Teacher's notes and answers to questions in the book - Hodder Plus ...

WJEC GCSE Additional Science Teacher’s Notes
2. Describe in words the pattern of each graph. This means that you have to describe how the voltage
(on the y-axis) varies with the current (on the x-axis).
Resistor – voltage is proportional to current (graph is linear with a positive gradient).
Bulb – voltage increases faster than current (graph is curved with increasing positive
gradient).
3. Explain how a variable resistor can be used in a circuit to control the current through and the voltage
across other components.
Altering the resistance of a circuit alters the current flowing through the circuit. If the
current is altered then the voltage across the other components will change due to Ohm’s
law.
Questions
7. Using the data that you collected for the Practical: Measuring current and voltage in circuits
controlled by a variable resistor, construct two tables. One table will be for the fixed resistor data,
and the other table will be for the bulb.
The values in the table will depend on the students’ results from the Practical.
8. Describe the patterns in your results for Question 7. How does the resistance of the fixed resistor vary
with current (or voltage) How does the resistance of the bulb vary with current (or voltage)
The resistance of the fixed resistor is constant for different values of current (or voltage).
The resistance of the bulb will increase with an increasing current (or voltage).
9. A 25 Ω fixed resistor has a current of 2 A through it. Calculate the voltage across the fixed resistor.
V = I × R; V = 2 × 25 = 50 V
10. In a mobile phone circuit, 1.5 V is applied across a keyboard circuit with a resistance of 5000 Ω. What is
the current in the keyboard circuit
V
I R
V
so I
R
1.5
5000
0.003 A
11. Figure 14.13 shows the electrical characteristic of a 12 V car bulb. Use the graph to calculate the
resistance of the bulb when the current through the bulb is:
a 0.2 A V = 0.4 V so R =
b 0.6 A V = 3.6 V so R =
c 1.0 A V = 10.0 V so R =
0.4
= 2
0.2
3.6
= 6
0.6
10 .0 = 10
1.0
12. Explain why the resistance of a bulb changes when more current is passed through it. (Hint: when the
bulb has more current going through it, it is brighter and hotter. How might this affect the structure of
the metal filament)
As more current flows through the filament, there will be more collisions of the free electrons
with the structure of the metal filament and the electrons themselves. This heats the filament
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