Electric energy and power for SQA National 5 Physics
This page covers the following topics:
1. Electric power
2. Power, voltage and current
3. Power, current and resistance
4. Electric energy and efficiency
5. Energy, power and time
Electric power is the rate at which energy is transferred. It is measured in Watts, where 1000 W = 1 kW. Power is calculated by the following formula: Power = Work Done/time, therefore each Watt is equal to 1 J of energy per second. The amount of money payed for electricity is calculated based on the amount of electrical energy transferred to an appliance of a specific power and the time during which it is switched on. The unit used to calculate the amount that must be paid for electricity is the kilowatt-hour (kWh), where 1 kWh is the energy converted by a 1 kW applicance used for an hour.
The relationship between power, voltage and current is given by: Power = Voltage × Current.
Ohm's Law can be used with the relationship of power, voltage and current to come up with a different relationship betweeen power, current and resistance, given by P = I²R.
Energy is transferred to an object to do work on it and is calculated in Joules. Work is done on an object whenever a force moves it. The work done can be calculated in Joules using the following formula: Work Done = Force × distance moved in direction of force. The efficiency is the proportion of the energy or power transferred to an object which is used in useful ways. It is calculated using the following ratio: Efficiency = useful energy or power out/total energy or power in. Since efficiency is a ratio, it has no units.
The energy transferred to an appliance can be related its power rating and amount of time for which it is switched on through the following formula: Power = Energy transferred/time.
Define electric power.
Electric power is the rate at which energy is transferred.
A man is pushing a shopping trolley of weight 330 N with a horizontal force of 50 N. He pushes the trolley 3.2 m forward at a constant velocity. Calculate the work done by the man.
The force that is doing work is the push of the man, and not the weight of the trolley. Therefore, Work Done = 50 N × 3.2 m = 160 J.
A car produces a driving force of 1800 N and travels 12 m in the direction of the force. Calculate the work done by the car.
Using WD = Fs, Work Done = 1800 N × 12 m = 21600 J.
A runner produces a force of 47 N in the forward direction and runs at a constant speed for 5 m. Calculate the work done by the runner.
Using WD = Fs, Work Done = 47 N × 5 m = 235 J.
A light bulb has a power rating of 100 W with a potential difference of 240 V across it. Calculate the current flowing through it.
P = IV
I = P ÷ V
I = 100 ÷ 240 = 0.42 A (2 d. p.)
End of page