The Potential Divider Electricity Lesson 10

Learning Objectives To know what a potential divider is. To derive and know how to use the potential divider equation. To know that the load affects the output of a potential divider To know the applications of potential dividers; to detect temperature or light levels.

Definition What is potential divider? A potential divider consists of two or more resistors connected in series with each other and a source of fixed potential difference. Draw a circuit diagram for this arrangement.

Circuit Diagram What is the pd across each resistor? We can use potential difference between points A and B as a supply for another component.

Uses of a Potential Divider To supply a fixed pd between any value between zero and the source pd. To supply a variable pd from a fixed one. To supply a pd that varies with physical changes e.g. position, temperature or light level

VSVS V1V1 V2V2 R2R2 R1R1 Another Example

Supplying a fixed pd The total resistance of the circuit = R 1 + R 2 The current through the resistors = V 0 /(R 1 + R 2 )

Deriving Potential divider equation For an unloaded potential divider the current is the same through both resistors So the voltage is proportional to the resistance

Potential divider equation If R1 >> R2 then V1 is more or less the supply voltage If R1 << R2 then V1 is close to 0 V. V 0 as an input to the potential divider and V1 as an output. The circuit itself provides a way to tap off a voltage between 0 V and V 0.

Confused? So what’s the difference between a variable resistor and a potential divider?

Confused? When a variable resistor is used the total resistance of the circuit is being altered. Also, the pd is not being taken across the variable resistor.

V INV V out

Using potential dividers Use as a volume, brightness or contrast control. Making and designing a circuit to use as a temperature sensor Making and designing a circuit to use as a light sensor Controlling logic devices

Effect of different resistive loads Output voltage is affected by load resistance Connecting a resistor across the output reduces the output voltage Shorting out across a bulb reduces the total resistance of the bulb – the wire is in parallel with the bulb

Questions A series circuit is connected as shown in the diagram. 1.What is the potential difference between A and B? 2.An additional resistor of 100 W is connected between the 50 W resistor and the cells. What is the potential difference between A and B now? 3.The additional 100 W resistor is now connected in parallel with the first 100 W resistor. What is the potential difference between A and B now?

Question How do we get a variable supply voltage from a fixed one? How can we measure physical changes in position, temperature or light level etc.

Answer A potential divider is one way of producing a variable p.d. A combination of a suitable sensor (angle / position sensor, thermistor or LDR) and a potential divider enables measurement / monitoring / control of physical changes.

Alternative formula The potential divider equation can be derived by rearranging the ratios above to give: V output = R1 / (R1+R2)  V input.