controlling current and voltage

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Controlling current and voltage


Resistance revision


Variable resistors

A variable resistor has two potential paths for current: one along a short, thick bar; another along a thin long coil.

The slider is a mobile point of contact between these two routes, and its position determines the path of the current.

A variable resistor, also known as a rheostat, allows the resistance of a circuit to be varied.

variable resistorvariable resistor

symbol

slider thick bar

coil


How do variable resistors work?


Ohm’s Law links current, voltage and resistance:

×

Ohm’s Law

voltage (V) = current (I) × resistance (R) Volts (V) Amps (A) Ohms (Ω)

A formula triangle can be used to rearrange this equation.

Ohm’s Law explains why resistance helps to control the current and voltage in a circuit. Any changes in resistance will have a knock-on effect on both the current and voltage.


A filament lamp has a current of 20 A running through it, with a potential difference of 100 V across it.

V = I × R

Ohm’s Law practice questions

R =

100 V = 5 Ω

What is the resistance of the filamentin the bulb?

V

57.5 Ω= 4 A

Calculate the current flowing through a 230 V kettle element which has a resistance of 57.5 Ω.

V = I × R

V

I=

20 A

I =R

230 V=


Voltage–current graphs

Ohm’s Law tells us that the gradient of a V–I graph can be used to calculate resistance:

cu

rre

nt

(A)

voltage (V)

Voltage–current graphs are a simple plot of voltage, on the x-axis, against current, on the y-axis.

change in current change in voltage

voltagecurrent

1gradientTherefore:

Voltage–current graphs can vary greatly in form depending on the properties of the substance conducting electricity.

with these axes.

gradient = resistance =

resistance =


Calculating resistance from line graphs

nichrome

voltage (V)

2

0

4copper

cu

rre

nt

(A)

2

0

4

0 5 10 15 20

Calculate the resistance of these copper and nichrome wires.

1gradient

change in current change in voltage

copper:

nichrome:

gradient = 2 ÷ 5

gradient = 2 ÷ 10

= 0.4

= 0.2

R = 1 ÷ 0.4 = 2.5 Ω

R = 1 ÷ 0.2 = 5 Ω

voltage (V)0 5 10 15 20

resistance = gradient =


Different types of V–I graph

While a resistor produces a constant resistance, and thus a straight line graph, other components show a variation in resistance at different levels of current and voltage.

Such variation in resistance leads to a curved V–I graph.

A light bulb has a curved graph: it warms up as more electricity passes through it, increasing resistance.

cu

rre

nt

(A)

voltage (V)

Such components are non-Ohmic – they do not obey Ohm’s Law.


V–I graphs for diodes

A diode is a component that stops current flowing in one direction, but allows it to flow readily in the other, providing it is over a certain voltage.

What would a V–I graph for a diode look like?c

urr

en

t (A

)

voltage (V)


V–I graphs for different components


Calculating resistance from curves


Ohm’s Law summary


Understanding voltage

When voltage is measured across a component it records the difference in electrical potential energy between the two sides of the component. This is also known as the potential difference.

Thus the voltmeter reading of 4 V tells us that there is 4 V more electrical potential energy on one side of the resistor than the other.

4.0

Voltage is an electrical pushing force.

The voltage of a cell describes how much electrical potential energy it gives the electrons: this pushes them around a circuit.


Controlling voltage

Imagine your alarm clock’s battery is flat.

It requires 4 V to run successfully, but you only have a 6 V battery. This will overload its circuitry.

However, you do have a selection of fixed resistors.

How can these resistors help you to run the alarm clock from the battery without damaging it?


If two resistors are connected in series with a power supply, then the voltage is shared out between them.

10Ω 20Ω

Series resistors and potential difference

The voltage is divided between components in proportion to their resistance. Thus the larger resistor has a larger share of the power supply voltage.

2.0 4.0

6 V


What is a potential divider?

10 Ω 20 Ω

2 V 4 V

6 V

VINThis principal can be used to power the alarm clock.

The clock itself has a resistance of 20 Ω.

When placed in series with a 10 Ω resistor, the battery’s voltage is split between the resistor and clock in a 2:1 ratio.

