Physics electricity

• What is potential difference?

  the energy transferred across an electrical component per unit charge passed
• What is the unit of volt?

  V, or J/C (joule per coulomb)
• What is the equation for voltage, relating energy and charge?

  V = E/Q
• Why is a voltmeter connected in parallel with a component?

  to measure the potential difference across it
• Describe what series circuits are:

  • the different components are connected in a line from +ve to -ve of the power supply
  • If you remove one component, the whole circuit will break
  • current is the same everywhere, but the voltage is shared
• What are the properties of series circuits?

  • there's a bigger supply voltage when more cells are in series
  • The current is the same everywhere but size depends on total voltage and resistance
  • The total voltage of the supply is shared between components, but the voltage of one component depends on its resistance
  • The total resistance increases when you add more resistors
• Describe what parallel circuits are:

  • each component is separately connected to both the +ve and -ve of the supply (so each loop only has one component, except ammeters)
  • If you remove one component, it will not affect the others
  • voltage is the same everywhere, but current is shared
• What are the properties for parallel circuits?

  • the potential difference is the same across all components
  • the current is shared between the loops (the total current of the circuit is the total of all currents going through each component)
  • the total current going into a junction has to equal the total current leaving -> CURRENT IS CONSERVED AT A JUNCTION
  • the total resistance decreases as you add more resistors (more options for current to flow)
• Why is an ammeter is connected in series with a component?

  to measure the current, in amp, in the component
• What is electric current in a circut?

  • The rate of flow of charge
  • The potential difference pushes the charge around
  • the current flowing through a component depends on the voltage and resistance of the component
• What is current in metals?

  Flow of electrons
• What is the equation involving charge, current and time?

  charge (coulomb, C) = current (ampere, A) × time (second, s)

  $Q =$$I*$$t$
• Explain how changing the resistance in a circuit changes the current

  • The greater the resistance of a component, the smaller the current that flows given that the voltage is constant
  • This is because resistance is how hard it is for the current to flow
• What is the equation relating voltage, current and resistance?

  potential difference (volt, V) = current (ampere, A) × resistance (ohm, Ω)

  V = I x R
• Explain why, if two resistors are in series, the net resistance is increased

  In series:

  • Two resistors have to share the total voltage (the bigger its resistance, the bigger its share of voltage)
  • So the voltage across each resistor is lower, so the current is also lower (V = I x R)
  • Since the current is the same everywhere in a series circuit, the total current decreases
  • Therefore the resistance increases
• Explain why, if two resistors in parallel, the net resistance is decreased (less than the smallest resistor)

  In parallel:

  • By adding another loop, the current has more than one direction to go
  • This increases the total current that can flow around the circuit
  • An increase in current means a decrease in resistance
• Explain the design and construction of series circuits for testing and measuring

  • The current is the same everywhere in a series circuit
  • An ammeter will measure the same current wherever it is placed in the circuit
• Core Practical: Construct electrical circuits to:

  1. investigate the relationship between potential difference, current and resistance for a resistor and a filament lamp


  use power supply, ammeter, voltmeter, resistor and filament lamp in a series circuit

  1. Place ammeter and resistor in series
  2. Place a voltmeter over the resistor
  3. Ammeter and voltmeter should have positive readings
  4. Set power supply from 1 volt to 10 volts
  5. Record readings for ammeter and voltmeter
  6. Repeat the same process but replace the resistor with a filament lamp
  7. Find resistance using R = V / I
• Core Practical: Construct electrical circuits to:

  2. test series and parallel circuits using resistors and filament lamps

  3 ammeters, 3 voltmeters, two lamps, connect in parallel

  1. vary voltage of power supply
  2. record current through each ammeter and the voltmeter across each component
  3. as voltage increases, so does the current through each lamp
  4. the voltage across each lamp is the same as the voltage of the power supply
  5. the total current is the sum of the current through the two loops

  
• Explain how current varies with potential difference in filament lamps and how this relates to resistance

