PHYA1 - Electricity

Current - The rate of flow of charge

• Current is due to the flow of charge carries (e.g. electrons or ions)
• To make current flow there must be a source of potential difference (e.g. battery)
• Conventional current flows around a circuit from the positive (+) to negative (-). However the electrons are flowing around the circuit in the opposite direction from the negative (-)  to positive (+)

Potential difference - Work done per unit charge

• The battery supplies each electron with electrical potential energy
• When an electron passes through a component it does work
• The amount of work done by an electron equals its loss in electrical potential energy

EMF - electrical energy produced per unit charge passing through the source

Resistance is defined as:

• unit - ohms(Ω)
• A component's resistance its magnitude of opposition to the flow of charge

Ohm's Law

• The p.d. across a metallic conductor is proportional to the current through it, provided physical conditions do not change

Measuring resistance

• Put resistor in series with an ammeter and in parallel with a voltmeter
• Change the resistance of the variable resistor in steps, measuring the current and p.d. at each step
• Plot a graph of V against I, gradient = resistance

Resistivity is measured in ohm metres (Ωm).

• ρ = the resistivity of the material in Ωm
• R = the resistance of the material in Ω
• A = cross-sectional area of the material m²
• L = length of the material in m

• Current is always shown from positive to negative in circuit diagrams
• All sources of light (except a LED) have the same symbol
• A diode allows current in one direction only, used as d.c protection
• The resistance of a thermistor decreases with increasing temperature
• The resistance of a LDR decreases with increasing light intensity

At any junction in a circuit the total current leaving the junction is equal to the total current entering the junction

For components in series:

• The current through each component is the same
• The sum of the voltages across each component is equal to the battery's voltage
• The voltage across each component is equal to its ratio of resistances
• Total resistance = the sum of the individual resistances

For components in parallel:

•  total current  in the circuit equals the the sum of the current through each component 
• P.d. across each component is the same
• Total resistance equals the inverse of the sum of inverse individual resistances

• The heating effect of current is due to the resistance of a component
• The charge carriers repeatedly collide with the metal ions
• The power/thermal energy/heat dissipated from a component= I²R

Internal resistance is due to the opposition to the flow of charge within the source

EMF is defined as the electrical energy per unit charge produced by the source

• pd across the terminalsrefers to the energy delivered to thecircuit(once the energy has exited the battery)
• Internal resistance is defined as the loss in p.d. per unit current that passes through the source

Terminal p.d. decreases when the current is increased because the lost volts (Ir) increases

• Diodes only let current flow in one direction. This is useful to protect d.c. circuits
• Diodes usually allows current to pass when the voltage is around 0.6v in the foward direction
• When the diode is reverse biased current will eventually flow at the breakdown voltage

• When current flows some energy is transferred as heat, causing the metal to heat up
• This increases the energy of the electrons so they collide more. Resistance increases
• When a lamp is first switched on its cold so more current than usual flows through it
• Rapid temp change could cause bulb to blow

Thermistors, diodes, LDRs

• Semiconductors aren't (intially) very good condutors
• They have fewer charge carriers than metals
• However, when energy is supplied to it (through heat etc.) more charge carriers are released and so current increases

NTC and PTC components

• NTC stands for Negative Temperature Coefficient
• PTC stands for Positive Temperature Coefficient
• For NTC components resistance decreases as temperature increases. Basically resistance and temperature are negatively correlated

If you cool a material down enough its resistivity reduces to zero and becomes superconducting.

• When a material's temperature is below its critical temperature, it's resistivity is zero
• When current passes through it there is no heating effect; no energy is wasted as heat

A potential divider consists of 2 or more resistors in series.

Using this:

• A particular voltage can be delivered across a component
• A variable p.d.
• P.d. that varies with a physical condition

Fixed p.d.

• The p.d. across each resistor is in proportion to the ratio of resistances of the resistors

Sliding contact can be moved to vary p.d. between one end of the wire and the contact

• Fig 4 - p.d. across thermistor can be varied with the variable resistor

Alternating current (a.c.) is a current that repeatedly reverses its direction

• The frequency of an alternating current is the number of cycles that pass a fixed point in one second
• The peak value is the maximum current/p.d. in either direction which is the same in each direction
• Peak to peak value is twice the peak value
• Graph is sinusoidal (resembles a sine curve)

How to use an oscilloscope

• Connect to y-input
• Adjust the time base so the screen displays however many waveforms you wish
• Adjust y-gain/sensitivity

How to investigate how resistance thermistor varies with temperature

• Thermistor connected with voltmeter in parallel, ammeter in series
• The candidate gives details of how the thermistor is heated in a beaker of 
• water or a water bath and a thermometer is used to measure the 
• temperature at small regular intervals. 
• The candidate states that the resistance is found at various temperatures 
• either directly with an ohmmeter or by dividing voltage by current. 
• The candidate may mention that the water must be stirred to ensure that the 
• thermistor is at the temperature measured by the thermometer. 
• The candidate may give some indication of the range of temperatures to be 
• used. 
• The candidate may refer to repetition of whole experiment. 
• The candidate may plot a graph of resistance against temperature. 
• The candidate may use a digital thermometer.

How are photoelectrons affected by lower frequency

• Photons have less energy so the maximum kinetic energy of the particles decreases

How are photoelectrons affected by greater intensity?

• There are more incident photons per second so more electrons emitted per second

• An excellent candidate will have a working circuit diagram with correct description of measurements including range of results and processing. 
• Uses a range of results and finds a mean value
• Uses a graphical method, eg I-V characteristics. They also mention precision eg use of vernier callipers. 

• length with a ruler 
• thickness/diameter with vernier callipers/micrometer 
• measure voltage 
• measure current 
• calculate resistance 
• use of graph, eg I-V or resistance against length 
• use of diameter to calculate cross-sectional area 
•  mention of precision, eg vernier callipers or full scale readings for V and I 
• flat metal electrodes at each end to improve connection 

The explanation expected in a competent answer should include a coherent account of the significance of discrete energy levels and how the bombardment of atoms by electrons can lead to excitation and the subsequent emission of photons of a characteristic frequency. 

• electrons bombard atoms of vapour and give energy to electrons in atom 
• electrons move to a higher energy level 
• electrons are excited 
• excited electrons move down to lower energy levels losing energy by emitting photons 
• photons have energy hf 
• photons of characteristic frequencies emitted from atoms of a particular element this is because atoms have discrete energy levels which are associated with particular energy values