Power and Resistance Heating

1. Power and Resistance Heating Electricity Lesson 4

2. Learning Objectives To calculate electrical power supplied to a device. To know how to calculate resistance heating.

3. Potential Difference The work done per unit charge is defined as the potential difference (pd) or voltage across the component. Potential difference is defined as the work done (or energy transfer) per unit charge. The unit of pd is the volt which is equal to 1 joule per coulomb.

4. Potential Difference If work W is done when charge Q flows through the component, the pd across the component V is given by:- In words:-

5. Current Remember that for a current I, the charge flow ΔQ in a time Δt is given by:-

6. Electrical Power Consider a component The symbol Δ is delta, a Greek capital letter Δ, meaning “change in”.

7. Resistance Heating The heating effecting of an electric current in a component is due to the resistance of the component. The charge carriers repeatedly collide with the positive ions of the conducting material. So there is a net transfer of energy from the charge carriers to the positive ions.

8. Resistance Heating When a current I passes through a component, the pd across the component is given by V=IR. The power supplied to the component is given by:- This is energy transferred per second as thermal energy. and rate of heat transfer to the surroundings (if the component is at constant temperature).

9. Resistance Heating If a component heats up, its temperature rise depends on the power the supplied to it and the rate of heat transfer to the surroundings. Energy transferred in time t = power × time = I2Rt The energy transferred does not depend on the direction of the current only its magnitude.