knowledge-10 C

• The law of conservation of charge states that in a closed system charge cannot be created or destroyed.
• charge does not get used up or lost when it flows round a circuit.
• The same amount of charge passes through every component per second.
• The current entering a component is the same as current leaving a component.
• The current passing through two or more components in series is the same through each component.
• At any junction in a component, the total current entering the junction is equal to the total current leaving the junction- this is known as Kirchhoff's first law.
• Kirchhoff's first law equation:

  • i1 = i2+i3+......+in+in-1
  • sum of current in = sum of current out
• electrical power supplied to a component can be calculated using :

  • P=iv =i^2xR = V^2/R
• Total energy transferred to the component in time t can be calculated using:

  E= ixtxV

  where i is current

  where t is time

  where V is voltage
• The potential difference between two points in a circuit is defined as the energy transferred per coulomb of charge that flows from one point to another.
• if the charge carriers lose energy , the p.d is a potential drop
• if the charge carriers gain energy, the p.d is the potential rise.
• Due to law of conservation of energy:

  energy transferred to the charge in a circuit = energy transferred from a charge in a circuit.
• For any complete loop of a circuit , the sum of e.m.f around any loop in a circuit is equal to the sum of the p.ds around the loop. This is known as Kirchhoff's second law.
• For two or more components in series, the total p.d across all components is equal to the sum of p.ds across each component.
• The p.d across components in parallel is the same.
• When two or more resistors are connected in series circuit:

  • current is the same through each resistor
  • p.d across any individual resistor can be calculated using V=iR
  • p.d is split between them in proportion to their resistance:
  • V=V1+V2+V3 =iR1+iR2+iR3
  • total resistance can be calculated using:
  • RTotal = R1+R2+R3
• When two or more resistors are connected in parallel circuit:

  • current from the supply is equal to the sum of the currents throughout each component.
  • p.d across components in parallel is the same
  • total resistance Rtotal can be calculated using:
  • 1/Rtotal = 1/R1 +1/R2 +1/R3
• A potential divider circuit can be used:

  • to supply a p.d fixed at any value between zero and the p.d of a source of fixed p.d.
  • to supply a variable p.d
  • in sensor circuits to supply a p.d that varies with physical conditions , such as temperature or light intensity.
• A simple potential divider provides a fixed p.d less than that of the p.d source. It comprises two or more resistors in series with each other and the source of the fixed p.d.
• The ratio of the p.d across each resistor is equal to the ratio of their resistances:

  • V1/ V out = R1/R2
  • A component parallel with R2 will have the same p.d across it as R2.
• A component parallel with R2 will have the same p.d across it as R2. This can be proven :

  • V out = (R2 /R1 + R2)V in
  • V out/ V in = output resistance / total resistance
• A variable p.d can be provided by replacing R2 with a variable resistor.
• A potentiometer is a variable potential divider that uses a single variable resistor connected in a way that allows V out to be varied from 0 v to the maximum source p.d. This is useful in volume and light controls that need a range from 0 to maximum.
• A temperature sensor consists of a potential divider made using a thermistor and a variable resistor. As the temperature goes up , the resistance of the thermistor goes down, so the output p.d goes down.
• A light sensor consists of potential divider made using a light-dependent resistor and a variable resistor. As light intensity goes up , the resistance of LDR goes down, so the output p.d goes down.
• Sensors can be used to turn on connected circuits when the output p.d goes below or above a certain p.d.
• Electromotive force of a power supply is the electrical energy per unit charge produced by the source. The unit of emf is the volt (v), because e.m.f is a potential difference.
• e.m.f:

  • e =energy / charge
• Internal resistance r of a power supply is the resistance to the flow of current inside the power supply due to collisions between electrons in the current and atoms in the supply.
• Power supplies can get hot when in use because of the energy transferred as a result of their internal resistance.
• When power supply or cell of e.m.f e and internal resistance r is connected to an external resistor of resistance R.

  • e = energy per coulomb supplied by the source
  • I = current through the whole circuit, including the power supply
  • v=Ir = voltage drop across internal resistance
  • V=IR = voltage drops across external resistor known as the terminal p.d
  • e = IR + Ir
• voltage = emf-Ir
• the terminal p.d V can be measured by connecting a high resistance voltmeter directly across the terminals of the power supply or cell.
• Since V= emf - Ir , a graph of terminal p.d V against current I for a power supply will have:

  • gradient of -r , the negative of internal resistance
  • y-intercept of e.m.f
• Connecting cells in series can provide more energy per coulomb to the charge flowing in the circuit.
• If cells are connected in series in the same direction in the circuit , the total e.m.f supplied to the circuit is the sum of the individual e.m.fs.
• If cells are connected in series in opposite directions in the circuit, the total e.m.f supplied to the circuit is the difference in the individual e.m.fs.
• The total internal resistance is the sum of the individual internal resistances in series.
• connecting cells in parallel can provide a longer lasting energy supply because total store of energy is greater.