Topic 10 - Electricity and Circuits

• What three subatomic components make up an atom?

  electrons, protons, neutrons
• Describe the structure of an atom

  A nucleus containing protons and neutrons, surrounded by electrons in shells.
• An atom contains two types of charged particles, name both of them and state their charges.


  Proton (+1)

  Electron (-1)
• In a closed circuit, if there is a potential difference in the circuit there will also be a ...


  Current
• How is current defined?

  As the rate of flow of charge (or electrons) around a circuit.
• What are the units of charge?

  Coulombs, C
• What are the two ways that a component can be connected in a circuit?

  1. Series (same loop)

  2. Parallel (adjacent loop)
• Voltage is also known as

  potential difference
• How does the potential difference across two components vary when connected in series and parallel?

  ● Series: Total P.D is shared between

  each component

  ● Parallel: P.D across each component

  is the same
• If two resistors are connected in parallel, what can be said about their combined total resistance?

  Their total resistance is less than the smallest of the two individual resistances.
• If two resistors are connected in series, what can be said about their total resistance?

  Their total combined resistance is equal to the sum of the two individual resistances.
• Where must a voltmeter be placed in a circuit?

  in parallel with the component that is being measured
• Give an equation relating potential difference with energy transferred and charge.

  Potential difference (V) = energy transferred (J) / charge (C)

  V=E/Q
• A volt can also be described as

  joule per coulomb
• What is an electric current?

  A flow of electric charge.
• State the equation linking charge, current and time. Give the units for the quantities involved.

  Q = I t

  Charge (Coulombs), Current (Amperes), Time (Seconds)
• What can be said about the value of current at any point in a single closed loop?

  Current is the same at all points in a closed loop.
• What two factors does the current in a circuit depend on?

  1. Potential Difference (V)

  2. Resistance (R)
• What equation should be used to calculate potential difference if current and resistance are known? State the units for all 3 quantities.

  V = I R

  Potential Difference (V), Current (A), Resistance (Ω)
• What is an ammeter and where must it be connected in a circuit?

  An ammeter measures current. It is placed in series with the component it is required to measured.
• What happens when current reaches a junction in a circuit?

  Current is conserved; the total current remains the same and is split between the two branches.
• How does resistance affect current in a circuit?

  As the total resistance of a circuit increases, the current flowing through the circuit decreases.
• How can current in a circuit be varied?

  Using a variable resistor
• Give the equation linking current with resistance

  p.d. (V) = current (A) x resistance (Ω)

  V = IR
• How is total resistance affected by two resistors in series?

  The total resistance increases; it is equal to the sum of the two resistors.
• How is total resistance affected by two resistors in parallel?

  The total resistance decreases; it is less than the resistance of the resistor with the lowest resistance.
• What is an 'Ohmic Conductor'? State the condition required.

  ● A conductor for which current and potential difference are directly proportional

  ● Resistance remains constant as current changes

  ● Temperature must be constant
• List four components for which resistance is not constant as current changes.

  1. Filament lamps

  2. Diodes

  3. Thermistors

  4. Light Dependant Resistors (LDRs)
• What happens to the resistance of a filament lamp as the temperature increases? Why?

  ● Resistance increases

  ● Ions in metal have more energy, so vibrate

  more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
• What is different about current flow through a diode?

  - The current only flows in one direction

  - Resistance is very high in the other direction, preventing current flow
• State what happens to the resistance of a thermistor as temperature increases.

  The thermistor's resistance decreases.
• Give two examples of when a thermistor may be used

  1. In a thermostat to turn a heater on below a certain temperature

  2. In a freezer to turn on a cooler when the temperature becomes too high
• State what happens to the resistance of an LDR as light intensity decreases.

  The LDR's resistance increases.
• Give an application for a LDR.

  ● Street lights often use LDRs

  ● When light levels become too low, the

  light gains sufficient current to turn on
• How do diodes work?

  A diode only allows current to flow in one direction. If current is flowing the right way, the resistance is large for small voltages (up until about 0.6V), but then at higher voltages becomes very small.
• What factors affect the energy transferred when charge flows through a component?

  ● Amount of charge

  ● The potential difference across the component
• Give an equation linking energy, current and p.d., giving all SI units

  energy (J) = p.d. (V) x current (A) x time (s)

  E = VIt
• Define potential difference in terms of charge.


  Work done per unit charge
• Give an equation relating potential difference to charge

  energy transferred (J) = charge (C) x p.d. (V)

  E = QV
• When an electrical current flows through a resistor why does it heat up?

  There are collisions between the electrons and the ions in the resistor's lattice. This causes a transfer of kinetic energy into thermal energy, which is released into the surroundings.