The
present perfect and the simple past
11
That is Mr Minus, who teaches me mathematics, but he (not have) time to
teach me much. I only (be) in his class for a week.
12
You (hear) his speech on the radio last night? -Yes, I . . . -
What you (think) of it?
13
I (not know) that you (be) here. You (be) here long?-Yes, I (be) here
two months. —
You (be) to the Cathedral? ~ Yes, I (go)
there last Sunday.
14
You ever (try) to give up smoking? -
Yes, I (try) last year, but then I (find)
that I was getting fat so I (start) again.
15
You (see) today's paper? -
No, anything interesting (happen)?
Yes, two convicted murderers (escape) from
the prison down the
road.
16
Mary (feed) the cat?-
Yes, she (feed) him before lunch. ~ What
she (give) him? -She (give) him some fish.
17
How long you (know) your new assistant?-I (know) him for two years. —
What he (do) before he (come) here? -I
think he (be) in prison.
18
I (not see) your aunt recently. -
No. She (not be) out of her house since
she (buy) her colour TV.
19
The plumber (be) here yet?-
Yes, but he only (stay) for an hour. -
What he (do) in that time? -
He (turn) off the water and (empty) the
tank.
20
Where you (be)?
I (be) out in a yacht. -
You (enjoy) it? -
Yes, very much. We (take) part in a race.
-
You (win)?~
No, we (come) in last.
21
How long that horrible monument (be) there?-
It (be) there six months. Lots of people
(write) to the Town Council asking them to take it away but so far nothing (be)
done.
22
I just (be) to the film War and Peace. You (see) it? -No, I
Is it like the book? -
I (not read) the book. -
I (read) it when I (be) at school. -
When Tolstoy (write) it? -
He (write) it in 1868. -
He (write) anything else?
23
Hannibal (bring) elephants across the Alps. -Why he (do) that?-
He (want) to use them in battle.
24
Where you (be)?-
I (be) to the dentist. ~
He (take) out your bad tooth? -
Yes, he . . . -
It (hurt)?-
Yes, horribly.
25
She (say) that she'd phone me this morning, but it is now 12.30 and she
(not phone) yet.
26
I just (receive) a letter saying that we (not pay) this quarter's
electricity bill. I (not give) you the money for that last week? ~ Yes, you . .
. but I'm afraid I (spend) it on something else.
27
How long you (be) out of work? -
I'm not out of work now. I just (start) a
new job. -
How you (find) the job? -
I (answer) an advertisement in the paper.
28
You (finish) checking the accounts? -No, not quite. I (do) about half so
far.
29
I (cut) my hand rather badly. Have you a bandage?-I'll get you one. How
it (happen)? -
I was chopping some wood and the axe
(slip).
30
How you (get) that scar? -
I (get) it in a car accident a year ago.
31
You (meet) my brother at the lecture yesterday? -Yes, I We (have)
coffee together afterwards. •
32
He (lose) his job last month and since then he (be) out of work. -Why he
(lose) his job? -
He
(be) very rude to Mr Pitt.
N - мощность горелки,
t - искомое время,
Q - затраченное количество теплоты.
Разберемся поэтапно с Q.
На что наша горелка будет затрачивать энергию?
- плавление льда: λ m(л)
- нагрев образовавшейся воды до температуры кипения от начальной - нуля: c m(л) (100 - 0) = 100 c m(л)
- нагрев воды, которая уже находилась в сосуде: c m(в) (100 - 0) = 100 с m(в)
Таким образом, Q = λ m(л) + 100 c m(л) + 100 с m(в).
Запишем найденную формулу Q в формулу мощности:
N = ( λ m(л) + 100 c m(л) + 100 с m(в) ) / t,
откуда искомое время t:
t = ( λ m(л) + 100 c m(л) + 100 с m(в) ) / N.
Упростим выражение (выносим сотню и удельную теплоемкость воды за скобки):
t = ( λ m(л) + 100 c (m(л) + m(в)) ) / N,
t = ( 335*10^3 * 35*10^-2 + 10^2 * 42*10^2 * 9*10^-1) / 1,5*10^3,
t = (117250 + 378000) / 1,5*10^3,
t = (117,25 + 378) / 1,5 ≈ 330,16 c ≈ 5,5 мин
Напряжение, как и ЭДС, измеряется в вольтах (В) . Установившиеся значения напряжения обозначают прописной буквой U, неустановившиеся значения строчной буквой u. По аналогии с током различают постоянное и переменное напряжения. Постоянное напряжение может изменяться по величине, не изменяя при этом своего знака. Переменное напряжение периодически изменяет и величину и знак.