiconductor) at room temperature is 500 Ω. At room temperature,
calculate
(i) the total resistance of the circuit;
(ii) the current flowing through the 750 Ω resistor. (20)
6V
PDST Resources for Leaving Certificate Physics
8
In the circuit diagram, the resistance of the thermistor (a semiconductor) at room
temperature is 500 Ω. At room temperature, calculate
(i) the total resistance of the circuit;
(ii) the current flowing through the 750 Ω resistor. (20)
6V
RT  R1  R2
 300  300
 600 
Rp
1
1
1
 
R p R1 R2

1
1

500 750

1
300
R p  300 
PDST Resources for Leaving Certificate Physics
9
In the circuit diagram, the resistance of the thermistor (a semiconductor) at room
temperature is 500 Ω. At room temperature, calculate
(i) the total resistance of the circuit;
(ii) the current flowing through the 750 Ω resistor. (20)
6V
V 6V
RT  600 
V  IR
6  I (600)
IT  0.01 A
500
0.01
750
2
I 750  (0.01)
5
 0.004 A
Resistances:
currents:
PDST Resources for Leaving Certificate Physics
500:750
2:3
3:2
3 2
:
5 5
10
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 Ω combination) decreases
(3)
the potential drop (V=IR) across the thermistor and 750 Ω combination also
therefore decreases
(2)
and the potential at A increases
(2)
PDST Resources for Leaving Certificate Physics
11