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Questions and Answers
What does the label 220 V, 12 W on an energy efficient light bulb signify?
What does the label 220 V, 12 W on an energy efficient light bulb signify?
- The bulb uses 12 W of power at a current of 220 A
- The bulb illuminates at 220 W and uses 12 V
- The bulb operates only at 220 W and 12 W
- The bulb transforms electrical energy into heat and light at a rate of 12 W at 220 V (correct)
How is power defined?
How is power defined?
- The resistance to energy flow
- The rate at which energy is transferred (correct)
- The amount of energy transferred per unit voltage
- The total energy consumed per unit time
Determine the current in a 220 V, 12 W light bulb when fully illuminated.
Determine the current in a 220 V, 12 W light bulb when fully illuminated.
- 0.6 A
- 18.33 A
- 1.83 A
- 0.055 A (correct)
Calculate the resistance of a 220 V, 12 W light bulb when fully illuminated.
Calculate the resistance of a 220 V, 12 W light bulb when fully illuminated.
What is the power of an electric stove that has a current of 11 A at 240 V?
What is the power of an electric stove that has a current of 11 A at 240 V?
A current of 11 A at 240 V is used for an average of 1 hour per day. How much energy in kWh is consumed per 30-day month?
A current of 11 A at 240 V is used for an average of 1 hour per day. How much energy in kWh is consumed per 30-day month?
Which rule correctly states Faraday’s law of induction?
Which rule correctly states Faraday’s law of induction?
If magnet Z moves closer to the solenoid and X becomes the south pole, in which direction will the electric current pass through a sensitive ammeter connected between A and B?
If magnet Z moves closer to the solenoid and X becomes the south pole, in which direction will the electric current pass through a sensitive ammeter connected between A and B?
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Study Notes
Power and Energy
- Power is the rate at which energy is transferred or converted.
- Energy efficient light bulb: 220 V, 12 W means it consumes 12 W of power when connected to a 220 V main supply.
- Calculations for the light bulb:
- Current: I = P / V = 12 W / 220 V = 0.054 A
- Resistance: R = V / I = 220 V / 0.054 A = 4074 ohms
Electric Stove
- Electric stove: 11 A, 240 V; power consumption: P = V x I = 240 V x 11 A = 2640 W
- Energy consumption per day: 2640 W x 1 hour = 2.64 kWh
- Energy consumption per 30-day month: 2.64 kWh x 30 = 79.2 kWh
- Cost per year: 79.2 kWh x 12 = 949.4 kWh/year; cost = 949.4 kWh/year x R2.10/kWh = R1983.74/year
Faraday's Law of Induction
- Faraday's law of induction: a changing magnetic field induces an electromotive force (EMF) in a nearby conductor.
- Induced polarity of the solenoid: when magnet Z moves closer, the induced polarity is such that X is the south pole.
- Direction of electric current: from B to A (through the ammeter).
- Polarity of Z: if the magnet is moving to the left, the polarity of Z is north.
Factors Affecting Induced Current
- Three factors to increase the magnitude of the current through the ammeter:
- Increase the strength of the magnetic field
- Increase the rate of change of the magnetic field
- Increase the number of turns in the solenoid
Direct Current Power Source
- When a direct current power source is connected between points A and B, the positive electrode is connected to A.
- Polarity of Y: north.
- Redrawn solenoid with induced magnetic field: [insert diagram]
- Effect of alternating current source: the polarity of X will change continuously.
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