Magnetism & Electromagntism (7)-pt2

Question 1

What do alternating currents look like on an oscilloscope?

Answer 1

-a wave that goes up and down, crossing the horizontal axis

Question 2

What do direct currents look like on an oscilloscope?

Answer 2

-it stays above the horizontal axis (the pd is always positive)

Question 3

How do microphones work?

Answer 3

• they generate currents from sound waves
• sound waves hit a flexible diaphragm attached to a coil of wire wrapped round a magnet
• causing the wire to move in the magnetic field which generates current
• it converts the pressure variations of a sound wave into variations of a current in an electric circuit

Question 4

Explain what transformers do?

Answer 4

• they change the size of the pd in an ac
• it has 2 coils of wire, primary & secondary, joined by an iron core
• iron is used because it’s easily magnetised

Question 5

How do transformers work?

Answer 5

• when an ac is applied across the primary coil the iron ore magnetises and demagnetises quickly
• this changing magnetic field induces an alternating pd in the secondary coil

Question 6

What do step-up transformers do?

Answer 6

• they increase the pd

- they have more turns in the secondary coil than the primary coil

Question 7

What do step-down transformers do?

Answer 7

• they decrease the pd

- they have more turns on the primary coil than the secondary

Question 8

What’s the transformer equation that links pd and number of turns in the coils?

Answer 8

V_p / V_s = N_p / N_s
N= number of turns in the coil
V=pd
They equation can be flipped

Question 9

What’s the transformer equation that links pd and the current in the coils?

Answer 9

V_s I_s = V_p I_p
I= current (A)
V = pd (V)

Question 10

Why is the national grid so efficient?

Answer 10

• a low current means less energy wasted hearing the wires

- V_s I_s = V_p I_p proves a high pd is needed for a low current