Electricity

What is drift velocity

• the velocity of electrons that travel in the opposite direction of a conventional current

• the average velocity go charge carriers travelling through the conductor

What is the equation f current and drift velocity

• I = nAvq

• This equation is used to work out current for a conductor

How to observe and estimate drift speed? (Experiment)

• soak filter paper with ammonium hydroxide

• A small crystal of copper sulfate and potassium permanganate are carefully placed near the center of the slide

• Switch on the power supply

• The movement if blue copper ions and purple permanganate ions can be observed and estimate velocities.

When switching on light, why light comes on instantly when the drift velocity of electrons is very small?

• electric field that causes electrons to move travel at near;y the speed of light

• So all electrons starts to move instantly

• As there are so many of the electrons

• The charge flowing per second equates of a significant current

What is the definition of potential difference

The electrical energy per unit charge converted into other forms of energy

What are the change in energy in a circuit from battery? Why this cause temperature in a lamp that’s connected to circuit to rise?

• a chemical reaction in the cells reates and elector,olive force

• This gives rise to an electrical potential difference across cell terminals. Electrons in the circuit experience a force

• When the switch is closed, this force causes the electrons to move around circuit and through the lamp as work is done on the electrons

• The chemical potential energy hence is converted to kinetic energy of atoms in the lamp, causing temperature to rise

What is the electromotive force?

• energy per unit charge converted into electrical energy by the source

• A form of energy transfer

What is a rheostat?

• control current in a circuit, by connecting between battery and lamp, so current/resistance can be changed

• Give a continuously variable potential difference I.e. a potential divider, by connecting across battery so changed the voltage of battery supplied to the lamp

How to investigate the I-V characteristics for a length of resistance wire/tungsten filament lamp/diode/thermistor

• connect 1m of nicotine wire which is taped to a meter rule using crocodile clips.

• P.d. Is set to minimum by sliding rheostat to the very end

• P.d. is gradually increased and the value of pd and current are recorded by reading from voltmeter and ammeter

• Experiment is repeated with terminals of cell reversed for negative data of current

• Plot current against voltage graph

• Should observe straight line through the origin

• For a semi-conductor diode, a resistor is connected to limit the current in the diode and prevent any damage.

What is Ohmns law

• for metals at a constant temperature, the current in the metal is proportional to the potential difference across it.

What caused resistance?

• the collision of electrons with vibrating lattice ions as the electrons drift through the material of the conductor.

• As a result of collisions, some energy dissipated to thermal energy.

Explain the IV graph for a filament bulb.

• the higher the current, the higher the temperature of the metal filament

• The higher the temperature, the higher the KE of metal ions so they vibrate more

• So resistance increases as this makes electrons/current harder to flow

Core Practical 2 - measuring the resistivity of graphite

• diameter of graphite lead is measured with a micrometer or digital calipers, at force places along the length at different orientations to give a good average

• Lead is taped to half meter rule, connect to circuit with crocodile clips

• Firm contact is made at different length, e.g. every 10cm

• Current and pd are recorded

• R can be calculated for each value of length

• A graph of R against length of wire is then plotted

• There might be random error caused by variation in the pressure applied when making contact with pencil lead or caused by the inconsistent in the composition of the graphite

• There might be systematic error caused by contact resistance due to poor contact between crocodile clips and lead.

• Graphical method minimises random error by averaging out the values and eliminates systematic error when gradient is taken

Why resistance increase with temperature in a metal conductor

• there is increase vibrations of lattice which reduces the drift velocity of electrons and current will decrease so resistance increases

• n I.e. number of electrons in a given volume, does not change with temperature in metal conductors

Why is it that in a thermistor/semiconductor, resistance decreases when temperature increases?

• In a thermistor, an increase in temperature can provide extra energy to release more charge carriers, so n increases with temperature (almost exponentially)

• This is because more conduction electrons are freed

• N increases therefore much more than relatively small decrease in velocity

• So the overall effect is that current increases so resistance decreases

Why if the temperature is high enough, even insulators can begin to conduct?

