Current, Potential Difference & Resistance

• potential difference = current × resistance
• Describe the action of the fuse in a circuit.

  if current is above 5 A the fuse melts breaking the circuit
• Name the component in the circuit used to change the potential difference across the resistor.

  variable resistor
• An electrical component which has a resistance that increases as the temperature increases is a filament bulb
• An electrical component which emits light only when a current flows through it in the forward direction is an LED
• Suggest two disadvantages of using this equipment as a thermometer compared to a liquid-in-glass thermometer.

  not portable

  cannot be read directly
• The resistance of the bulb increases as the potential difference across the bulb increases. Why?

  temperature of the bulb increases
• Why is it important that ammeters have a very low resistance?

  Reduces current as little as possible
• It would not be safe to connect the electric cooker hob to the mains electricity supply by plugging it into a standard power socket. Why?


  current exceeds maximum safe current for a 2.5 mm2 wire a 2.5 mm2 wire would overheat
•  

  What is the difference between an alternating current and a direct current?

  a.c. is constantly changing and d.c. flows in one direction only
• An 18 V battery could not be used as the input of a transformer. Explain why?

  need a.c.  battery is d.c.
• What is the difference between an alternating current and a direct current?

  a.c. is constantly changing direction and d.c. flows in one direction only
• Cell

  Circuit symbol
• Battery


  Circuit symbol
• Lamp

  Circuit symbol
• Fuse


  Circuit symbol
• Voltmeter


  Circuit symbol
• Ammeter


  Circuit symbol
• Diode


  Circuit symbol
• Resistor

  Circuit symbol
• Thermistor

  Circuit symbol
• Variable resistor


  Circuit symbol
• LDR (Light Dependent Resistor)


  Circuit symbol
• LED (Light Emitting Diode)


  Circuit symbol
• Electric current

  The flow of electrical charge
• Equation linking charge, current and time

  1. Q = I t

  2. Charge (Coulombs)

  3. Current (Amperes)

  4. Time (Seconds)
• Current is the same at all points in a closed loop
• Factors current in a circuit depends on

  • 1. Potential Difference (V)
  • 2. Resistance (R)
• Equation to calculate potential difference if current and resistance are known

  1. V = I R

  2. Potential Difference (V)

  3. Current (A)

  4. Resistance (Ω)
• Ohmic Conductor


  • A conductor for which current and potential difference are directly proportional
  • Resistance remains constant as current changes
  • Temperature must be constant
• Components for which resistance is not constant as current changes

  • Lamps
  • Diodes
  • Thermistors
  • Light Dependant Resistors (LDRs)
• As temperature of a filament lamp increases

  Resistance increases
• Current flow through a diode

  • The current only flows in one direction
  • Resistance is very high in the other direction, preventing current flow
• As temperature of a thermistor increases

  Resistance decreases
• When a thermistor may be used

  • In a thermostat to turn a heater on below a certain temperature
  • In a freezer to turn on a cooler when the temperature becomes too high
• As light intensity on a LDR decreases

  Resistance increases
• Application of a LDR

  • Street lights often use LDRs
  • When light levels become too low, the light gains sufficient current to turn on