Topic 12: Electromagnetic induction 12.3: Transmission of electrical power 12.3: Transmission of electrical power

Energy losses Energy is lost when it is transmitted Energy is lost when it is transmitted At the power transmission lines At the power transmission lines transformers transformers

Large amounts of electrical energy Are transmitted each second, from the power stations to the consumers, often over large distances. Large amounts of electrical energy Are transmitted each second, from the power stations to the consumers, often over large distances. Since power = current x voltage, we could use: Since power = current x voltage, we could use: either a) a low voltage and a high current, either a) a low voltage and a high current, or b) a high voltage and a low current. or b) a high voltage and a low current.

Why does the National Grid always use method (b)? Why does the National Grid always use method (b)? Remember that a current always produces heat in a resistor. Remember that a current always produces heat in a resistor. If the cables have resistance R, and carry a current I, the energy converted to heat each second is I 2 R If the cables have resistance R, and carry a current I, the energy converted to heat each second is I 2 R P = I 2 R P = I 2 R

This means that in method (a) the high current produces a lot of heat in the cables and little of the energy from the power station gets to the consumer. This means that in method (a) the high current produces a lot of heat in the cables and little of the energy from the power station gets to the consumer. Method (b) is used because the low current minimises the power loss. Method (b) is used because the low current minimises the power loss. Transformers at each end of the system step the voltage up and then down. Transformers at each end of the system step the voltage up and then down.

Losses in transformers Copper losses: the wires have some resistance Copper losses: the wires have some resistance Hystereis loss: Magnetising and demagnetising uses power Hystereis loss: Magnetising and demagnetising uses power Eddy currents: small currents form in the core Eddy currents: small currents form in the core

Transmission of Power

At the power station side: At the power station side: Voltage is stepped up with a transformer to V Voltage is stepped up with a transformer to V This reduces electrical loss in the transmission lines This reduces electrical loss in the transmission lines

At the end of the line At the end of the line Voltage is stepped-down with a transformer to Voltage is stepped-down with a transformer to V: heavy industry V: heavy industry V: Light industry V: Light industry 230 V : Homes 230 V : Homes

Health risks How many transformers are there in your home? How many transformers are there in your home? How many electric fields are you exposed to everyday? How many electric fields are you exposed to everyday? What about wireless internet? What about wireless internet? Can these pose a threat to our health? Can these pose a threat to our health?

Electric fields from power lines and mobile phone masts are all around us Electric fields from power lines and mobile phone masts are all around us Electric fields are known to interact with tissues by inducing electric fields and currents in them. Electric fields are known to interact with tissues by inducing electric fields and currents in them. Some studies have found a higher rate of cancer in people living close to power lines Some studies have found a higher rate of cancer in people living close to power lines

How can these fields do this? Results from animal studies conducted so far suggest that electric fields do not initiate or promote cancer. Results from animal studies conducted so far suggest that electric fields do not initiate or promote cancer. Electric fields and magnetic fields were classified as possibly carcinogenic to humans based on epidemiological studies of childhood leukaemia Electric fields and magnetic fields were classified as possibly carcinogenic to humans based on epidemiological studies of childhood leukaemia

"Possibly carcinogenic to humans" is a classification used to denote an agent for which there is limited evidence of carcinogenicity in humans and less than sufficient evidence for carcinogenicity in experimental animals. "Possibly carcinogenic to humans" is a classification used to denote an agent for which there is limited evidence of carcinogenicity in humans and less than sufficient evidence for carcinogenicity in experimental animals.

What about high-voltage power lines? Do not touch them!! Do not touch them!! Again no risk of cancer has been found Again no risk of cancer has been found