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Broadoak School GCSE Physics

Electricity Calculations Practice

- This file contains questions for you to practice on - it assumes that you already have been taught
     about how to do them.
- The computer won't keep score.
- You shouldn't need a calculator, but it may help to write down your working rather than try to do
     it all in your head. You ought to write out your working anyway, because it's often  worth marks!

Contents

   - Click on these headings :.

Resistance - Ohm's Law questions

Power - Watts

Energy - Joules

Electricity bills and kilowatt-hours

Transformer calculations                   (Higher paper only)

Charge, energy, voltage and time     (Higher paper only)

Click on "File", "Exit" to finish.


No - try again


No

Resistance isn't Volts x Amps - it's Volts DIVIDED by Amps


Yes

Because 12 Volts divided by 2 Amps = 6 Ohms


No

You've added them together - try again


Yes

Because 100 Volts divided by 10 Amps = 10 Ohms


Yes

Because 2 Amps x 50 Ohms = 100 Volts


No

Voltage is not Ohms divided by Amps - it's Ohms x Amps


No

Because Amps are  NOT  Volts x Ohms

 - its Volts DIVIDED by Ohms


Yes

Because Amps = Volts divided by Ohms


Resistance
Resistance questions

Resistance = Voltage divided by Current, So OHMS = VOLTS divided by AMPS
                                                                            AMPS = VOLTS divided by OHMS
                                                                           VOLTS = AMPS x OHMS

QUESTION 1
     If the Voltage is 12 V, and the current is 2 Amps, what's the resistance?

24 Ohms

6 Ohms

12 Ohms

14 Ohms

Use the scrollbar to see the rest ------->>

QUESTION 2
     If the voltage is 100V and the current is 10A, what's the resistance?

100 Ohms

110 Ohms

1000 Ohms

10 Ohms

QUESTION 3
     If the resistance is 50 Ohms and the current is 2 Amps, what's the voltage?

100 Volts

25 Volts

50 Volts

QUESTION 4
     If the resistance is 10 Ohms and the Voltage is 150V, what's the current?

150 Volts

1500 Amps

15 Amps

Use the "Contents" button at the top of the screen


NO

Current is measured in Amps - not Volts !!


Power

Power = Voltage x Current,   So WATTS = VOLTS x AMPS
                                                        AMPS = WATTS divided by VOLTS
                                                      VOLTS = WATTS divided by AMPS

QUESTION 1
     If the current is 2 Amps and the voltage is 10 V, what's the power?

20 Watts

20 Volts

5 Watts

Use the scrollbar to see the rest ---->>

QUESTION 2
     If the power is 100 Watts, and the voltage is 50 V, what's the current?

5000 Amps

2 Amps

0.5 Amps

QUESTION 3
     If the power is 10 Watts and the current is 2 Amps, what's the voltage?

5 Volts

20 Volts

Use the "Contents" button at the top of the screen


Yes

Because Power (Watts) is VOLTS x AMPS


No

Power is in Watts, not Volts!


No

Power is not Volts divided by Amps

it's Volts x Amps


No

Current is not Watts x Volts

it's Watts divided by volts


Yes

Current is Watts divided by Volts.

This comes in handy for working out fuses - see the section on electricity bills.


No

Current is not Volts divided by Watts

it's Watts divided by Volts


Yes

Because Volts = Watts divided by Amps


No

Voltage is not Watts x Amps

it's Watts divided by Amps


Energy

Energy = power x time
         (Joules = Watts x seconds)

ALSO
Energy = Voltage x Current x time
         (Joules = Volts x Amps x seconds)

QUESTION 1
     A 100 Watt heater is switched on for 10 seconds. How much energy is turned into heat?

1000 Watts

1000 Joules

10 Joules

Use the scrollbar to see the rest ---->>

QUESTION 2
     A 1 kW lamp (that's 1,000 Watts) is run for 1 minute. How much energy gets converted?

1,000 Joules

16.6 Joules

60,000 Joules

QUESTION 3
     If the voltage is 5 V and the current is 2 A,  how much energy will be converted in 20 seconds?

0.5 Joules

100 Joules

40 Joules

200 Joules

QUESTION 4
     If the voltage is 10 V and the current is 2 A, how long will it take to convert 200 Joules of energy?

10 seconds

100 seconds

1 minute

10 minutes

Use the "Contents" button at the top of the screen


Bills
Electricity bills

Domestic appliances use a lot of energy.
So we use "kilowatt-hours" rather than Joules to measure the energy.
A kilowatt-hour is the amount of energy used by a 1,000W appliance in 1 hour.