A circuit that splits the voltage between components, to produce a specific output voltage, is a potential divider.

The voltage across the resistor is 2 V, while the voltage across the clock is 4 V. The clock can now run safely.


Potential dividers are drawn in a slightly different way to other circuits.

6 V

4 V

0 V 0 V

10 Ω

20 Ω

Drawing potential dividers

VIN

VOUT

R1

R2

potential divider diagram

This arrangement is designed to visually demonstrate the change in potential difference across the resistors.

The distance between the horizontal lines represents the potential difference between different parts of the circuit.

A potential divider uses series resistance to produce an output voltage (VOUT) that differs to the input voltage (VIN).


Fixed output potential dividers


The output voltage (VOUT) of a potential divider depends on the size of the resistors, and also the input voltage (VIN).

VOUT and VIN are measured in volts (V).

The potential divider equation

R1 and R2 are measured in ohms (Ω).

VOUT can be calculated using the potential divider equation:

VOUT = VIN × R2 (R1 + R2)

VIN

VOUT

0 V 0 V

R1

R2


10 V

VOUT

0 V 0 V

15 Ω

60 Ω

Calculate the output voltage, VOUT, for this potential divider.

R2

(R1 + R2)

60

15 + 60

60

75

= 8 V

Potential divider questions

= 10 × 0.8

R1

R2

VOUT = VIN ×

= 10 ×

= 10 ×


10V

VOUT

0 V 0 V

75 Ω

300 Ω

Calculate the output voltage, VOUT, for this potential divider.

R2

(R1 + R2)

300

75 + 300

300

375

= 8 V

Potential divider questions

= 10 × 0.8

R1

R2

VOUT = VIN ×

= 10 ×

= 10 ×


Variable resistors in potential dividers


Potential dividers with a variable output

If a variable resistor is used in a potential divider,VOUT becomes variable.

If R1 is a variable resistor…

VOUT is low when the resistance of R1 is high.

VIN

VOUT

0 V 0 V

R1

R2

R1 has a high proportion of the resistance, and thus a high proportion of the voltage.


Potential dividers with a variable output

What happens when R2

is a variable resistor?

VOUT is high when resistance of R2 is high.

VIN

VOUT

0 V 0 V

R1

R2

R2 has a high proportion of the resistance and thus a high proportion of the voltage is at VOUT.

In this arrangement, the relationship between the resistance of the variable resistor and VOUT inverts.


Potential divider summary


A semiconductor is a material which has electrical properties somewhere between an insulator, such as wood, and a conductor, such as iron.

Some semiconductors are able to vary their conductivity in response to changes in temperature or light intensity.

Semiconductors

Semiconductors are usually made from silicon.

computer processor light dependent resistor light emitting diode

Uses for semiconductors include:


The resistance of a light dependent resistor (LDR) is not fixed. It is dependent on the intensity of incident light.

res

ista

nce

(k

)

light intensity

LDRs: light and resistance

An LDR has a high resistance in the dark but a low resistance in the light.

This means that LDRs can be used in light sensing circuits, because their output is dependent on the light conditions.

The graph shows how the resistance of an LDR decreases as the light intensity increases.

LDR symbol


The resistance of a thermistor varies depending on temperature.

Thermistors: temperature and resistance

This is unusual, as resistance normally increases with increasing temperature.

Thermistors are useful in the sensor circuits of a thermostat, as their output varies with temperature fluctuations.

It has a high resistance when cold but a low resistance when hot.

res

ista

nce

(k

)

temperature (°C)

thermistor symbol


How do semiconductors work?


10 V

VOUT

0 V 0 V

R1

thermistor

The combination of a potential divider and a thermistor creates a temperature sensor.

Semiconductors as sensors

The thermistor’s resistance will vary with temperature, resulting in a VOUT that is temperature dependent.

If the thermistor is in the R2 position, VOUT will be high at low temperatures, as the thermistor’s resistance will be high relative to R1.

How will VOUT change if the thermistor is in the R1 position?

To produce a light sensor, replace the thermistor with an LDR.

R2


Comparing conductors and semiconductors


Glossary


Anagrams


Controlling current and voltage quiz

controlling current and voltage

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