Physics (electrical energy and power)

• What is the formula to calculate energy in joules?

  Energy = power × time, or $E =$$P \times t$
• What is the formula to calculate power in watts?

  Power = current × voltage, or $P =$$I \times V$
• What units are used for energy, power, current, voltage, and time?


  Energy in joules (J), power in watts (W), current in amps (A), voltage in volts (V), and time in seconds (s)
• What is the mechanical power formula for a machine?


  The mechanical power is defined as the rate at which energy is transformed, $P =$$\frac{E}{t}$
• What are the two other forms of the power equation?

  Power = energy/time, or $P =$$\frac{E}{t}$ and time = energy/power, or $t =$$\frac{E}{P}$
• How can electrical power be calculated using current and voltage?

  Electrical power can be calculated using the formula $P =$$I \times V$
• If a study lamp is rated at 60W and 230V, how much current does it carry?


  The current is $I =$$\frac{P}{V} =$$\frac{60}{230} \approx 0.26 \text{ A}$
• What is the power of an electric heater that takes a current of 4A when connected to a 12V supply?


  The power is $P =$$I \times V =$$4 \times 12 =$$48 \text{ W}$
• What happens to electrical energy when there is current in a resistor?


  Some of the electrical energy is lost as heat or kinetic energy to the atoms in the wire
• How does increasing the current affect heat energy produced in a resistor?


  More current results in more resistance and thus more heat
• What is the effect of increasing voltage on heat energy produced?


  Increasing voltage results in more heat energy produced
• How does increasing the circuit's resistance affect heat energy produced?

  More resistance leads to more heat due to more collisions
• How much energy does a colour TV use in 60 seconds if it runs off a 240V supply with a current of 0.35A?


  The energy used is $E =$$P \times t =$$84 \times 60 =$$5040 \text{ J}$
• How do you calculate the current and resistance for domestic appliances operating at 240V?


  • Current: $I =$$\frac{P}{V}$
  • Resistance: $R =$$\frac{V}{I}$
• What is the current drawn by a microwave with a power rating of 650W operating at 240V?


  The current drawn is $I =$$\frac{650}{240} \approx 2.71 \text{ A}$
• What is the resistance of a toaster with a power rating of 1200W operating at 240V?


  The resistance is $R =$$\frac{240}{5} =$$48 \Omega$
• How much power does a kettle with a resistance of 48Ω use when connected to a 240V supply?


  The power is $P =$$I \times V =$$5 \times 240 =$$1200 \text{ W}$
• What are the correct quantities for the following descriptions?

  • Potential difference: voltage is current x resistance
  • Energy: charge is current x time
  • One Ampere: 1 Ampere is one Coulomb every second
  • Voltage: potential difference another term for voltage
  • Charge: energy is power x time
• What is Joule's Law in relation to electrical power?


  Joule's Law states that $P =$$I^2 \times R$ or $P =$$\frac{V^2}{R}$
• What is the difference between alternating current (a.c.) and direct current (d.c.)?

  Direct current (DC) flows in the same direction, while alternating current (AC) changes direction periodically
• What type of current does a cell or battery provide?

  A cell or battery provides a constant (steady) direct current (DC)
• How is direct current (DC) described in terms of direction?

  Direct current (DC) is described as being unidirectional
• What does a graph of voltage against time look like for direct current (DC)?


  A graph of voltage against time for DC shows a constant voltage level