5. Electricity

• Electric current is the rate of flow of charge and is measured in amperes (amps)
• Potential difference is the work done moving a unit charge between 2 points in a circuit, represented by V = W / Q
• Resistance is how difficult it is for current to flow through an appliance. A component has a resistance of 1Ω if 1A flows through it when a p.d of 1V is applied across it, represented by R = V / I
• An ohmic conductor is a conductor that obeys Ohm’s law, meaning that current is directly proportional to potential difference providing physical conditions (such as temperature) remain constant
• Current in a circuit can be measured with an ammeter connected in series with the component
• Potential difference across a component can be measured using a voltmeter connected in parallel across the component being measured
• The gradient of a current-potential difference graph represents the rate of change of current with respect to voltage, not the same as 1/R
• A graph represents an ohmic conductor if the line has a constant gradient and passes through the origin, showing that voltage is directly proportional to current
• A graph with higher resistance requires a higher voltage for the same change in current compared to a graph with lower resistance
• A curve representing a filament lamp shows that as current increases, resistance also increases. A big increase in voltage produces only a small increase in current
• Increasing current on a filament lamp causes an increase in resistance as electrical energy is converted to heat energy, causing metal ions to vibrate and impede electron movement
• A diode is an appliance that only allows current to flow in one direction
• Voltmeters are assumed to have infinite resistance unless stated otherwise, ensuring all current flows through the appliance being measured
• An ammeter is assumed to have zero resistance unless stated otherwise, so there is no potential difference across it and it does not affect the circuit
• A Light Dependent Resistor (LDR) is a semiconductor sensitive to light, with resistance decreasing as light intensity increases
• A thermistor works similarly to an LDR, but as temperature increases, resistance decreases due to its negative temperature coefficient
• Resistivity is the resistance of a 1m cylinder with a cross-sectional area of 1m^2, an inherent property of a material represented by ρ = RA/L
• An experiment to determine the resistivity of a metal involves measuring the resistance, length, and cross-sectional area of a wire, then calculating resistivity using the formula ρ = RA/L
• A superconductor is a material with zero resistivity at or below a critical temperature, which is an inherent property of the material
• In a series circuit, the total resistance is the sum of the individual resistances of each component
• If 6 cells, each with a voltage of 5V, are arranged in parallel, the voltage in the circuit is 5V
• If 6 cells, each with a voltage of 5V, are arranged in series, the total voltage provided to the circuit is 30V (5V x 6)
• In a series circuit, the current through all components is the same
• In parallel components of a circuit, the current can vary as each branch can have different currents according to Kirchhoff’s first law
• Kirchhoff’s first law states that all the current going into a junction is equal to the current leaving the junction
• Kirchhoff’s second law states that for any path (loop) of a circuit, the sum of all potential differences must equal the total emf of the circuit
• If two cells in a series circuit are connected negative to negative, their emf will cancel out, resulting in a total emf of ε total = ε 1 - ε 2
• Power is defined as the rate of energy transfer
• The equation for power in terms of current and voltage is Power = Current x Voltage (P = IV)
• The purpose of a potential divider is to provide either a variable potential difference or a constant specific potential difference
• In a circuit with a cell providing 12V to resistors of 6Ω and 7Ω in series, the voltage across the 7Ω resistor is 6.5V
• EMF (Electromotive force) is the electrical energy transferred by a power supply per unit charge
• Two applications of superconductors are power cables to reduce energy loss during transmission and strong magnetic fields for applications like maglev trains and medical uses
• The total resistance of a parallel arrangement of 3Ω, 7Ω, and 9Ω resistors is 1.7 Ω
• RP06 - Method:

  • Set up the apparatus as shown in the diagram.
  • With the switch open, record the reading V on the voltmeter.
  • Set the variable resistor to its maximum value, close the switch and record V and the reading I on the ammeter.
  • Open the switch between readings.
  • Decrease the resistance of the variable resistor and repeat this, obtaining pairs of readings of V and I over the widest possible range.

  
• Rearranging for y = mx + c:

  Ɛ = I(R + r)

  Ɛ = IR + Ir

  As V = IR; Ɛ = V + Ir

  Ɛ - Ir = V

  V = -rI + Ɛ

  where:

  • y = V
  • x = I
  • gradient is -r
  • y-intercept is Ɛ
• Explain what is meant by resonance

  • Idea that (at resonance) frequency of forced vibrations equals natural/resonant frequency
  • Idea that amplitude (of vibrations/oscillations) is at a maximum
• State what is meant by the emf (electromotive force) of a battery.

  Work done in moving 1 C of charge the whole way around a circuit
• Explain how the law of conservation of energy applies in this circuit. You should consider the movement of one coulomb of charge around the circuit.

  • (1 C of) the charge gains ε J on passing through cell
  • ε = IR1 + IR2 + Ir

  
• Explain, in terms of electron movement, why the resistance of the filament lamp changes as the voltage changes as shown in Figure 3.

  • As the voltage increases more electrons move through the wire per second
  • More collisions per second between the conduction electrons and the lattice ions
  • The rate of vibration of the lattice ions increases causing a greater number of collisions per second causing increased resistance