Current II Questions

1.  A battery is rated 120 AH. How long will it work if it steadily supplies a current of 4A.

2.  The current capacity of an accumulator is 40Ah. Find the amount of current flowing if the  accumulator is used for 600 minutes

3.  (a) A student hung a magnet next to a coil of wire to make a door chime as shown in figure 6:-

When the switch S was put on, the magnet hit the chime bar which made some noise.

(i) Explain how the current made the magnet move towards the chime bar

(ii) What should the student do to make the magnet hit the chime bar harder?

(iii) The student was asked to describe the energy changes inside the device. State the changes:

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(b) A coil of wire is connected in series with a battery, a rheostat and a switch as shown in figure 7:

(i) Draw on the diagram, the shape of the magnetic field inside and outside the coil when the

switch is closed   If the jockey J on the rheostat is moved towards Q what’s the effect on:-

(ii)The resistance of the circuit

(iii) The current through the coil

(iv) The magnetic field in the coil

(i) Explain why a transformer will only transform alternating voltages and not direct current voltage

(ii) Explain why transformers are widely used throughout the national grid system

4.  Determine the current passing through L1 in the figure shown below, given that 0.8A passes

through the battery, 0.28A through L2 and 0.15A through L3.

5.  State two advantages of generating an alternating current (a.c) to direct current (d.c) in a power

station.

1.  120 = 4t

t = 120
1 mk

4

= 30 hrs 1 mk

2.  I = Q = 40 = 40 = 4A

t 600 10

60

3.  (a)  (i) – When the switch is closed current flows through the coil causing a magnetic field. This repels the magnet towards the chime bar.

– The end of the coil adjacent to the sound pole of the bar magnet acquires a south pole.

(ii) – Increase he number of turns in the coil.

• Increase d.c source.
• – Introduce a soft iron core in the coil.

(b) (i) Electrical – magnetic – potential – sound energy

(ii) Resistance in the circuit increase.

(iii) Current reduces.

(iv) Become less stronger.

(c) (i)  – In alternating current there is a change in the magnetic flux of the primary coil linked  to the secondary soil.

– In direct current there’s no charge in the flux therefore no inducement of e.m.f in it.

(ii). – Transformers step up voltage to higher value for transmission which minimizes

power loss.

– They also step down higher voltage to the voltage required by the consumers.

4.

(a) Current tough L1:

0.8 = 0.15 + 0.28 + L1

0.8 = 0.43 + L1

L1 = 0.37A

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