energy, power and resistance

• What is the difference between potential difference and emf?

  Potential difference transfers energy from electrical to other forms
• How is emf defined?

  Energy transferred from other forms to electrical per unit charge
• What is often omitted when answering exam questions about potential difference and emf?

  The statement of per unit charge
• In a simple circuit, what does potential difference represent?

  Energy transferred across a resistor
• What is the formula for potential difference?

  Potential difference = energy transferred / charge
• What is the unit for potential difference and emf?

  Volt
• How can we express the energy transferred in terms of potential difference?

  Energy = potential difference * charge
• What is the base unit for energy?

  kg m² s⁻²
• How is the volt defined in terms of base units?

  Volt = kg m² s⁻³ A⁻¹
• How do you calculate the speed of particles accelerated by a potential difference?

  Using the equation $ev =$$\frac{1}{2}mv^2$
• What does the equation $ev =$$\frac{1}{2}mv^2$ represent?


  Electrical energy equals kinetic energy
• How do you rearrange the equation $ev =$$\frac{1}{2}mv^2$ for speed?


  $v =$$\sqrt{\frac{2ev}{m}}$
• What is the speed of an electron accelerated by 5000 volts?

  Approximately $4.2 \times 10^7 \text{ m/s}$
• What is the definition of resistance?

  Resistance is potential difference divided by current
• What are the units for resistance?

  Ohms
• How do you derive the base unit for ohms?

  By dividing potential difference by current
• What does Ohm's law state?

  Current is directly proportional to potential difference
• What is the mathematical expression for Ohm's law?

  $V =$$I \times R$
• What is the procedure to investigate Ohm's law experimentally?

  1. Connect a cell to a variable resistor.
  2. Add a fixed resistor, ammeter, and voltmeter.
  3. Vary the resistance and measure current and potential difference.
  4. Plot a graph of V against I.
  5. A straight line through the origin confirms Ohm's law.
• What does a straight line through the origin in a V-I graph indicate?

  Current is directly proportional to potential difference
• What do the IV characteristics of a resistor look like?

  A straight line through the origin
• What does it mean if a filament lamp's IV graph is not a straight line?

  The lamp does not obey Ohm's law
• How does the resistance of a filament lamp change with current?

  Resistance increases as current increases
• What happens to the resistance of a thermistor as current increases?

  Resistance decreases as current increases
• What is the IV characteristic of a diode?

  It looks very different from previous IV characteristics
• How does the IV characteristic of a filament lamp differ from that of a resistor?

  Filament lamp's graph is not a straight line
• What happens to the resistance of a thermistor as temperature increases?

  Resistance decreases with increasing temperature
• What is the relationship between current and temperature in a thermistor?

  Current increases as temperature increases
• What happens to the resistance of a diode when the potential difference is applied?

  Resistance changes significantly compared to other components
• How does the IV characteristic of a diode differ from that of a resistor?

  Diode's IV characteristic is nonlinear
• What law does a thermistor not obey?

  Ohm's law
• How does the current behave in a thermistor?

  The current increases faster than the pd
• What happens to resistance in a thermistor as current increases?

  Resistance decreases
• How does the resistance of a thermistor compare to a filament lamp?

  It behaves oppositely to a filament lamp
• What happens to the number density of conducting electrons as temperature increases?

  It increases
• What is the behavior of a diode's current when the pd is reversed?

  There is no current
• At what point does a diode start to conduct?

  At the threshold voltage
• How does resistance change as a diode conducts?

  Resistance decreases as current rises
• What is the relationship between resistance and length of a wire?

  Resistance is proportional to length
• How does cross-sectional area affect resistance?

  Resistance is inversely proportional to area