Electricity

Current and charge

Conventional Current - positive to negative (made before the discovery of electrons)

Q=IT

• Insulator - each electron is attached to the atoms, cannot move. No current flows when a voltage is applied, because no electrons can move through the insulator

• Conductor - Delocalised electrons, when voltage is applied, they move and carry the charge (they are attracted to the positive end of the metal)

• Semiconductor - Number of charge carriers increase with the temperature. So resistance decreases as you increase temperature. (increase temperature, liberate more electrons from atoms, so more can move)

Potential Difference and Power

• Electrons pass through the battery and gain a fixed amount of energy, when they flow around the circuit and pass through a component, they transfer energy to that component, they then pass back through the positive terminal of the battery to be resupplied with energy

• It transfers energy to the component because they have to do work to pass through the component. (So the work done by the electron is equal to the loss of energy)

• The work done per unit charge is the Potential Difference

W = QV = ITV …..so….. P = IV

Resistance

• Caused by the repeated collisions with the charge carriers in the material, and the fixed positive ions in the material

• R = V/I

• VOLTMETER - INFINITELY HIGH RESISTANCE, OTHERWISE SOME CURRENT WILL PASS THROUGH IT, AND THE AMMETER WILL NOT READ THE EXACT CURRENT THROUGH THE COMPONENT

Ohms Law - PD across a conductor is directly proportional to the current through it, provided that temperature is constant. (V=IR)

• Resistivity is an intrinsic property of a metal

• 1) measure the diameter at several points on the wire and calculate a mean area

• 2) Measure the R at different lengths and plot a graph of R against L

• 3) Resistivity is given by the gradient x Area

Superconductors - zero resistance below a critical temperature.

• When a current passes through it, no PD flows through at as there is no Resistance, so there is no heating effect.

• Loses superconductivity if the temp is raised above the critical temperature

• used to make high power electromagnets - ie for MRI scanners or Particle Accelerators

Resistance increases with temperature (METAL)

• Positive ions move and vibrate more when temperature is increased

• So there are more collisions between electrons and positive ions

• so the electrons cannot flow through the metal as easily when a PD is applied across a conductor

• METALS = POSITIVE TEMPERATURE COEFFICIENT

Resistance decreases with temperature (SEMICONDUCTOR)

• Number of charge carriers increase with temperature

• SEMICONDUCTOR = NEGATIVE TEMPERATURE COEFFICIENT

• percentage change of resistance per kelvin is much greater than the metal (so graph of R against Temp is a decreasing curve, whereas for a metal it is an increasing straight line)

• (FROM THE SAME GRAPH) the resistance of thermistor decreases non-linearly, whereas the metal resistance increase (at a much smaller rate that the thermistor decreases) along the same temp range

Components

• Diode - only allows current to flow in the forward direction by having a really high current in the backwards direction

• LED - emits light when current flows

• Thermistor - Increase temp, Decrease Resistance (semiconductor)

• LDR - Increase Light Intensity, Decrease Resistance

IV Characteristics

• Wire - straight line through the origin. So resistance is constant always (and is given by 1/gradient) - PROPORTIONAL RELATIONSHIP

• Filament lamp - curve with a decreasing gradient, as voltage increases, current increases, so temp increases.

• Thermistor - Straight line at a constant temperature. Higher temp, Greater Gradient (as resistance falls with a greater temperature) - same graph as a wire, different explanation

Current Rules

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

Series

• Current entering a component is equal to the current leaving the component

• The current is the same through each component (same amount of charge pass through per second)

Potential Difference Rules

• Series - The total PD across all the components is equal to the sum of the potential differences across each component (so emf = v1+v2+v3 etc)

• Parallel - potential difference is the same across each component in parallel

• For any complete loop of a circuit, the sum of EMF’s around the loop is equal to the sum of potential drops around the loops

Resistance

• Series - total resistance is the sum of the resistance of each component

• Parallel - total resistance is less than the smallest resistor

HEATING - Charge carriers repeatedly collide with positive ions of the conductive material - net transfer of energy from the charge carriers to the positive ions.

P = IV = I²R = v²/R

EMF and Internal Resistance

• Internal resistance - in the source and resists the flow of charge through the source - electrical energy id dissipated in the source when charge flows through

• EMF - the energy per unit charge transferred to the charge carriers, in converting chemical energy to electrical energy

• Terminal PD - the electrical energy provided by the source per unit charge when it is connected in a circuit

• EMF = I(R+r)

• Lost volts = the electrical energy per unit charge that is lost due to the internal resistance

• P supplied by the cell = I²R + I²r

• Max power is supplied to the load when the load resistance is equal to the internal resistance of the source

Rules for circuits with a single cell and one or more resistors

1. Sketch the diagram

2. Calculate the current passing through the cell (emf/Rtotal)

3. Calculate the total circuit resistance

4. work out the current and PD for each resistor in series (V_{AcrossEachResistor} = Current x R

5. work out the current and PD through the resistors in parallel (I_{ThroughEachResistor} =PD_{acrossParallelCombo}/ R)

The Potential Divider

• 2 or more resistors in series with each other - and a source of fixed PD

• PD of the source is divided between the components because they are in series with each other.

• It can supply a PD which is fixed at any value between 0 to the source PD

• It can supply a variable PD

• Supply a PD that varies with temperature or pressure

The ratio of PD’s across each resistor is equal to the resistance ratio of the two resistors

Potential dividers can be used for volume dials and adjustable light bulbs (e.g dimming)

Sensor Circuits - use a thermistor and and a variable resistor