Resistors and resistance
How they all add up
Test dates!
• I am proposing that the test is done on the following dates;
• 12AWaPH1 Mon 19 Oct 1+2 with DJF
• 12CWaPH1 Tues 20 Oct 5+6 with JT
These are provisional, but assume they are correct
Starter motorhttp://www.youtube.com/watch?v=al6Yz3Nv7dY
What we are going to cover today
• Explanation of thermistors (finishing off)• Superconductors• Revision of resistances of different components• Resistors in series• Resistors in parallel• Calculating current from a known resistance• Starter motors• Potential divider circuits and uses
Semiconductor
Thermistor at T = 0K
Full of electrons
Full of holes (free space for electrons)
Thermistor above absolute zero
energy
At the hotter temperature electrons can get into the conducting band
SuperconductorsSome special material can become superconductors
The superconductor has to be cooled down to its critical temperature (Tc)
At the critical temperature the resistance fall to zero
These can be used to make low loss power lines and very good electromagnets
Resistance
Temperature (k)0
Tc
The world record Tc of 138 K
String the resistanceresistor
V
I
bulbV
I
Diode
V
I
0.7
Make sure you get the right axis
V =IR
Temperature dependant resistance
thermistorR
Temperature K0
resistorR
Temperature K0
Superconductor
R
TemperatureK0 Tc
Answersresistor
V
I
bulbV
I
Diode
V
I
0.7
Make sure you get the right axis
V =IR
Temperature dependant resistance
thermistorR
Temperature K0
resistorR
Temperature K0
Superconductor
R
TemperatureK0 Tc
Resistors in series
R1R1 R2R2 R3R3
The total resistance of the resistors are
Rt =R1 + R2 +R3
Resistors in parallel
R1R1
R2R2
R3R3
Rt R1 R2 R31 1 1 1= + +
Calculate the resistances and then measure them in real
circuits to prove it
R1R1 R2R2 R3R3
R1R1
R2R2
R3R3
What happens to the total resistance in the parallel circuit?
Quick questionsCalculate the resistance of these circuits
4Ω 12Ω
4Ω
12Ω
2Ω
6Ω
3Ω
Rt =R1 + R2 +R3
AnswersCalculate the resistance of these circuits
4Ω 12Ω
4Ω
12Ω
2Ω
6Ω
3Ω
4 + 12 = 16Ω
1/R = 1/12 + ¼R = 3Ω
Parallel part first1/R = 1/3 + 1/6R = 2Plus the 2 ohms in seriesR = 2 + 2R = 4Ω
Another question (because I could not fit it on the last slide!)
9Ω
4Ω2Ω
3V
1)Calculate the total resistance of the circuit2)The battery current3)The power supplied from the battery4) The power supplied to the 9 ohm resistor
P = I2R
V = IR
P = IV
P = V2 /R
Another question (because I could not fit it on the last slide!)
9Ω
4Ω2Ω
3V1)Calculate the total resistance of the circuit2)The battery current3)The power supplied from the battery4) The power supplied to the 9 ohm resistor
P = I2RV = IR
1) 6 ohms in parallel with a 9 ohm resistor1/6 + 1/9 = 0.277R = 1/ 0.277 R = 3.6 Ω
2) I = V/R I= 3/3.6 I= 0.83
3) P = IV P = 3 x 0.83 P = 2.5W
4) P = V2 /R P = 32 /9 P = 1 W
Flow of currentThe same number of electrons will flow in as they flow out
R1R1
R2R2
R3R3
A A
Measure the current, to discover how the electrons flow through each line
A
HomeworkPage 52 Q1 complete the table and answer b
A potential divider
R1
R2
V1
Vout
Vin