# Measuring Current, Voltage and Resistance

#### Students’ Learning Outcomes

·         Explain the use of ammeter in electric circuits.
·         Measure current using different devices.
·         List electrical hazards and precautionary measure to ensure the safe use of electricity at home.
·         Describe why electricity is hazards to humans.

#### Information for Teacher

·         The current is measured by ammeter or ampere-meter and galvanometer. Ammeters are of different ranges. Smaller current is measured with mille ammeter. A minute current is measured with galvanometer.
·         A low value resistance is connected in parallel to the galvanometer. This provides an easy path to the current and it is converted into an ammeter.
·         To measure the current in the components. The ammeter is always connected in series otherwise it will be damaged.
·         Considerable increase of current in a circuit may occur due to overloading, short circuit and random voltage fluctuation.
·         Often fire is caused due to short circuit.
·         A fuse or circuit breaker is used to protect the electric appliances.
·         Some time live wire of faulty appliance comes in contact with the metallic body.
·          A person touching the body will get electric shock.
·         Earthling the metallic body saves the person from the shock.
·         It is dangerous to touch the device with wet hands.
·         Electricity is a good servant but a bad master.
·         One must observe proper safety precautions while using the electricity or electrical devices.
·         Some precautions are as:
Ø  Proper insulation of wires and joints.
Ø  Proper electrical connections.
Ø  Use of circuit breaker, fuse and earth wire.

#### Material / Resources

Bulb (3.OV), connecting wires, bulb holder, ammeter, dry cells and holder

#### Worm up activity

·         Draw two simple circuits on the board, circuit 1 consist of one bulb and two dry cells, and circuit 2 consists of two bulbs and two dry cells connected in series. Ask the students:
·         Img
Q. In which circuit bulb will be brighter?
(Expected response: The bulb in circuit 1)
Q. Why it lights brighter?
(Because the current passing through the bulb in circuit 1 is larger than that in circuit 2)
Q. How can we measure the current?
(By connecting a device like ammeter or galvanometer)

#### Activity 1

Using an ammeter:
·         Divide the students in groups and provide an ammeter to each group.
·         Ask the students to look at this apparatus and observe it carefully.
·         Connect a bulb, key and cell in series as shown in fig.1
·         Step 1
·         Cut the connecting wire at p (we can take point p anywhere in the circuit) and name two ends of connecting wire as p+ and p- as shown in fig. 2.
·         Step2:
·         Connect the end p+ o the positive terminal of the ammeter and p- to the negative terminal as shown in fig3.
·         Step3:
·         As a result, the ammeter is connected in series with the bulb.
·         After the explanation, the teacher will ask:
Q. Why the colors of the terminal are red and black?
(Expected response: Red is positive terminal and black is negative terminal)
·         Q. if we want to measure the current passing through the bulb, how should we connect the ammeter?
·         (Expected response: In series)
·         Q. Why we connect the ammeter in series?
·         (Expected response: So that the current may flow through the ammeter with damaging it)
·         Q. What is the unit of the current?
·         (Expected response: Ampere)
·         Draw the circuit as shown in fig.8 on the writing board and ask:
·         Q. If we connect the ammeter at this position as shown in fig.5 what will happen?
·         (Never connect the ammeter in parallel. It will be damaged)

#### Activity 2

·         Divide the class in four groups:
·         The teacher will provide all the necessary materials to each group and instruct each group to make simple circuits.
·         Circuit-1 one bulb and two dry cells connected in series.
·         Circuit-2 Two bulbs and two cells connected in series.
·         Ask each group to measure the currents. 11 and 12 which pass through each circuit and fill the following table on the writing board after collecting the information from the groups.
 Group Current 11 (Circuit 1) Current 12 (Circuit 2) G 1 G 2 G 3 G 4
Q. What do you find from the table?
(Expected response: (12 is larger than 11)
Q. Why the bulb in circuit 1 lights brighter than the bulb in the circuit 2?
(More current is passing in circuit1)

#### Sum up / Conclusion

·         Ammeter and Galvanometer are used to measure the current.
·         Ammeter is always connected in series.
·         Never connect these meters in parallel to the load.

#### Assessment

Q. Why the ammeter should not be connected in parallel?
(Expected response: Ammeter will be damaged)
Q. How the ammeter is connected in each circuit?
(Expected response: The ammeter is connected in series with the load (bulb) in the circuit)
Q. what is the function of ammeter?
(Expected response: To measure the flow of current)
Q. Name the devices used to measure the current?
(Expected response: Ammeter & galvanometer)