RESISTANCE and SEMI CONDUCTORS

Breaking the Law!

(Ohm’s Law)

Learning Intentions

Recall how some devices do not obey Ohm’s Law

State the use of these devices in an electrical circuit

What is the resistance of the wire? At the molecular level what is physically

happening?

Calculating Resistance

Voltage is directly proportional to current so long as temperature and all other physical conditions remain constant.

I

VR

voltage in volts

resistancein ohms

currentin amps

Ohm’s Law states that –

The gradient of a voltage / current graph gives the resistance (R = V/I)

The V / I graph for an ohmic conductor is a straight line through the origin. This shows that its resistance is constant and that voltage current.

V (V)

I (A)

10

2

52

10

X

YgradientR

Types of Resistors Page 124 Not all components are designed to

obey Ohm’s Law (V will not be proportional to I)

They can control when the current can flow and how much can flow.

R

These components are also known as SEMI CONDUCTORS or Non-Ohmic Devices

Vol

tage

RI

VGRADIENT

Current

In the bulb as the voltage goes up, so will the heat energy

(vibrations of the atoms) and therefore the electrons find it

difficult to flow.

(1) Filament BulbResistance (shown by the gradient of the graph) is seen to increase as current increases

Current is not proportional to voltage!

Cur

rent

Voltage

Ohm’s Law is clearly not obeyed as voltage is not directly proportional to current.

The diode allows current to flow in one direction only – known as forward bias but only at a definite voltage (usually about 0.6V).

(2) Diode

Pump

Load

Valve

(3) Thermistor (ntc)Negative temperature coefficient (n.t.c.) thermistors have a high resistance when cold but a much lower resistance when hot. They are used as electrical thermometers to detect temperature changes. As the temperature rises, more charge carriers become available and the resistance falls.

Resi

stance

Temperature

(4) Light Dependent Resistor (LDR)

LDRs have a high resistance in the dark but a much lower resistance in bright conditions.

The light-sensitive part of the LDR is a wavy track of cadmium sulphide.

Light energy triggers the release of extra charge carriers in this material, so that its resistance falls as the level of illumination increases.

Cu

rren

t (fi

xed

PD

)

Light Intensity

They are used in electrical circuits to switch on components automatically.