O Level Notes : Physics - Practical Electricity

We are going to learn more about electricity. But before that, have you ever noticed the power ratings on the electrical appliances in your home? And wondered how they are measured? Don’t worry, we’ll discuss that! Starting with power, we have learned in the tutorial on Work, Energy and Power that power is the rate of work done or rate of energy converted.

Introduction:

Hey everyone! How are you? We are going to learn more about electricity. But before that, have you ever noticed the power ratings on the electrical appliances in your home? And wondered how they are measured? Don’t worry, we’ll discuss that!

 

Measuring Electrical Energy:

Starting with power, we have learned in the tutorial on Work, Energy and Power that power is the rate of work done or rate of energy converted. In symbols, it’s

P = W / t

or                                                                                      P = E / t

Now, the equation for potential difference V between two points in a circuit is given by:

V = E / Q

which is equal to                                                  E = QV

where V is the potential difference or voltage, E is the energy consumed (or work done in Joule) and Q is the charge in C (Coulomb).

 

If we substitute the above two equations, we’ll get P = QV / t.

Now substitute Q = It into the equation we just formed: P = VI

 

These equations will be very helpful to find the unknown (power in most cases).

  • P = VI
  • P = I2R (since V = IR)
  • P = V2 / R (since I = V / R)

You know that the SI unit of power is Watt, right?

 

Q1. A bulb is connected to 3 V battery of negligible internal resistance. The ammeter reading was 0.7 A. What is the power used by the bulb?

 

Coming to electrical energy, we know that P = E / t , we can write it as E = Pt or E = VIt (since P = VI).

Other formulas for energy are:

  • E = VIt
  • E = I2Rt (since V = IR)
  • E = V2t / R (since I = V / R)

 

 

Cost of electrical energy:

Now if you have ever noticed how the amount of electricity consumed by your house is calculated, you will know that it is calculated in kilowatt-hours (kWh) of electrical energy used. One kilowatt-hour is the amount of electrical energy used by a 1kW device in one hour. Solve this question and you’ll get clear about this:

 

Q2. A 1.5 kW electric heater is used to heat a large container of water for 2 hours. Calculate the amount of electrical energy used by the heater in (a) kWh, (b) J.

 

Safe use of electricity:

Before jumping to the real thing, you should all know what a live and a neutral wire is. Electricity is supplied by a cable containing these two wires. Live wire is usually at higher voltage of 240 V while neutral is at 0 V. The neutral wire completes the circuit by forming a path for the current back to the supply.

Now we’ll learn the safety features installed in our homes, these are:

  • Circuit breakers

These are the safety devices that can switch off the electrical supply in a circuit when there is an overflow of current.

The circuit breakers have a current rating written on them, and when the current exceeds these values, the circuit breaker will trip (it’s a switch which will turn off itself).

  • Fuses

A fuse is a safety device included in an electrical circuit to prevent excessive current flow. It is basically a thin piece of wire which becomes hot and melts when the flowing through it is greater than its rated value. This is how a typical fuse looks like:

Now how it is installed? The  current rating of the fuse should be slightly higher than the current the appliance will use under normal conditions. Let’s say an appliance uses 4 A normally, the fuse rating should be 5 A. Also, the fuse should be connected to the live wire so that the appliance will not become charged after the fuse has melted down.

 

  • Correct placement of switch in the circuit

The switch is fitted on the live wire so that switching it off disconnects the high voltage from the appliance. If the switch is placed wrongly on the neutral wire, the appliance will remain live even if the switch is off. And whoever touches the appliance will get an electric shock.

This diagram shows how a fuse is wrongly placed which can cause electric shock if touched because the metal casing is live.

  • The three-pin plug

 

This is how a typical power socket is used in a house. The live wire is brown in colour, earth wire is green while neutral is blue. If excessive current flows, the fuse will blows and breaks the circuit.

  • Earth wire

The earth wire (green in colour) is a low-resistance wire. It is usually connected to the metal casing of the appliance. If there is a fault let’s say the live wire is not properly connected and it touches the metal casing, the user could get an electric shock.Earthing it will prevent this from happening.

 

 

 

Answers:

Q1. 2.1 W

Q2. (a) 3 (b) 1.08 x 108.