Questions 5 - Electricity

1. When correctly connected to a 9 volt battery the wire has a current of 0.30 amperes flowing through it.

(i) Give the equation that links current, resistance and voltage. (1)

V = I x R (or R = V/I, same thing)

(ii) Calculate the resistance of the wire.  Show clearly how you work out your answer and give the unit. (3)

Tip: use the 123 method...always...but especially when it's 3 marks!

1. I = 0.30 A       2. R = V      3. R = 9 ÷ 0.30

    V = 9 V                     I              = 30  Ω (ohms)

(iii) When the wire is heated, the current goes down to 0.26 amperes.  State how the resistance of the wire has changed. (1)

The resistance has increased to about 34 Ω (ohms), whereas before it was only 30. It goes up.

2. An adaptor can be used to connect up to four appliances in parallel to one 230 V mains socket. The adaptor is fitted with a 13 A fuse. The table gives a list of appliances and the current they draw from a mains socket.

Appliance	Current
computer	1 A
hairdryer	4 A
heater	8 A
iron	6 A
television	2 A

(i) What current will flow to the adaptor when the television, computer and hairdryer are plugged into the adaptor? (1)

2 + 1 + 4 = 7 A (always mention unit.)

(ii) Write down the equation which links current, electrical power and voltage.(1)

P = I x V 

power = current x voltage

(iii) Calculate the electrical power used when the television, computer and hairdryer are plugged into the adaptor. Show clearly how you work out your answer and give the unit. (2)

Tip: use 123 method

1. I = 7 A          2. P = I x V      3. P = 7 x 230

    V = 230 V                             = 1,610 W (watts)

(iv) What would happen to the fuse if the heater is also plugged into the adaptor?

Give a reason for your answer. (2)

Current is greater than 13A, so the fuse would melt. (in fact it would be 15A)

Note: the stuff that's underlined is more or less where you'd get the marks. In some cases, because of formatting, I was unable to underline some stuff, so here it is below.

In question 1 (ii), you get marks for the correct re-arranging of the equation, the units (ohms) and the right answer. Note that in question 2 (iii), you can change the answer to 1.61 kW (kilowatts) and still get the mark.

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Questions - not answered

1. A set of Christmas tree lights is made from twenty identical lamps connected in series. 

a) Each lamp is designed to take a current of 0.25 A. The set plugs directly into the 230 V mains electricity supply.

(i) Write down the equation that links current, potential difference and resistance. (1)

(ii) Calculate the resistance of one of the lamps. Show clearly how you work out your final answer and give the unit. (4)

(iii) What is the total resistance of the set of lights? (1)

(b) How does the resistance of a filament lamp change as the temperature of the filament changes? (1)

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2.  Burning fuels produce smoke particles and waste gases. An electrostatic smoke precipitator can remove the smoke particles from the waste gases.

The following sentences A to E describe how an electrostatic smoke precipitator works. The sentences are in the wrong order. Arrange the sentences in the right order. Start with sentence C. (3)

A The smoke particles stick to the positively charged collecting plates.

B The smoke particles are given a negative charge.

C The waste gases and smoke pass through a negatively charged metal grid.

D The collecting plates are knocked so the smoke particles fall and can be taken away.

E The smoke particles are repelled from the grid and attracted to the positively charged collecting plates.

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3. The underground storage tanks at petrol stations are filled from tankers. A static electric charge can build up on the tanker as the petrol flows through the pipe to the storage tank. This could be dangerous. 

Why is the static electric charge dangerous and what should be done to stop the charge building up? (2)

No copyright infringement intended - I did not write any of the questions above. All rights go to the appropriate author and owner. However, all answers are completely 100% totally written by me. I may have modified the question number to fit formatting but that's it.