• energy associated with the very high temperature breaks down the atomic structure so that more charge carriers are released

Investigatiom of the effect of temperature on the resistance of a thermistor

• start with thermistor immerse in crushed ice

• Lowest steady temperature and corresponding resistance are recorded

• Beaker of ice heated up for intervals of 10 degree

• Ensure leads are kept away from flame and hot gauze

• Water is stirred thoroughly before taking readings

• Record till 100 degree

• Plot resistance against temperature

What is internal resistance

• opposes flow of charge through cell

• Some energy converted by the cell or power supply will be used up inside the cell to overcome this resistance

• EMF = Ir+IR

Core Practicl three -investigation of emf and internal resistance for a cell

• starting with rheostat at ta highest value

• Current in cell and of across cell terminals are recorded for different settings of the rheostat

• Graph of v against I is plotted

• Internal resistance is the gradient and emf is the intercept

How is a solar cell used for generating power?

• when light strikes photocell, light gives photocell energy to free electrons in the semiconductor material of the cell. An electric field within the cell provides a force on the electrons

• Electrons flow which proivides current and cells electric field causes a voltage, hence we have power

• Characteristics of a soar cell depress on illumination. If cell were illuminated strongly, the emf of cell would be much greater o internal resistance would decreases. The max power generated would be much greater

Why we should se ammeter with very small resistance and voltmeter with very large current

so both device does not affect readings

Core practical 12-use terms tie in a potential divider circuit to activate a thermostatic switch

• ohmmeter used to measure resistance of thermistor at room temperature

• Resistor is selected such that its resistance R2= R1

• Output voltage for temperature ranging to 60 degree are measured and a graph of Voltage out against temperate is plotted

• Graph can be used to determine about at specific temperatur

• A simple electronic circuit can be designed to operate a switch when output from circuit reaches a predetermined value

• E.g if a switch is set to operate at 40 degree, then the circuit could be used o switch on a lamp when temperature reaches 40

Why LDR material has a high resistance

• there are very few electrons that are free to conduct , mainly locked into the crystal lattice and unable to move, only vibrate

Why in LDR, light increases resistance decreases?

• when intensity of light increases

• light photons incident upon LDR, which release conduction electrons/ gives electrons sufficient energy to break free from the crystal lattice and conduct electricity

• Number of lecterns per volume,n, increases

• Hence resistance decreases

• The more the light shines on LDR, the more number of electrons are released

• LDR can be used as a potential divider

Why does emf of a supply differ from its terminal potential difference

Some of the energy that is transferred to charges is def to overcome the internal resistance of the power supply

Desribe a method to determine the terminal potential difference of a battery

Attach a voltmeter directly across the terminals of the artery when it is isolated. Since no current flows, no potential is lost due to internal resistance and so voltmeter will read the clue of terminal pd.

What is semiconductor

A component that can change its resistance based on external conditions

What is a potential divider

A pair of resistors send to split the pd over resistors in a given ratio.

Explain why bulb 1 in series with bulb 2 and switch 1 changes when switch 2 that’s in series with bulb 3 but parallel to bulb 1 is turned on also

• clobefore turning on switch 2, both bulb 1 and bulb 2 have the same brightness as current/voltage is the same

• closing switch 2 allows current to pass through switch 3

• Bulb 1 and bulb 3 in parallel, so combination of resistance decreases

• pd across 1 decreases, across 2 increases

• current in bulb 2 increases, so brightness increases/ current in bulb 1 decreases, so brightness decreases

• bulb 1 is not as bright as bulb 2

what is terminal pd

what is the supply pd

• emf - Ir, internal resistance causes terminal pd to be less than emf

• emf

When ask to figure out which circuit with potential divider/ variable resistor is more effective…

• talk about range of pd, sometimes minimum pd can be 0V, but sometimes like when potential divider is in series with the bulb, the minimum pd cannot be 0V, so the range is shorter

what is kirchoff’s second law

when energy is conserved so the sum of pd = emf supplied