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SCIENCE FACT FILE TEACHER’S GUIDE 156

Resistance: p 61

1. too much current flowing through the bulb

2. The thin filament of the bulb gets hot, burns and melts when a large current flows through it.

3. a) copper, nichrome

b) Copper has the lowest resistance.

4. The resistance of a wire decreases as its thickness increases.

Test yourself: p 62–63

1. a) A conductor is a material that allows electricity to pass through it.

b) iron wire, aluminium foil

c) The connections might be loose or the battery might be low. He could tighten the connections
or change the battery.

2. a) 0.25 A in each bulb.

b) 0.25 A in each bulb

c) 0.25 A in each bulb.

3. a) i) 0.3 A ii) 0.3 A iii) 0.3 A iv) 0.6 A

b) 0.3 A

c) i) 0.6 ii) 0.2 A iii) 0.2 A

4. a) electric fire: electricity to heat

table lamp: electricity to light

hairdryer: electricity to heat and mechanical energy

television: electricity to light and sound energy

toaster: electricity to heat

b) i) electric fire ii) table lamp

c) A toaster needs thicker wires for greater resistance to cause the element to heat up.

d) i) 13 A ii) 5 A iii) 3 A

e) A fuse is made of a wire which has a high resistance. When too much current flows through it,
the wire melts and breaks the circuit. When this happens the supply of electricity is cut off and
the rest of the circuit, including any device attached to it, remains safe.

5. a) bedroom 1

b) bathroom

c) Sometimes both switches may be on or off at the same time.

d) i) It is called a ring main.

ii) The circuit is made in the form of a ring, and devices can be placed anywhere in the ring
between the two wires.

6. a) i) towards A ii) towards B

b) A thicker wire has lower resistance and so the bulb will glow more brightly.

c) i) It is called a variable resistor because the value of the resistance can be changed by moving
the clips.

ii) It helps to control the amount of current flowing through a circuit. It also controls the voltage
at different points in a circuit.

Workbook 1 Chapter 6

1.

switch bulb or
lamp

motor battery
(one cell)

wire buzzer ammeter bell

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57SCIENCE FACT FILE TEACHER’S GUIDE 1

2.

3. a) 0.4 A

b) (i) The brightness decreases.

(ii) The total current is divided equally between the two bulbs.

(iii) It becomes difficult for the current to flow in the circuit.

(iv) The cell is pushing the current through two bulbs.

4. a) i) B2

ii) The circuit passing through B2 is complete.

b) They will glow with equal brightness.

c) a parallel circuit

d) in homes, schools, offices, etc.

5. a) i) The reading on the ammeter also increases.

ii) The wire wool becomes hotter.

b) The wire wool should not be touched.

c) It tries to stop the current from flowing through it.

d) in an electric bulb, in fuses

6. a) i) 13 A

ii) 3 A

b) If the current flow becomes too large, the 13 A fuse will not break the circuit, and the food
processor may burn out.

c) i) 13 A

ii) 950 / 230 = 4.13 A

iii) 5 A

Project
Varying resistance

You will need a battery, 3 wires, a bulb, a soft pencil

Soak the pencil in water for one day. Strip the wood from the pencil until you have the bare lead. Use one
piece of wire to connect the bulb and battery. Attach the other two wires to the bulb and battery so that their
ends are free. Touch the pencil lead with the free ends, close together but not in contact. The bulb should
light up. Move the wires apart so that there is more pencil lead in the circuit, increasing the resistance and
making the bulb glow less brightly.

Multiple Choice Questions
1. Which of the following is not a conductor of electricity?

A a copper wire B an iron wire C a nichrome wire D a rubber band

2. Electric current is a stream of flowing through a conductor.

A electrons B molecules C neutrons D protons

3. Electric wires are usually made of

A gold B silver C copper D iron

4. A safety device which prevents the passage of too large a current is called

A a fuse B a switch C an ammeter D a motor

a) b) c)

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113SCIENCE FACT FILE TEACHER’S GUIDE 1

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