P13 dc circuits

• at any junction in a circuit, the total current entering the junction is equal to the total current leaving the junction
• the current entering a component is the same as the current leaving the component, components do not use up current
• the current passing through two or more components in series is the same through each component, as the rate of flow of charge will always be the same
• the rule for potential difference in series is: VT = V1 + V2 + V3 + ...​
• the rule for potential difference in parallel is: potential difference across each component is the same
• for any complete loop of a circuit, the sum of the emfs is equal to the sum of the potential drops
• the rule for resistance in series is: RT = R1 + R2 + R3 + …​
• the rule for resistance in parallel is: 1/RT = 1/R1 + 1/R2 + 1/R3 + ...
• if you combine P = VI and V = IR you get P = I^2R and P = V^2/R
• power is energy transferred over time, so using P = VI you get E = IVt
• internal resistance is caused by electrons colliding with atoms inside the battery, so some energy is lost before the electrons even leave the battery
• to represent internal resistance you draw a small resistor inside a battery
• electromotive force is the energy transferred by a cell per coulomb of charge which passes through it
• the equation for emf is epsilon = E/Q

  epsilon = electromotive force in V

  E = electrical energy in J

  Q = charge in C
• the potential difference across the terminals of a source is the electrical energy per unit charge delivered by the source when it is in a circuit
• the terminal potential difference is less than the emf when current passes through the source because of internal resistance
• the equation for emf in a circuit is: epsilon = I(R+r)

  epsilon = electromotive force in V

  I = current in A

  R = resistance of the circuit in ohms

  r = internal resistance in ohms
• the lost volts is the potential difference across the resistor in the battery, it is equal to the energy wasted by the cell per coulomb of charge
• emf is the sum of the terminal potential difference and the lost volts
• always sketch a diagram of the circuit so you can more easily understand it and label it, then use the circuit rules to work out the necessary values
• a potential divider consists of two or more resistors in series with each other and a source of fixed potential difference
• the potential difference in a potential divider is divided between all components in the circuit, as they are all in series with each other
• purposes of potential dividers:

  • supply a potential difference which is fixed at any value between zero and the source potential difference
  • supply a variable potential difference
  • supply a potential difference which varies with a physical condition such as temperature or pressure
• a temperature sensor consists of a potential divider made using a thermistor and variable resistor

  when the temperature of the thermistor is constant it has constant resistance, so you can change the potential difference across it by adjusting the variable resistor

  when the temperature of the thermistor changes, its resistance changes, so the potential difference across it changes
• a light sensor consists of a potential divider made using an LDR and a variable resistor

  when the light intensity on the LDR changes, the potential difference across it changes