ElectricityAS level Physics

Contents:

• Static electricity• Electric fields• Electric charges• Electric current• Potential Difference• Resistance• Kirchhoff's Laws & Combination of

resistances• Resistance and resistivity

Static electricity

• You walk across the rug, reach for the doorknob and..........ZAP!!!

• ?????????• Everything in the world consists of tiny

particles called "atoms." Atoms contain even tinier particles called "protons," "neutrons" and "electrons."

Static electricity

• Sometimes, however, the outer layer of an  atom gets rubbed off. This creates atoms with a slightly more positive charge.

• The item which rubs off the outer layer of the  atom steals" some of the extra electrons, making it slightly negative.

Static electricity

• As you walk over carpet in socks, your feet rub electrons off the carpet, leaving you with a slightly negative static charge. When you reach for a doorknob, you get a shock  as electrons jump from you to the knob, which conducts electricity.

Different methods for charging an object

• Charging by conduction• Charging by induction • Charging by friction

Electric field

• It is the region around a charged particle or object within which a force would be exerted on a test charge.

• Every charged object will experience the effect electric field.

Electric field

texthttp://www.learnerstv.com/animation/animation.php?ani=86&cat=physics

Electric charge

• Charge is the fundamental quantity of electricity. (Electricity is all about charge.)

• No one can tell you what charge is. They can only tell you how charges interact.

Electric charges• Electric charge (often just called charge) comes

in two and only two types.• positive (+) and • negative (−) • The term neutral does not refer to a third type of

charge, but to the presence in a region of positive and negative charges in equal amount.

• The sum of identical positive and negative quantities is zero (0). This is what it means to be electrically neutral.

Charges

• Elementary Charge• 1.60 × 10−19 coulombs• the magnitude of the charge on an

electron or proton• Charge is quantized in multiples of the

elementary charge

Electric current

• Electric current is defined as the rate at which charge flows through a surface (the cross section of a wire, for example).

• It is a scalar quantity.• The direction of electric current is always

taken as from positive to negative.• Current is the flow of charged particles.

Potential differences

• Potential difference between two points in a circuit is the work done in moving unit charge (i.e. one coulomb) from one point to the other

• Unit of potential difference is volt.

Resistance and Resistivity

• What is resistance• I-V characteristics & Ohm’s Law• Superconductivity• Resistance and temperature• Resistivity

Resistance

• Resistance is the measure of opposition to electric current.

• This resistance serves to limit the amount of current through the circuit with a given amount of voltage supplied by the battery, as compared with the “short circuit” where we had nothing but a wire joining one end of the voltage source (battery) to the other.

More about resistances• For an electron, the journey from terminal to

terminal is not a direct route. Rather, it is a zigzag path that results from countless collisions with fixed atoms within the conducting material. The electrons encounter resistance - a hindrance to their movement. While the electric potential difference established between the two terminals encourages the movement of charge, it is resistance that discourages their movement. 

I-V Characteristics

• When the potential difference across(p.d) the conductor is varies, the current through the conductor will also vary.

• These two quantities are found to be linearly related to a metallic conductor.

• A graph plotted between these two quantities is called the I-V characteristics.

Ohm’s law

• For a metallic conductor at normal temperatures, the current flowing through the conductor is directly proportional to the potential difference across the terminals.

• This relationship is given as OHM’S LAW.• V = I R• Where V is the p.d and I is the current.• R the constant of proportionality is the

resistance of the conductor.

More on I- V characteristics

• I-V characteristic curves are generally used as a tool to determine and understand the basic parameters of a component or device and which can also be used to mathematically model its behaviour within an electrical circuit.

Metallic conductor• A resistor R satisfies Ohm’s law, I=V/R, so

its I-V characteristic goes through the origin and has slope 1/R.

• Resistance = 1/ gradient of the graph

The conductors which obey Ohm’s law is called ohmic conductors.

Resistance and temperature.

• A conductor which disobeys Ohm’s law is said to be a non- ohmic conductor.

• Metal filament of a lamp is an example for that.

• The filament has a very high resistance and so when current flows through the circuit the filament heats up and start to glow.

I-V characteristic for a non- ohmic resistor

Similar to the case of an ohmic resistor, the line passes through the origin

When the current and voltages are small, the graph is roughly a straight line.

When the voltages are high it start to curve showing the resistance value high.

Graph in detail

Temperature dependence of the resistance

• When the voltage is more the resistance is more in case of a filament lamp.

• It can be interpreted as the resistance variation for the filament is due to the increase in temperature.

Thermistors

• The name thermistor is a shortening of the words thermally sensitive resistor. 

• Thermistors are the temperature dependant resistors.

• Their resistance value changes rapidly with the temperature.

• They are mainly the oxides of manganese and nickel.

Thermistor circuit symbol

This thermistor circuit symbol is widely used within circuit diagrams.

Types of thermistors

• Negative temperature coefficient (NTC thermistor)   This type of thermistor has the property where the resistance decreases with increasing temperature.

• Positive temperature coefficient (PTC thermistor)   This type has the property where the resistance increases with increasing temperature.

Uses Of Thermistors• Thermistors can be used in a wide variety of

applications. They provide a simple, reliable and inexpensive method of sensing temperatures. As such they may be found in a wide variety of devices from fire alarms to thermostats.

Use of a negative temperature coefficient thermistor in industry.• Another thermistor application is as temperature

compensation devices. Most resistors have a positive temperature co-efficient, their resistance increasing with increasing temperature. In applications where stability is required, a thermistor with a negative temperature co-efficient can be incorporated.

Diodes

• Semi conductor diode is another non-ohmic resistor.

• A semi conductor diode allows current flow only when it is forward biased.

