ELECTROMAGNETISM
1. Explain why transformer are
used to reduce the current passing through the transmission cables of the
National Grid
[Show answer]
The transformer (Step –up) reduces the current passing through the transmission cables of the
National Grid so as to reduce the energy wasted (Power Loss) by heating up the cables.
[Addition Answer]
This is why very high potential difference are used to transmit electrical power over long
distances.
Refer
REASON:
Electricity is transmitted at a very high potential difference (Voltage) and low currents to reduce energy loss and improve efficiency
Refer
- NECTA 2019, Physics 1, Qn 11(a)
- NECTA 2003, Physics 1, Qn 7(a)
REASON:
Electricity is transmitted at a very high potential difference (Voltage) and low currents to reduce energy loss and improve efficiency
2. Explain why transformer
does not work using a d.c. supply.
Why is an a.c. supply necessary to make a transformer work?
[Show answer]
According to Faraday Law of Electromagnetism that says “wherever there is a change of magnetic
flux (linking the coils) e.m.f is induced, so if the d.c is supplied to the primary coil of the
transformer the magnetic field is constant (No change of magnetic flux). Then no potential
difference (voltage) induced in the secondary coil.
Why is an a.c. supply necessary to make a transformer work?
[Show answer]
When a.c (its magnitude varies periodically) is supplied to the primary coil, a changing magnetic field is produced. The iron core becomes magnetized and carries the changing magnetic field to the secondary coil which will induce potential difference (e.m.f) in the secondary coil.
3. Explain why a bicycle lamp
attached to the to the dynamo does not light when the bicycle is stationary?
[Show answer]
A bulb in dynamo system will light if the e.m.f will be induced by changing magnetic field (of the
rotating permanent magnet) linking the coil. If the bicycle is stationary (permanent magnet
joined to the Shaft do not rotate) the magnetic field will be constant (No change of magnetic
flux), no potential difference (voltage) induced in the coil and the bulb will not light.
4. A student is trying to test
a transformer using battery. Explain why the voltmeter connected to the
secondary coil reads 0 V?
[Show answer]
Battery supply direct current, so if the d.c is supplied to the primary coil of the transformer the magnetic field is constant (No change of magnetic flux). Then no potential difference (voltage) induced in the secondary coil.
5. A wire is made to move
through a magnetic field so that a potential difference is induced across its
ends. State two ways in which the size of induced potential difference can be
increased.
[Show answer]
Ways to increase the size of induced e.m.f
- Increase speed of wire (Move wire faster)
- Increase strength of the magnet
- Add more turns to the coil
6. Explain why very high
potential differences are used to transmit electrical power over long distances
[Show answer]
Electricity is transmitted at a very high potential difference (Voltage) and low
currents to reduce energy loss and improve efficiency
7. Figure below shows simple
transformer
a) Is the transformer being used as step-up or step-down
transformer?
b) Why must the wire used to make the coils be insulated?
[Show answer]
The wire should be insulated in order to prevent the insulated wire's current from coming into contact with other conductors. It preserves the wire material against environmental threats and resists electrical leakage.
ii) Why must the core be made from this material?
[Show answer]
Soft iron core is magntized and demagnetized easly hence minimize hysteresis power loss
8. Figure below shows part of
a bicycle dynamo which is in contact with the wheel
a) Explain fully why a current flow through the lamp when the
bicycle wheel turns
[Show answer]
When the bicycle wheel turns the e.m.f will be induced by changing magnetic field (of the
rotating permanent magnet) linking the coil according to Faraday Law of Electromagnetism that says “wherever there is a change of magnetic flux (linking the coils) e.m.f is induced".
b) Why does the lamp get brighter as the cycle moves faster?
[Show answer]
As you go faster, the wheel spins faster and the voltage of the electricity increases, making the light brighter.
c) Why does the lamp not work when the bicycle is stationary?
[Show answer]
A lamp in dynamo system will light if the e.m.f will be induced by changing magnetic field (of the
rotating permanent magnet) linking the coil. If the bicycle is stationary (permanent magnet
joined to the Shaft do not rotate) the magnetic field will be constant (No change of magnetic
flux), no potential difference (voltage) induced in the coil and the bulb will not light.
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