You are Here: Home >< Physics

# Circuit Question! watch

1. If there's problem with reading the question, see the attachment.
Attached Images

2. I think what the mark scheme is saying, is that when you first start this test on the lamps, you don't know which one is faulty or what the fault is. So if the fault was lamp A shorted, then the second test, where you close switch 1, would produce a short circuit.
This could damage the power supply.
Of course, as it turns out, bulb A was ok.
It was just a safety measure and is a good reason for using the ohm-meter and not a power supply.
3. (Original post by Stonebridge)
I think what the mark scheme is saying, is that when you first start this test on the lamps, you don't know which one is faulty or what the fault is. So if the fault was lamp A shorted, then the second test, where you close switch 1, would produce a short circuit.
This could damage the power supply.
Of course, as it turns out, bulb A was ok.
It was just a safety measure and is a good reason for using the ohm-meter and not a power supply.
Okay, but how could a shorted bulb blow a power supply but not an ohm meter?
4. (Original post by Zishi)
Okay, but how could a shorted bulb blow a power supply but not an ohm meter?
Because a power supply has very low internal resistance. If you connect it to a short circuit, with no resistance, a very large current will flow. This could melt the wires or damage the power supply. Usually, a power supply will have a fuse that blows to prevent this happening.
An ohm-meter is a standard volt/amp meter that contains a small battery. When you set it to ohms it measures the current that flows through the component you are measuring and calculates its resistance from Ohm's law. It is designed with a large resistance in series (internal resistance) that ensures that, even if connected to a short circuit, only a small current flows. This is often called the current for full scale deflection, and the meter would be calibrated such that when it is connected to a short circuit, it produces a known current, determined by its internal resistance. When connected to an open circuit, no current flows.
5. (Original post by Stonebridge)
Because a power supply has very low internal resistance. If you connect it to a short circuit, with no resistance, a very large current will flow. This could melt the wires or damage the power supply. Usually, a power supply will have a fuse that blows to prevent this happening.
An ohm-meter is a standard volt/amp meter that contains a small battery. When you set it to ohms it measures the current that flows through the component you are measuring and calculates its resistance from Ohm's law. It is designed with a large resistance in series (internal resistance) that ensures that, even if connected to a short circuit, only a small current flows. This is often called the current for full scale deflection, and the meter would be calibrated such that when it is connected to a short circuit, it produces a known current, determined by its internal resistance. When connected to an open circuit, no current flows.
Thank you!

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: March 29, 2011
Today on TSR

### Uni league tables

Do they actually matter?

### University open days

• University of Warwick
Sat, 20 Oct '18
• University of Sheffield
Sat, 20 Oct '18
• Edge Hill University
Faculty of Health and Social Care Undergraduate
Sat, 20 Oct '18
Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE