Activity: Electrical power

Aims

• Derive the formula P = IV
• Measure the electrical power for different examples
• Estimate power loss in cables.
• Have an idea of magnitudes of domestic electrical power usage

Power converted by the battery • What is the definition of EMF?
• How much chemical energy is converted to electrical PE when charge q flows in the battery?
• If charge q flows in time t what is the power delivered by the battery?

You should now understand why the power delivered by the battery is given by:

P = εI

Power dissipated in a resistor

• What is the definition of potential difference across a resistor?
• How much electrical energy is converted to heat when charge q flows?
• If charge q flows in time t what is the power dissipated in the resistor?

So power dissipated is give by:

P = IV

From Ohm's law V = IR.

• Substitute for V to give an equation in terms of I and R.
• Substitute for I to give an equation in terms of V and R.

Measuring power

Electric kettle

An electric kettle converts electrical energy into heat.

• Use a temperature sensor to measure the rate of temperature rise for an electric kettle and calculate the power delivered to the water.
• How does your value compare to the makers value?

Light bulb A light bulb converts electrical energy into light but also produces heat.

• Use the special "light bulb in water" apparatus to measure how much energy is converted to heat per unit time.
• Measure the current through and pd across the light bulb and calculate the power dissipated.
• Calculate the % of electrical energy converted to light per second.

Electric motor

An electric motor converts electrical energy to mechanical energy

• Devise a method for measuring the power of a battery power drill. Discuss the method with your teacher and if possible perform the experiment.

Heat loss in cables

Ideal wires have zero resistance, real ones don't

• Measure the resistance of the cable connecting the kettle to the mains supply. The cable must be disconnected from the mains supply when you do this.
• Calculate the heat loss per second in the cable when the kettle is in use.

If too much heat is generated in a cable it will get hot and the insulator will melt.

DO NOT TRY THIS!

Different electrical devices need different amounts of current.

• Use the "power meter" to measure the power of different devices.
• Is there a relationship between the power of the device and the thickness of the cable?

When wiring a house some devices need more power than others so different cables must be used. To protect the cables from overload a circuit breaker is installed, this will switch off if the current becomes too big for the cables. Have a look in the switch cupboard. • What is the current value for the largest circuit breaker?
• What sort of devices are connected to this circuit?
• What sort of device is the lowest current circuit used for?
• Estimate the total current that can be supplied.
• Calculate the maximum power that could be delivered.
• Electrical energy is measured in kWh, how many Joules is equivalent to 1 kWh?

Batteries

The total amount of energy that can be convert from chemical to electrical during the life of a battery can be calculated from the Ah (Amp hour) rating. A 1 Ah rating means the battery can deliver 1 A for 1 hour. A typical 1.5 V AA has a rating of about 2 Ah.

• Calculate the total energy that can be delivered by an AA battery (in Joules).
• Find out the rating of different makes of battery. Do the expensive ones contain more energy?

Exercises on page 228 - 229

Power and potential dividers multiple choice

All materials on this website are for the exclusive use of teachers and students at subscribing schools for the period of their subscription. Any unauthorised copying or posting of materials on other websites is an infringement of our copyright and could result in your account being blocked and legal action being taken against you.