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1 The power of lamps Specification references: P2.4.1 Power P2.4.2 Energy transfers in everyday appliances (part) MS 1a, 2a, 3b, 3c, 3d WS 1.2, 2.6, 3.1, 3.3 Aims In this practical, you will observe the effect of potential difference and current on a lamp s brightness and power. Then you will practise using equations relating to charge, current, time, potential difference, energy, and power in electrical circuits. Learning outcomes After completing this practical, you should be able to: identify the factors that affect the energy transfer in a circuit state that a battery or power supply provides energy to a current whereas a resistor causes a transfer of energy to the surroundings calculate the energy transferred by a charge passing through a potential difference perform calculations involving rearrangement of the equations Q = I t and E = V Q apply the law of conservation of energy in a circuit use algebra to combine the equations Q = I t and E = V Q to form the relationships E = V I t and P = I V. Safety The lamp will get hot when it is switched on, and will stay hot for some time after it has been switched off. Do not touch the bulb (or the shade, if the lamp has one). Equipment 12 V or 6 V lamp Power supply Variable resistor Ammeter Voltmeter This resource sheet may have been changed from the original. 1

2 Method 1 With the power supply switched off, make sure its potential difference setting matches what is needed by the lamp. 2 Build a series circuit including the power supply, lamp, and variable resistor. 3 Connect the ammeter and voltmeter so that they will measure the current through the lamp and the potential difference across the lamp. 4 Switch on the power supply, and adjust the variable resistor to make the lamp as dim as possible. 5 Record the current and potential difference measurements. 6 Repeat step 5 with the variable resistor on three other settings, including the setting which makes the lamp as bright as possible. Results Create a suitable table for your results, including a column for calculated powers. Your table should clearly show which results are for the lamp at its brightest, and which are for the lamp at its dimmest. Questions 1 Calculate the power of the lamp at each different brightness, and then write the powers in your table. 2 Describe the relationship between the power and the brightness of the lamp. (4 marks) 3 Cross out the incorrect words to complete the sentence below. The power supply transfers energy to/from the current, and the lamp and resistor transfer energy to/from the current to/from the surroundings. 4 What processes make energy transfers happen at: a the lamp? b the resistor? This resource sheet may have been changed from the original. 2

3 5 Calculate how much energy is transferred by the lamp in 5 minutes, when it is at the brightest setting at which you made measurements. Student follow up To answer these questions you will need to use the following equations: power = potential difference current, or P = V I energy transferred = charge flow potential difference, or E = QV charge flow = current time, or Q = I t Show your working, and write units with your numerical answers. 1 State the units used in the above equations for the following quantities: a Power b Energy transferred c Charge d Time 2 In a series circuit containing a battery, resistor, and bulb, the potential difference across the battery is 6 V and the potential difference across the bulb is 4 V. a Which of the components in the circuit does work on the current? b Which of the components in the circuit have work done on them by the current? This resource sheet may have been changed from the original. 3

4 c How many joules of energy are transferred from the battery to each coulomb of charge that passes through it? d How many joules of energy are transferred from each coulomb of charge to the bulb? e Use the law of conservation of energy to work out the number of joules of energy transferred from each coulomb of charge to the resistor. f State the potential difference across the resistor. 3 An electric fan has a current of 5 A and a potential difference of 10 V. a Calculate the charge which flows through the electric fan in 60 s. b Calculate the power of the fan. c Calculate the energy transferred to the fan in 60 s using the following equations: i energy transferred = charge flow potential difference This resource sheet may have been changed from the original. 4

5 ii energy transferred = power time d Compare your answers to c i and c ii. e Which of the two equations that you used in part c can be used to calculate the energy transferred: i in any device, whether it is electrical or mechanical? ii in electrical devices only? 4 In this question, all of the quantities are given to two significant figures, so you should round each of your answers to two significant figures. a Calculate the charge flow when a current of 0.15 A flows through a lamp for 10 s. b Calculate the potential difference across the lamp if it has a resistance of 40 Ω. c Use your answers to a and b to calculate the energy transferred in 10 s. This resource sheet may have been changed from the original. 5

6 d Calculate the power of the lamp. 5 Calculate the energy transferred when a charge of 3 mc moves through a potential difference of 1 kv. Extension 6 a A defibrillator used in a hospital has a potential difference of 500 V, and transfers 200 J of energy when it is used to restart a patient s heart. Calculate the charge that flows. b The charge flows in a time of 8 ms. Calculate the average current in the defibrillator. 7 Show that the equations Q = I t (which gives the definition of current) and E = V Q (which gives the definition of potential difference) can be combined to form the following equations: a E = V I t b P = V I This resource sheet may have been changed from the original. 6

7 8 Use the definitions of current and potential difference to explain why the power of an appliance can be found by multiplying the current flowing through it by the potential difference across it. This resource sheet may have been changed from the original. 7