I-V characteristics of a diode

Diodes

• Forward junction potential (threshold potential) is the voltage applied to a forward biased junction so that the diode start conducting (resistance decreases).

• It has a value 0.7 V for silicon diodes and 0.3 V for germanium diodes.

• Breakdown voltage is the reverse voltage which a diode cannot withstand and it start conducting beyond that.

Resistance of a pure and an impure metal.

• Resistance also depends on the material of the conductor. If you pass electricity through a wire made of a pure metal it will  have less resistance than a wire which is made up out of a metal  alloy. This is because the atoms in a pure metal are all equal in size and the gaps between the atoms are also equal and so a current can  pass easily through the gaps, whereas in an metal alloy the atoms are of different sizes and so the atoms are at unequal intervals making it more difficult for a current to pass through the gaps.

Metals and alloys

Pure metal has an orderly arrangement while alloys or impure metals have a disorderly arrangement because of the different sized atoms.

Resistivity

• If the dimensions of a conductor(length and area of cross section of a conductor) do not change, it’s resistance will not change.

• If two conductors of exactly the same dimensions have a different resistance, indicates that resistance depends on one more factor other than length and area of cross section.

What is resisitivity

• Electrical resistivity (also known as resistivity, specific electrical resistance, or volumeresistivity) is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current.

• S. I unit is Ω -m

Resistance of a conductor depends on

• Length l• Cross-sectional area A• The material the wire is made from• The temperature of the wire

It can be written as ….

• resistance = resistivity × length / area

Where, ρ is the resistivity of the material.

http://phet.colorado.edu/en/simulation/resistance-in-a-wire

Graphical representation of resistivity of different materials

Graphics to illustrate how resistance varies with l, A and rho.

Resistivity of some materials

• CONDUCTORS• Aluminium 2.7 x 10-8

• Copper 1.7 x 10-8

• Iron 10.5 x 10-8

• Mercury 96 x 10-8

• voltage cables.)

INSULATORS•P.V.C. 5.4 x 1015

•Glass 1014

•Quartz 1012

•P.T.F.E 1012

(P.T.F.E.= polytetrafluoroethylene used to insulate high voltage cables.)

Resistivity and temperature

• For metals, resistivity increases as temperature increases.

• For semiconductors and many insulators, resistivity decreases with temperature.

With increase in temperature for a metal the collision frequency increases which results in increase in resistance.

CIRCUITS

• The invention of the battery -- which could produce a continuous flow of current -- made possible the development of the first electric circuits. Alessandro Volta invented the first battery, the voltaic pile, in 1800. The very first circuits used a battery and electrodes immersed in a container of water. The flow of current through the water produced hydrogen and oxygen.

Voltaic pile

• Constructed of alternating discs of zinc and copper, with pieces of cardboard soaked in brine between the metals, the voltaic pile produced electrical current. The metallic conducting arc was used to carry the electricity over a greater distance. Alessandro Volta's voltaic pile was the first battery that produced a reliable, steady current of electricity.

Voltaic pile

EMF – electromotive force

• The electromotive force (e) or e.m.f. is the energy provided by a cell or battery per coulomb of charge passing through it, it is measured in volts (V). It is equal to the potential difference across the terminals of the cell when no current is flowing.

Internal resistance

• When electricity flows round a circuit the internal resistance of the cell itself resists the flow of current and so thermal (heat) energy is wasted in the cell itself.

Internal resistance

• e = electromotive force in volts, V• I = current in amperes, A• R = resistance of the load in the circuit in

ohms, W• r = internal resistance of the cell in

ohms, W

For a power supply internal resistance may be due to the wires and components inside while for the cell it is due to the chemicals inside it.

If we plot a graph of terminal potential difference (V) against the current in the circuit (I) we get a straight line with a negative gradient.

A graph of terminal p.d. against current

Internal resistance and emf from the plot.

• We can them rearrange the e.m.f. equation from above to match the general experession for a straight line, y = mx +c.

• V= - Ir+ε• The slope from the plot gives –r(the

internal resistance of the cell).• The Y- intercept gives the e.m.f of he cell.

Due to internal resistance

• When the charges flows through external resistance and internal resistance of the power supply, some energy is lost as heat.

• The heating effect of cell when we use the cell to supply power for an electrical component is because some energy is used by the charges to do work against the internal resistance.

Effects of internal resistance

• A battery with low internal resistance delivers high current on demand. High resistance causes the battery to heat up and the voltage to drop. The equipment cuts off, leaving energy behind.

Low resistance, delivers high current on demand; battery stays cool.

High resistance, current is restricted, voltage drops on load; battery heats up.

Internal resistance of cell

• http://phet.colorado.edu/en/simulation/legacy/circuit-construction-kit-ac-virtual-lab

• Download circuit construction kit and make a circuit containing a cell, a switch and resistor .

• Connect an ammeter and voltmeter in the circuit.

• Right click the cell and change it’s internal resistance of the cell.

Internal resistance of the cell…

• With switch open measure the voltage across the cell. It gives the emf of the cell.

• Close the circuit and measure the voltage using the voltmeter. It gives the terminal voltage of the cell.

• Vary the resistance value by right clicking the resistance and repeat the experiment.

Potential dividers.

• A potential divider is a simple circuit that uses resisters(or thermistors / LDR’s) to supply a variable potential difference.

• They can be used as audio volume controls, to control the temperature in a freezer or monitor changes in light in a room.

• Two resistors divide up the voltage supplied to them from a cell. The p.d. that the two resistors get depends on their resistance values.

• Vin = p.d. supplied by the cell• Vout = p.d. across the resistor of interest• R1 = resistance of resistor of interest R1

• R2= resistance of resistor R2