Task Use the following equations to answer the questions. You may need to rearrange the equations and convert the units. An example has been done for you. P = I x V V = I x R P = I 2 x R E = P x t E = Q x V Q = I x t power, P (watts, W) current, I (amperes, A) potential difference, V (volts, V) resistance, R (ohms, Ω) energy, E (joules, J) charge flow, Q (Coulombs, C) time, t (seconds, s) Example question A 6 V battery delivers 0.5 A of current. What is the power rating of the circuit? Step 1 Write down the variables you are given. V = 6 I = 0.5 P =? Step 2 Choose the formula that has those variables. Step 3 Substitute the numbers into the equation. P = I x V P = 0.5 x 6 3 W Step 4 Give the answer with the correct units. Questions 1. How much energy can an electric motor produce when it is plugged into a 110 V outlet with a total charge of 50 C passing through the motor? 2. If 80 J of energy is transformed over 1 hour by a button battery, what is its power? 3. If a light on a dolls house draws a current of 0.83 A, and has a resistance of 3.6 Ω, what is its power? www.teachitscience.co.uk 2018 30255 Page 1 of 5
4. In a lightning flash, the potential difference between a cloud and the ground is 1.5 x 10 9 V and the amount of charge transferred is 25 C, calculate the change in energy. 5. If a light bulb has a power of 4 W and the voltage is 230 V, what is the current flowing through it? 6. A heater has 230 V running through it and uses 8.3 A. a. How many watts is this heater? b. What is its resistance? (to 2 significant figures) 7. If a generator produces 120 V with a charge of 60 C, how much energy is produced? 8. Calculate the power dissipated in a circuit that contains a 20 Ω and 4 A of current. 9. 10 J of energy (work) is required to transfer 2 C of charge from X to Y. What is the potential difference between these two points? 10. What is the power in a circuit if the current is 3 A and the potential difference is 12 V? 11. How much energy is used in 2 minutes by a 50 W motor? 12. If a stage light is rated at 500 W and has a supply of 250 V. Calculate: a. The current flowing through it. b. The resistance of the bulb. 13. Now calculate the current and resistance through a 24 W, 12 V car headlight. Compare this with the stage light in question 12. 14. If 1.25 kj of energy is used to transfer 1500 C, what is the potential difference? www.teachitscience.co.uk 2018 30255 Page 2 of 5
15. How many volts would it take to power a television if it draws 2 A and is rated at 160 W? 16. Work out the power if the energy supplied is 1200 J over 12 s. 17. What is the current if the power is 120 W and the voltage is 240 V? 18. If the current through an oven is 55 A, what charge flows in 60 s? 19. How many watts would a mobile phone charger be rated at if it was supplied with 0.5 A and 5 V? What would its resistance be? 20. What is the voltage of a wire that supplies 2 A to a 250 W light bulb? 21. What current flows through a 5 Ω lamp rated at 80 W? 22. What is the power rating of the light bulb in this circuit? 200 Ω 36 volts 23. Calculate the power of a washing machine if the current is 80 A and the potential difference is 200 V. 24. A lamp in a circuit has a potential difference of 12 V when the current is 2 A. a. Calculate the charge flow through the lamp in 300 s. b. The energy supplied to the lamp in 300 s. Brain teaser: A kitchen contains three 2 A light bulbs, one 10 A microwave, a gas stove, one 6 A toaster and one 8 A kettle. Everything is powered by 120 V. How many watts of electricity are consumed if someone boils some water on the stove in the dark? www.teachitscience.co.uk 2018 30255 Page 3 of 5
Answer help sheet Step 1 Write down the variables you are given V = 6 I = 0.5 P =? Step 2 Choose the formula that has those variables Step 3 Substitute the numbers into the equation P = I x V P = 0.5 x 6 3 W Step 4 Give the answer with the correct units www.teachitscience.co.uk 2018 30255 Page 4 of 5
Answers 1. 5 500 J 2. 0.022 W (0.02 W) 3. 2.5 W 4. 3.75 x 10 10 J 5. 0.017 A 6. a. 1 909 W b. 28 Ω 7. 7 200 J 8. 320 W 9. 5 V 10. 36 W 11. 6 000 J (6 kj) 12. a. 2 A b. 125 Ω 13. 2 A, 6 Ω. Same current, much lower resistance. 14. 0.83 V 15. 80 V 16. 100 W 17. 0.5 A 18. 3 300 C 19. 2.5 W, 10 Ω 20. 125 V 21. 4 A 22. 6.48 W 23. 16 000 W (16 kw) 24. a. 600 C b. 7 200 J Brain teaser none, no electricity is being used! www.teachitscience.co.uk 2018 30255 Page 5 of 5