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