Overview, Year 9, Formulas

Overview, Year 9, Formulas

A. Jørgensen

This document gives an overview of the formulas that were introduced in Term 1, by giving examples of how to solve exercises.

1. We have made the following exercise on class: Electrons have a charge of1.602×10⁻¹⁹C.

a) How many electrons pass through a wire per second, if the current i 1 A?

I = 1 A, t= 1s I = Q

t ⇔Q=I·t= 1 A·1s = 1C

s ·1 s = 1 C.

number of electrons = 1 C

1.602×10⁻¹⁹C = 6.2×10¹⁸

b) Calculate the number of electrons that flow though the wire every minute.

2. By the way, the exercise above resembles another kind of exercise that we have done: If every atom has a mass of9×10⁻²⁶kg, how many atoms are then in one 1 kg of the considered material?

m_total

m_atom = 1 kg

9×10⁻²⁶kg = 1,1×10²⁵.

3. If every atom has diameter of10⁻¹⁰m, how many atoms does it take to make a line atoms that is 1 m long?

1

4. You have a transformer, where the primary coil has 50 turn, and 5 A flow through this coil.

a) If you need the current through the secondary coil to be 10 A, what is the number of turns of the secondary coil then?

I₁= 5 A, I₂ = 10 A, N₁ = 50

I₁ I2

= N₂ N1

⇔N₂ =N₁I₁ I2

= 505 A 10 A = 25

b) The electric potential across the primary coil is 100 V. What is the electric potential across the secondary coil?

There are several ways to solve this, as you knowI1,I2,N1andN2. In any case, use that

U₁ = 100 V

U₁ U2

= I₂ I1

= N₁ N2

⇔U2=U1

I₁ I2

= 100 V 5 A

10 A = 50 V

c) Calculate the power.

U₁ = 100 V, I₁ = 5 A, U₂ = 50 V, I₂ = 10 A P =U₁·I₁=U₂·I₂ = 100 V·5 A = 500 W.

5. You have a transformer, whose primary coil has 500 turns, and whose secondary coil has 250 turns. The electric potential across the secondary coil is10 V

a) calculate the electric potential across the primary coil.

b) The power 2000 W. Calculate the current through the secondary coil.

6. Consider¹⁶O. Oxygen is the eighth element in the periodic table.

a) FindA,Z andN.

A= 16, Z = 8, A=Z+N ⇔N =A−Z = 8 (1) b) How many electrons does neutral¹⁶O contain?

The number of electrons and protons is the same, in a neutral atom. Thus, there are 8 electrons.

c) How many electrons are there in the ion¹⁶O²⁺.

The total charge is 2+. So there are two protons more than there are electrons; there are thus 6 electrons.

7. Consider³⁵Cl. Chlorine is the seventeenth element in the periodic table.

a) FindA,Z andN.

b) How many electrons does neutral³⁵Cl contain?

c) How many electrons are there in the ion³⁵Cl⁻.