CURRENT ELECTRICITY

Energy dissipation in resistors

when current flows through a resistor, electrical potential energy is lost athe charges move through the material. This energy is dissipated as heat. The power dissipated in resistor R carrying current I, is given by; P = R. If the resistor has a fixed voltage V applied across it. P = . This is because resistors are never perfect, this power dissipation causes the components to heat up. The maximum safe power a resistor can handle without burning is call its power rating e.g. 0.5wtt, 1wtt etc.

Total Dissipated Energy Over Time

If a device dissipates power P for a time t, the total energy E lost (Mainly as heat) is calculated as E = Pt

Using the different forms of power, this becomes

E = VIt = Rt =  

4.3.1 Examples

1. Calculate the current in a 60W light bulb when it is connected to a 230V power supply.

2. A large power station supplies electrical energy to the grid at a voltage of 25kV. Calculate the output power of the station when the current it supplies is 40kA

3. A 12V car battery can supply a current of 10A for 5.0 hours. Calculate how many joules of energy the battery transfers in this time.

4. A lamp is operated for 20s. The current in the lamp is 10A. In this time, it transfers 400J of energy to the lamp. Calculate:

a. how much charge flows through the lamp

b. how much energy each coulomb of charge transfers to the lamp

c. the p.d. across the lamp.

A small electronic device is rated at 0.25 W when connected to 120 V. What is the resistance of this device?  

A calculator is rated at 0.10 W and has an internal resistance of 22 Ω. What battery potential difference is required for this device?