Meter Mishaps

Use of Ammeters and Voltmeters

Some V (energy) gets dissipated in ammeter.

Some current leaks through voltmeter.

Not really ideal

Potential Divider Circuits

• Sometimes elements on circuit need less than the total voltage potential (pd) .

• A circuit that can be adjusted to divide the p.d. called potential divide.

• Which type of circuit divides voltage among resistors?

• Series circuits divide total input voltage between resistors.

12 V

R 2R

4V 8V

The total 12-V is divided proportionally by resistors.

Potentiometers, Rheostats,variable resistors

• Placed in circuit to vary voltage available to load.

Variable R can dim bulb by increasing R using more E

• Can it make bulb go off?

• What resistance would be needed to extinguish bulb?

0 - 40 20

9-V

If R = 40, it gets 2/3 or Vtot, or 6 V. Bulb gets 3V.

If r = 0, it gets 0 or 0 V. Bulb gets 9V.

Sliding Contact Variable Resistors

6.0 V V

• Envision a long resisting wire.

• It’s as if wire is many resistors in series.

• The voltmeter can read the pd around some of the wire by touching specific points.

• What does it read in this position?

Simplest potentiometer has slider contact.

• At top (point S) V = 2 V• Halfway V = 1 V• At bottom V = 0V.

2V

• This is equivalent.

• Why?

2V

S

Ex: If there are 30cm above & 70cm below the contact, what is the voltmeter reading?

• Find the V reading around 70 cm.

• Can set up proportion.• Partial R = 70 cm• Total R = 100 cm

• V = 2V x (70/100)• 1.4 V

2V30cm

70cm

Ex 1: If there are 40cm above & 60cm below the contact, what is the voltmeter reading?

• The reading around 60cm.

• Partial R =• 60 cm.

• Total R =• 100 cm.

• V = 2V x (60/100)

• 1.2 V

2V40cm

60cm

I can add another resistor to the circuit and determine the reading on the voltmeter.

Ex 2. The maximum resistance of this variable resistor is 100 . It is placed in series with the 100 R. When the sliding contact is at point A, what will the voltmeter read?

• It will read 6V (the variable resistor is bypassed): the maximum reading of voltage in this circuit.

• What is the reading of the voltmeter when the sliding contact is moved to B?

We have, in effect, the following situation.

Therefore, the voltmeter will read 3V.

It is not possible to make the reading of the voltmeter vary from zero up to the full voltage of the supply using this circuit.

Ex 3: Now all 3 connections of the 100 variable resistor are used as below, what will the voltmeter reading be? X is a wire, the contact is at B.

• Zero. The contact bypasses the bulb like a short circuit. The wire x is in parallel with the bulb. Wire and bulb have 0V.

Ex 4. The variable resistor is 100 . Approximately what resistance does it supply on the circuit below? What will the voltmeter read?

• 50 , since resistance is directly proportional to length.

• 4 V

Ex 5:What is the voltage around the bulb?

9V

40

20

9 V

Uses of potentiometers

• To control volume.

• To control lights (dimmer switch).

Kerr 145 potentiometer

• Demo, explanation, problem.

• Nov 04 pg 15 - 18

• Kerr pg 150 #32.

• N10 S2 A3 Circuits.

• S. S2 A2. Sketch Circuit.

Use your data booklets to sketch the circuit with the potentiometer,

voltmeter, and bulb, using the IB symbols on the boards.

Common Applications of Potential DividesElectrical Sensors

• Devices whose resistance changes with changing physical conditions.

Light Sensor

LDR – light dependent resistorR decreases when light shines

Burglar Alarm electric eye

Symbol

In order to increase the reading on the voltmeter theA. temperature of R should be increased.B. temperature of R should be decreased.C. light intensity on R should be increased.D. light intensity on R should be decreased.

1. The diagram shows a potential divider circuit.

D

Thermistor – R varies with T. Usually the R decreases w/increasing temperature (counter to our wires).

Symbol.

Strain Gauge

When stretched R increases b/c it gets longer &

skinnier.

Find symbol put in notes.

When too much bend R increases w stretch, goes down

with compression

Using Potential Divide with Sensors

12V

LDR - When light stops shining, its resistance increases and V2 increases. If it V2 gets high enough, the switch will activate perhaps putting on lights. The p.d. divide is needed to create a p.d. to activate switch.

switch

Use of Sensors

Ex: The cell has an emf of 12-V and no internal resistance. The p.d. required to activate the switch is 5-V. Find the

value of R1 that will cause the switch to activate when the resistance of the LDR is 200 k.

switch

Rearrange the potential divide equation:

Vout = (Vin) R2

R1 + R2.

solve for R1.

R1 = (R2) Vin - Vout

V out.

Solve for R1.

280 k .

Which would be a good sensor to use with a fire alarm?

What would be a good use for a strain gauge?

Hwk. Read Hamper.115 – 123

do pg 123 #21