We call this "1 unit"

So a 1,000W appliance would use 1 unit per hour.
       In 30 minutes it would use 0.5 units,
            in 2 hours it would use 2 units,
              in 3 hours it would use 3 units, etc

QUESTION 1
     How many units would a 2kW appliance use in 2 hours?   (Remember that 2kW = 2,000W)

2 units

4 units

2,000 units

4,000 units

QUESTION 2
     If electrictiy costs 10p per unit, what would it cost to run a 3kW appliance for 1 hour?

30p

£30

QUESTION 3
     Imagine that you've just received an electricity bill, and you've used 500 units since the
     last time you had a bill. Electricity costs 10p per unit, and the "standing charge" is £10.
     What's your bill going to be?

£60

£550

£50

To find out more, run the EXCEL program and look at the file "ELECBILL.XLS" on Drive M.

Use the "contents" button at the top of the screen


No

Energy is measured in Joules, not Watts!!


Yes

Because Joules  = Watts x seconds


No

You need to multiply them - not divide.


No

Remember that there are 60 seconds in a minute!


No

You need to multiply, not divide


Yes

1,000 Watts x 60 seconds = 60,000 Joules


No

Muliply them all together.


Yes

5V x 2A x 20sec = 200 Joules


Yes

200 Joules divided by (10V x 2A) = 10 seconds


No

Try
       10V x 2A = 20.
       Now divide 200 Joules by that 20 and see what you get.
       Remember to work in seconds!


No

Try again


Yes

A 2,000W appliance uses 2 units per hour, so in 2 hours it'll use 4 units.


Yes

A 3kW appliance uses 3 units per hour, so at 10p per unit that's 30p


Yes

500 units at 10p = £50.
Add on the £10 standing charge and you get £60


Transformers

CALCULATIONS about transformers will be on the Higher paper only

QUESTION 1
       A transformer has a primary coil of 100 turns and a secondary coil of 300 turns.
       What will be the voltage out of the secondary coil if we apply 20 V to the primary coil?

300 V

33 V

60 V

40 V

QUESTION 2
      If we want to transform 240 V down to 12 V, and we want our transformer to have
     2000 turns on its primary coil, how many turns should it have on its secondary coil?

100 turns

240 turns

40,000 turns

QUESTION 3
      A battery charger for a 6Volt camcorder battery has 4000 turns on its primary coil and
      100 turns on its secondary.
      Is this charger made for use in the U.K, where the mains supply voltage is 240V, or is it for use
      on the island of Klatch, where the supply is at 120V?

Made for U.K.

Made for Klatch

Use the "Contents" or "Back" buttons at the top of the screen


Yes


Yes

20 Volts x 300 turns/100 turns  = 60 Volts out


Yes

2000 turns x 12V/240V = 100 turns


Yes

6 Volts x 4000 turns/100 turns = 240 Volts, so it's for the U.K.

    (If you used it on the island of Klatch it would only give
   120 Volts x 100 turns/4000 turns = 3 Volts, so it wouldn't
    be much use for charging a 6 Volt battery. That's why you'd
    need a different charger.)


charge
Charge, Voltage and Energy

Higher paper only

There are two equations to remember :

                         Q = I x t                              and                            E = V x Q

                 Charge = Current x time               and                     Energy = Voltage x Charge

            Coulombs = Amps x Seconds          and                     Joules = Volts x Coulombs

(Note : put them together and you get   Energy = Volts x Amps x time ......  sounds familiar?)

QUESTION 1
          A current of 3 Amps flows through a heater for 30 seconds.
          How much charge has passed through it?

10 Coulombs

90 Coulombs

90 Volts

QUESTION 2
          60 Coulombs of charge passed through an Ammeter in 1 minute. What was the current?

60 Amps

3600 Amps

1 Amp

QUESTION 3
          If a solar cell sends out 50 Coulombs of charge along a wire at a voltage of 10 V,
          how much energy has been transferred?

500 Amps

500 Joules

5 Joules

QUESTION 4
          A laboratory power supply is set to 12.5 V and switched on for a short time.
          In that time, it supplies an experiment with 100 J of energy at a current of 2 A.
          How long was it switched on for?

4 seconds

10 seconds

25 seconds

Use the "contents" button at the top of the screen


No

Charge is measured in COULOMBS


No

Charge = current TIMES time, not divided!


No

Remember that the time must be in SECONDS


No

Not bad - you've remembered that the time
                must be in seconds, not minutes.
                But remember that if Q = I x t, then I = Q / t


No

Energy is in JOULES!


No

You need to use both of the equations to crack this one!