Question 20S: 2: Sensing, Questions, Short Answer, Starting Out, electric current, potential difference, resistance, electrical power, Revision questions

These questions are on electric current, potential difference and power.

Estimating your electricity bill

Sandra wants to budget for paying the electricity bill on her small new flat. For cooking she mainly uses a microwave oven rated at 1 kW to reheat chilled meals, 'bake' potatoes etc. On average she will use this cooker for 10 minutes a day. She also assumes she will use a kettle (rated at 3.3 kW) and a toaster (rated at 500 W) daily for the about the same period of time.

1.Calculate how many kilowatt-hours Sandra uses daily for preparing meals and snacks.

2.She remembers to include the cost of lighting: she has 100 W bulbs throughout the flat and expects to have one of the lights on in the evening for 3 hours.

Work out the number of kilowatt-hours Sandra uses daily on lighting.

3.Estimate Sandra's quarterly bill assuming one quarter = 90 days and the cost of electricity is 8 p / kilowatt-hour.

4.If you wish, add on your own estimates for weekly ironing and vacuuming

Cost of an electrically heated shower

5.In your student 'digs', you have to put a 10 p coin in a slot if you want to have a five minute shower. You note that 'Power Rating = 9 kW' is marked on the shower fitting. The last electricity bill to your home stated that one unit of electricity cost 8 p. Is the shower good value? Justify your answer with a calculation.

Torch Bulb

3 V; 0.5 A is written on the packet of torch bulbs.

6.Use your ideas about electrons to describe the mechanism of the energy transfer when the torch is 'on'.

7.Calculate the power conversion for the bulb in normal use.

8.The life of the bulb is approximately 10 hours.How much energy will it have dissipated in its lifetime?

Question 30S: 2: Sensing, Questions, Short Answer, Starting Out, electric current, potential difference, resistance, electrical power, Kinds of light bulb

People use electric light bulbs for many purposes, from a torch used to light up a path home, to aircraft searchlights. These lamps differ tremendously in the power they use.

1.All bulbs are stamped with two different values, for instance 36 W, 12 V. What do these numbers tell you?

2.You can also use these values to calculate the current and the resistance of the bulb filament. The table below shows these values for five different bulbs. Use a suitable formula to calculate the missing values.

Bulb / Power / W / p.d. / V / Current / A / Resistance / 
Headlamp / 36 / 12 / 4
Torch bulb / 0.09 / 3 / 0.03
Filament bulb / 100 / 230 / 529
Flashlight bulb / 4.5 / 9 / 0.5
Energy Saving bulb / 24 / 230

Fuse protection

3.Explain why appliances are protected by a fuse and explain how the fuse provides this protection.

4.The table shows the power rating and voltage as marked on a number of appliances. Calculate the operating current of each appliance. Suggest a suitable fuse value for each appliance choosing from the fuse values given.

Appliances / Power rating / p.d. / V / Operating current / A / Suggested fuse values choosing from 3 A; 13 A
Iron / 1200 W / 230
Vacuum cleaner / 900 W / 230
Headlamp / 48 W / 12
Jug kettle / 2.4 kW / 230
Radio / 100 W / 230
Travel kettle / 340 W / 120
Microwave
cooker / 1.4 kW / 230

Measuring potential difference

A pupil wants to measure the potential difference across a battery connected to a circuit:


5.What instrument should he/she use?

6.The pupil notices that when the meter is put across the terminals AF, BE, CD in turn, the reading is always the same. Why is that so?

7.State and account for the voltmeter readings when placed across FE or AC.

A portable radio

You buy a new portable radio. It is powered by eight cells and there is a diagram printed on the battery chamber to show you how to fit the cells:


8.What is the total potential difference of this arrangement of cells?

9.This radio can also be connected to the 240 V a.c. mains supply which is far too large for this radio to be used directly. What component must be included inside the radio to change the incoming supply to 12 V?

10.Battery and mains supplies vary in potential difference. State one other significant difference.

Question 50S: 2: Sensing, Questions, Short Answer, Practice, potential difference, electric charge, electron, electric current, Ions in x-ray machines: Large and small numbers 2

These problems are intended to help you practice calculations with large numbers of conducting particles and small amounts of charge found in a variety of examples of electrical conduction.

X-ray machines

An x-ray machine works by accelerating electrons from a hot cathode through a high voltage in a vacuum tube. The fast electrons crash into a metal target, producing a lot of heat and some x-rays, which can be used for medical purposes. The tube current (more electrons per second) determines the quantity of x-rays produced and their penetration is determined by the tube potential difference (more energy per electron). The current, potential difference and time of exposure are varied by the radiographer to examine different parts of the human body.

Some typical values are:

Examination / Voltage / kV / Current / mA / Time / s
pelvis / 65 / 350 / 0.8
hand / 40 / 80 / 0.1

1.How much charge and energy do the electrons in the tube deliver during each exposure?

2.Explain why there are differences in the energy and penetration required.

3.For the pelvis example, find the number of electrons arriving per second, and the energy of each electron.

4.Hence find the number of electrons arriving during the 0.8 s exposure and check that the total energy they deliver agrees with your answer to question 1.

- 1 -Chapter 2 – Sensing – Set 1