Chapter 16,17,18

• Metallic conductor (Ohmic)


  A fixed resistor is considered to be an Ohmic conductor if we assumed its resistance to be constant
• Metallic conductor (Ohmic)


  • The I-V graph shows a straight line passing through origin
  • Current is directly proportional to potential difference
• Conducting wires are assumed to have negligible (zero) resistance and hence, no energy is transferred to the internal store of the wires as current flows through
• Tungsten Filament lamp (Non-ohmic)


  • The filament gets hotter as more current flows through it
  • I/V ratio decreases with current
  • V/I ratio increases with current
  • Its resistance increases with current
• Semi-conductor diode (Non-ohmic)


  • When potential difference applied is in the reverse direction, the diode has extremely high resistance. Current is negligible even at high potential difference (applied in reversed direction)
  • When the potential difference applied in the forward direction (above a minimum operating value), the resistance will be very low. The current will be very large
  • I/V ratio will be very small in reverse bias
  • I/V ratio will be very large in forward bias
• Resistance of the semi-conductor diode when placed in the forward direction is very low. Current passes through as per normal
• Resistance of the semi-conductor diode when placed in the reverse direction is very high. Current will be almost zero
• Current


  1A (ampere) of current is produced when one coulomb of charge passes a point in a circuit in one second
• Conventional current


  Flows from positive terminal to negative terminal
• Electron flow


  Flows from negative terminal to positive terminal
• Q


  Quantity of electric charge in C
• E


  Potential difference across a component in an electric circuit, the work done to drive a unit charge through the component

  
• Electromotive force (EMF)


  The work done by the source to drive a unit charge around a complete circuit
• Resistance


  Ratio of voltage across the material to the current flowing through it (V/I)
• Variable resistor


  • Allows the current in the circuit to change so that the sufficient voltage and current reading can be obtained to plot a V/I graph from which the resistance can be determined

  
• Ammeter


  • Measures the current flowing through a circuit + has very low resistance so that potential difference across is negligible
• Voltmeter


  • Measures potential difference + has very high resistance so that current flowing through it is negligible
• Procedure to determine unknown resistance (ohmic)

  1. Adjust rheostat to maximum value

  2. Record the ammeter and voltmeter readings

  3. Adjust rheostat to get 5 other current readings at regular intervals

  4. Record the 5 current values and their corresponding potential difference readings

  5. Plot a graph of potential difference against current

  6. Calculate the gradient of graph to obtain resistance

  
• Procedure to determine unknown resistance (non-ohmic)


  The resistance is determined by the ratio of potential difference to current, which can be read off the V/I graph
• Ohm's Law


  The current passing through a metallic conductor is directly proportional to the potential difference across it, provided that the physical conditions remain constant
• Ohmic conductors


  • Voltage is directly proportional to the current (V/I graph) + straight line that passes through the origin

  
• Resistance of filament changes with temperature rise (non-ohmic)

  • 1) Temperature of filament rises with increasing current
  • 2) From the graph, with increasing I, the ratio of I/V decreases
  • 3) Since resistance is equal to V/I, it shows that the resistance of the filament increases with the temperature rise
• Resistivity


  Ω m, a fixed property of the wire's material (metal) that affects the wire's resistance
• Resistance


  R is directly proportional to L + R is inversely proportional to A
• Same material + same length: thinner conductor (smaller cross-sectional area) has a higher resistance (steeper gradient)
• Series circuit


  • Current - Only one path in which current flows, Current is the same through every component in circuit, Current will cease to flow if there is a break anywhere in the circuit
  • Potential difference - Sum of potential difference across individual components = potential difference across the whole circuit
  • Resistance - Sum of resistance across the components in the circuit as (V=IR)

  
• Parallel circuit


  • Current - Has multiple paths in which charge can flow, Current splits then merges when switch is closed, Sum of currents in the separate branches = current from source
  • Potential difference - Each component in parallel has the same potential difference across it
  • Resistance - Inverse of 1/R of individual components added up together

  
• Sliding from end back to start, instead of start to end


  Causes maximum voltage to be applied across diode first, then decreasing the voltage → diode may overheat
• Existence of resistors in potential divider circuits

  Limits current → appliance may blow due to overheating
• Thermistor


  Resistor that changes resistance depending on the temperature, Lower resistance when temperature increases (NTC thermistors)
• LDR/ light dependent resistor

  Changing resistance based on amount of light it is exposed to, More light → LDR resistance decreases, Less light → LDR resistance increases
• NTC used as a fire alarm

  Temperature increases → resistance of thermistor decreases + the potential difference across fixed resistor will increase. This would trigger a secondary circuit, switching on the fire alarm
• Why voltage across thermistor decreases as temperature increases

  • 1) Temperature rises → resistance decreases
  • 2) As thermistor and fixed resistor are functioning as a potential divider
  • 3) Voltage across thermistor is found by Vout= Rth/R1+Rth * Vin
  • 4) Since resistance of thermistor has decreased, the voltage of thermistor decreases
• What happens to current in the circuit when temperature rises

  • 1) Room temperature rises → resistance of thermistor decreases
  • 2) With same voltage supply, current flowing in circuit increases
• How a motor operates when there is a relay coil

  • 1) Temperature rises → resistance decreases
  • 2) Total resistance of circuit decreases
  • 3) With same voltage supply, current in primary circuit increases
  • 4) Increase in current flow causes relay coil to be magnetised strongly enough to close relay switch to operate motor
• Why connecting the motor in parallel to source is better than in series

  Can operate normally with __V across, however in series, thermistor will take up some of the voltage from __V supply so motor will operate at voltage below __V
• Heating effect of electricity

  • Energy is transferred electrically to the internal stores of the heating elements of appliances such as kettles or electrical irons
  • Heating elements: made from long and thin wires of nichrome or iron-chromium-aluminium alloy (High resistivity—can withstand high temperatures + does not oxidise easily even at high temperatures)
  • Electric current passes through heating elements, they heat up rapidly
• An appliance is labelled as 240V, 60W = will function properly 240V e.m.f is supplied to it, and it will have a power rating of 60W, with current 0.25A flowing through under normal working conditions
• Power dissipated in the circuit: sum of all power dissipated by individual appliances whether they are connected in series or parallel (brightness/ heating effect is related to the power dissipated by the appliance)
• Will brightness in bulb change when additional bulb is added in parallel to existing bulbs?

  • 1) The voltage across A and B will not change
  • 2) With the same R for each bulb, using the formula V^2/R, the working power remains the same hence brightness remains the same