Units of Measurements and Electrical Continuity Testing – SS1 Physics Lesson Note (Third Term)

Measurement is the process of comparing a physical quantity with a standard unit, so that it is given a number and a unit.

Behavioural objectives

By the end of the lesson, students should be able to:

1. State the two things every measurement requires and give the SI units of length, mass and time.
2. Name the instruments used to measure length, time, mass, weight and volume.
3. State the smallest length that the vernier callipers and the micrometer screw gauge can measure.
4. State the differences between mass and weight and use the relationship W = mg.
5. Explain how to read a vernier calliper.
6. Describe how to test a wire, fuse or bulb for electrical continuity.

What is measurement?

Measurement is the finding of the size of a quantity by comparing it with an agreed standard unit.

• Number: the size of the quantity, for example 20.
• Unit: the standard used for comparison, for example metres.
• Complete measurement: a number and a unit together, for example a length of 20 metres.
• Importance: physics is based on exact measurements, and a law is only accepted if it can be measured and quantified.

What are the SI units of measurement?

SI units are the internationally agreed units used for measurement in science.

How is length measured?

Length is measured with a metre rule, tape rule, callipers, vernier callipers or micrometer screw gauge.

The metre rule

• Use: measures distances from a few centimetres to some metres, such as the sides of a table or room.
• Smallest graduation: 0.1 cm or 1 mm.
• Longer distances: a tape rule is used.

Callipers

• Use: measure distances on solid objects where a metre rule cannot be applied directly.
• Structure: hinged steel jaws.
• Method: close the jaws (external callipers) until they touch the part being measured.
• Reading: measure the gap between the jaws against a graduated scale such as a metre rule.

The vernier callipers

• Accuracy: measures small lengths to the nearest 0.1 mm (0.01 cm), more accurately than a metre rule.
• Parts: two sets of jaws, a main scale and a sliding vernier scale.
• Uses: thickness of a metre rule, internal and external diameters of a tube, diameter of a rod.

The micrometer screw gauge

• Accuracy: reads to 0.01 mm or 0.001 cm, finer than the vernier callipers.
• Uses: diameter of a wire, thickness of a sheet of paper, diameter of a small ball such as a pendulum bob.

How do you read a vernier calliper?

A vernier calliper reading is the main scale reading plus the vernier scale reading, taken in centimetres.

1. Close the jaws and check for zero error. If the zeros do not line up, correct every reading.
2. Clamp the object between the jaws.
3. Read the whole number and the first decimal digit from the main scale, at the zero mark of the vernier scale.
4. Find the vernier mark that lines up exactly with a main scale mark.
5. Read that vernier mark as the second decimal digit.
6. Add the two readings. Example: main scale 2.3 cm plus vernier mark 6 (0.06 cm) gives 2.36 cm.

How is time measured?

Time interval is the difference between the times at which two events occur.

• Minutes or hours: clocks and watches, which show the time of day.
• Seconds: stop clocks and stopwatches, used for short intervals such as the time of swing of a pendulum.

How are mass and weight measured?

What is mass?

Mass is the quantity of matter in a body.

• Method: compare the body with standard masses using a balance.
• Beam or chemical balance: balances the body against standard masses on a pan.
• Lever balance: uses a system of levers.
• Dial spring balance: gives a direct reading on a dial.

What is weight?

Weight is the force acting on a body due to the earth’s gravitational pull.

• Instrument: the spring balance.
• Unit: the newton (N).

What are the differences between mass and weight?

What is the relationship between mass and weight?

Weight equals mass multiplied by the acceleration due to gravity.

W = mg

• W: weight in newtons (N).
• m: mass in kilograms (kg).
• g: acceleration due to gravity in m/s².

How is volume measured?

The volume of a liquid is measured with a graduated cylinder.

• Accuracy: reads to about the nearest 1 cm³.
• Irregular solids: the volume of an object such as a stone is found with the displacement (eureka) can.

What is electrical continuity testing?

Electrical continuity testing is checking whether a complete path exists for current to flow through a wire, fuse, switch or component.

• Continuity: the path is unbroken, so current can flow.
• Open circuit (break): the path is broken, so no current flows.
• Common faults: a broken wire, a blown fuse, a burnt bulb filament or a faulty switch.

What instruments are used for a continuity test?

• Multimeter: set to the continuity (buzzer) setting or the lowest resistance (ohm) range.
• Simple tester: a cell, a small bulb and connecting wires joined in series with the component.

How do you test continuity with a multimeter?

1. Switch off the supply and disconnect the component from any power source.
2. Set the multimeter to the continuity or lowest ohm setting.
3. Touch the two probes together. The buzzer should sound or the reading should be close to zero.
4. Place one probe at each end of the wire or component.
5. Observe the buzzer or the reading.

How do you interpret the result?

What safety rules apply?

• No live circuits: never test continuity on a circuit connected to the mains.
• Isolate first: unplug the appliance or switch off at the source.
• Dry hands: hold the probes by the insulated handles.

Evaluation

1. State the two things that every measurement requires.
2. Name the instrument used to measure the diameter of a wire and state the smallest length it can read.
3. State two differences between mass and weight.
4. Write the relationship between mass and weight and state what each symbol stands for.
5. Describe how to test a fuse for continuity with a multimeter and state what a buzzer sound means.

Assignment

Explain, in steps, how a reading is taken from a vernier calliper. Then name the instruments used to measure time intervals of minutes or hours, time intervals of seconds, mass and weight.