  Increasing current increases temperature of the filament, making the resistance increase
• Explain how current varies with potential difference in diodes and how this relates to resistance

  the current will only flow through the diode in one direction

  The diode has a very high resistance in the opposite direction
• Explain how current varies with potential difference in resistors and wires and how this relates to resistance 


  current is directly proportional to voltage (if temperature is constant)

  different resistors will have different I-V graph gradients because they have different resistances
• Describe how the resistance of a light-dependent resistor (LDR) varies with light intensity

  In bright light, the resistance decreases

  In darkness, the resistance increases
• Describe how the resistance of a thermistor varies with change of temperature (negative temperature coefficient thermistors only)

  In hot conditions, resistance decreases

  In cool conditions, resistance increases
• Explain how the design and use of circuits can be used to

  explore the variation of resistance in the following devices

  1. filament lamps

  • Connected to a increasing d.c of 2-12V (in increments of 2)
  • Connect the filament lamp to ammeter in series/voltmeter in parallel.
  • Measure the current for each voltage.
  • Plot a graph to show relationship between the voltage and current.
  • Non-linear relationship shows the variation of resistance.
• Explain how the design and use of circuits can be used to

  explore the variation of resistance in the following devices

  2. diodes

  • Connect to an ammeter in series and voltmeter in parallel.
  • Measure the current for each voltage.
  • Switch the diode the other way round to record current for the same voltages (but negative)
  • No current can flow through the diode anymore
  • Plot graph for the positive and negative potential differences to show the non-linear relationship and the variation of resistance.
  • Negative voltages have 0 current
• Explain how the design and use of circuits can be used to

  explore the variation of resistance in the following devices

  3. thermistors

  • Constant voltage of 12V
  • Connect to an ammeter
  • Place in water with thermometer
  • Measure current at room temperature.
  • Add hot water and stir, measuring current at 30 to 60 degrees.
  • Calculate the resistance.
  • As temperature increases, current through thermistor increases and resistance decreases
• Explain how the design and use of circuits can be used to

  explore the variation of resistance in the following devices

  4. LDRs

  • Constant voltage of 12V
  • Connect to ammeter.
  • Shine lamp immediately onto thermistor and measure current.
  • Move the lamp by 10cm intervals and measure the current until 50cm
  • Calculate resistance at each light intensity
  • As light level gets brighter, current through LDR increases and resistance decreases
• When there is an electric current in a resistor, there is an energy transfer which heats the resistor
• Electrical energy is dissipated as thermal energy in the surroundings when an electrical current does work against electrical resistance when it flows through a component
• Some electrical energy is dissipated to the thermal energy stores of the resistor, making the resistor heat up
• What is the energy transfer from work against resistance a result of?

  Collisions between electrons and the ions in the lattice that makes up the component
• Why do resistors heat up?

  Electrons collide with the ions in the lattice that makes up the resistor as they flow through it

  This gives the ions energy, causing to vibrate and heat up (friction from collisions)
• Why does the resistance of a resistor increase when the resistor heats up?

  • The harder the ions vibrate when the resistor heats up, the harder it is for the electrons to get through the resistor (more collisions)
  • This means that for a given voltage, current decreases as the resistor heats up and therefore the resistance increases
• Explain ways of reducing unwanted energy transfer through low resistance wires

  A smaller resistance means that there are fewer collisions between electrons and ions in the lattice.

  This means there will be less work against resistance and less thermal energy will be transferred to the surroundings
• Describe the disadvantages of the heating effect of an electric current

  • Heating up a component reduces its efficiency - less energy is transferred to useful energy stores because more of it is transferred to the thermal energy store of the component
  • If the temperature gets too high, it can cause components to melt - meaning the circuit will stop working
• Describe the advantages of the heating effect of an electric current

  • Fuses use the heating effect to protect circuits - they melt and break the circuit if the current gets too high
  • It is useful for appliances that heat up objects - e.g toasters (these appliances should have high resistance)
• How do you find the energy transferred to an electrical component?

  energy transferred (joule, J) = current (ampere, A) × potential difference (volt, V) × time (second, s)

  E = I × V × t
• The larger the current through, or the voltage across, a component, the more energy is transferred to it
• What is power and its unit?

  the energy transferred per second

  it is measure in Watts