Friday, 27 September 2013

Dewey Cheetem & Howe Electricity

Bill for 10^th February to 10^th April 2011

Mrs E Windsor, 1 The Mall, London

Reading last time	18366	Reading this time	19617
Units used		Cost at 8p per unit
		Standing Charge	£5.50
		Total

Fill in the empty boxes.

Why was the next bill less?

Mrs Windsor and her family went away to Scotland for the whole of the summer and use no electricity. Did she have no bill to pay?

PAYING FOR ELECTRICITY

Number of kWh= power (in kW) x time (in hours)

1000 w= 1 kW

Assume 1 kWh costs 8p

Calculate the cost of running

1. A 1 kW fire for 5 hours

2. A 4 kW immersion heater for 3 hours.

3. A 3kW oven for ½ hr.

4. A 2 kW kettle for 15 min

5. A 600 W TV for 5 hrs

6. A 240 W music system for 3 ½ hours.

7. A 50 W clock radio for 24 hours

8. A 1.5 W battery charger for 10 hours.

9. Six 100 W bulbs for 6 hours

10. Six 25W energy saving bulbs for 6 hours (one of these will replace a 100W bulb)

Friday, 20 September 2013

Transformers caculations

Transformer Effect

V_in = N_p or V_out = N_s

V_outN_sV_inN_p

A step down transformer reduces voltage from 240 V (V_in) to 6 V. If there are 1000 turns on the primary windings (N_p), how many are there on the secondary windings ?

V_out = N_s 6V/240V = N_s/1000 N_s/1000 = 0.025 N_s = 1000 x 0.025 = 25 turns

V_inN_p

A step down transformer reduces voltage from 240 V (V_in) to 12 V (V_out). If N_p= 200 turns calculate N_s.

V_out = N_s12V/240v = Ns/200 Ns = 12/240 x 200 = 10 turns

V_inN_p

A transformer has 1000 turns on it’s primary windings and 100 in the secondary . If V_in is in 20V calculate V_out.

V_out = N_sV _out/20v = 100/1000 V_out= 20 (100/1000) = 2 V

V_inN_p

4. In a transformer N_p = 2000 turns and N_s = 50 turns. If V_in =400 V, calculate V_out.

V_out = N_sV _out/400v = 50/2000 V_out= 400 (50/2000) = 10 V

V_inN_p

5. A step up transformer increases voltage from 4 V to 40 V. If N_p = 100 turns calculate N_s.

V_out = N_s40V/4v = Ns/100 Ns = 40/4 x 100 = 1000 turns

V_inN_p

6. A step down transformer reduces voltage from 100 V to 10 V. If N_p = 2000 turns calculate N_s.

V_out = N_s10V/100v = Ns/2000 Ns = 10/100 x 2000= 200 turns

V_inN_p

7. In a transformer N_s = 10 turns and N_p = 500 turns. If V_out = 10 V calculate V_in.

V_in/V_out = N_p/N_s rearranging V_in = V_outN_p/N_s V_in = 10(500/10) = 500V

8. If N_s = 40 turns and Np = 4000 turns calculate V_in if V_out = 10 V.

V_in/V_out = N_p/N_s rearranging V_in = V_outN_p/N_s V_in = 10(4000/40) = 1000V

Thursday, 12 September 2013

Markscheme for physics homework

1. (a)

They do not depend on the weather.
There are no fuel storage problems.
They do not cause atmospheric pollution.
They can be sited anywhere.
There are no problems with transporting waste.
One wind farm can generate enough electricity to supply all of England.

[If more than 3 ticked – deduct 1 mark for each incorrect answer] 3

(b) A description to include:
1. magnet / magnet field / N-S poles;
2. movement (inside generator context important);
3. coil / wire;
4. induce V / I; 6

[8]

2. A

[1]

3. Direct current is only in 1 direction (1)

Alternating current periodically changes direction (1)

Transformers only change alternating voltages /current

transformers will not work on direct current (1)

[3]

4. C

[1]

5. (i) An explanation to include:

          1   energy / power / work;
2   input – out put / transfer;
3. input –output different / ratio /                                                  2
correct comparison;

(ii) friction / heat energy produced / wires get hot / sound / noise; 1

[4]

6.       Any two from:
large area needed
people unaffected by noise
wind more reliable at sea than on land;;                                                                     2
plus 1 communication mark for ensuring that spelling, punctuation                          1
and grammar are accurate, so that the meaning is clear;

[3]

7. (a) lower (fuel) costs / cheaper (fuel); (do not allow environmental effects) 1

(b) One from A and one from B
A
typical start up time is long
correct comment re specified power source and start up time;
B
general comment on output to match demand
correct comment re specified power source and power output ; 2

[3]

8. (i) changing polarity, 1

(ii)     Any two from:
     · stronger magnet;
     · more turns;
     · increase speed rotation;
     · placing coil on soft iron core;                                                                  2

[3]

9. (a) (i) number of units = 220;
220 × 2.5 = (£) 5.50 / 550p ;
[Accept (£)5.5 for both marks]
[If contradictory answers given in table and answer space, no marks] 2

(ii) (£)68.35; [ecf from any answer given in part (i)] 1

(b)   A suggestion to include one from:
1.      (night units are) cheaper / less expensive / eq;
2.      day units are more expensive;
3.      saves money / more economical;
[Reject less units used]                                                                                     1

[4]