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Define resistance - Leaving Cert Physics - Question 9 - 2012
Question 9
Define resistance. (i) Two resistors of resistance $R_1$ and $R_2$ are connected in series. Derive an expression for the effective resistance of the two resistors i... show full transcript
Worked Solution & Example Answer:Define resistance - Leaving Cert Physics - Question 9 - 2012
Step 1
Derive an expression for the effective resistance of the two resistors in terms of $R_1$ and $R_2$
Answer
For two resistors connected in series, the total or effective resistance is simply the sum of the individual resistances. Therefore, the formula is given by:
This means that the total resistance increases as more resistors are added in series.
Step 2
Draw a circuit diagram to show how another 4 Ω resistor could be arranged to give an effective resistance of 6 Ω
Answer
To achieve an effective resistance of 6 Ω with two 4 Ω resistors in parallel and an additional 4 Ω resistor in series, the arrangement can be illustrated as follows:
4 Ω
+-----+
| |
| 4 Ω |
| |
+-----+
In parallel, the effective resistance of the two 4 Ω resistors is:
rac{1}{R_P} = rac{1}{4} + rac{1}{4}
This gives:
When the 2 Ω is then placed in series with another 4 Ω:
Step 3
How does a fuse operate?
Answer
A fuse operates as a safety device by being integrated in the live part of the circuit. Its function is to protect the circuit by melting or breaking the connection if the current exceeds a certain rated value (which is predetermined and specified). This action stops the flow of current, thus safeguarding the circuit components from damage.
Step 4
What test would you use to determine that the bridge is balanced?
Answer
To determine that the Wheatstone bridge is balanced, I would connect a galvanometer (G) or a milliammeter (mA) across points A and C. The bridge is considered balanced when there is no deflection in G (or in the milliammeter), indicating equal potential at both ends.
Step 5
Step 6
What type of resistor is it? Give a use for this type of resistor.
Answer
The unknown resistor, when covered by black paper and causing the bridge to go out of balance, is most likely a light-dependent resistor (LDR). This type of resistor changes its resistance based on the intensity of light falling on it. A common application for LDRs is in light meters used to measure ambient light levels.