Physics 1.3 JaskiratK

Physics - P1.3

Using Electrical Energy

We can calculate the amount of electrical energy transferred by an appliance and how much it costs to run. This is useful for comparing the advantages and disadvantages of using different electrical appliances for a particular purpose.

Electrical Energy Calculations:The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours, kWh. One unit is 1 kWh.E = P x T• E – Is the energy transferred in kilowatt-hours, kWh.• P – Is the power in kilowatts, kW.• T – Is the time in hours, h.

Power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you must divide by 1,000.Time is measured in hours here, instead of the more usual seconds. To convert from seconds to hours you must divide by 3,600.

E.G - A 2 kW electrical fire is switched on for 3 hours. It uses 2 × 3 = 6 kWh of electrical energy.Joules, Watts And Seconds:

You can also use the equation, E = P x T when:• E – Is energy transferred in Joules, J.• P – Is power in watts, W.• T – Is time in seconds, S.

Transferring Electrical Energy

Cost Of ElectricityElectricity meters measure the number of units of electricity used in a home or other building. The

more units used, the greater the cost. The cost of the electricity used is calculated using this equation:Total Cost = Number Of Units x Cost Per Unit.

Example-If 5 Units are used at a cost of 8p per unit.The total cost will be 5 x 8 = 40pRemember, that the number of units can be calculated using this equation:Units (kWh) = Power (kW) x Time (h) …so…Total Cost = Power (kW) x Time (h) x Cost Per Unit

Example-Double-glazing might cost £2,500 and save £100 a year. What is the payback time? Payback Time = Cost Of Energy-Saving Measure ÷ Money Saved Each YearPayback Time = 2,500 ÷ 100 Payback Time = 25 Years

Physics - P1.3

Cost Of ElectricityElectrical Energy Calculations:

The amount of electrical energy transferred to an appliance depends on its power and the length of time it is switched on. The amount of mains electrical energy transferred is measured in kilowatt-hours,

kWh. One unit is 1kWh.The equations below shows the relationship between energy transferred, power and time:

Energy Transferred(kWh) = Power (kW) x Time (h)Power is measured in kilowatts here instead of the more usual watts. To convert from W to kW you

must divide by 1000. E.g.- 2000W = 2000 ÷ 1000 = 2kW. Time is measured in hours here, instead of seconds. To convert from seconds to hours you must divide

by 36000. E.g.- 1800s = 1800 ÷ 3600 = 0.5 hours.

Physics - P1.3

The CostElectricity meters measure the

number of units of electricity used in a home or other building. The more units used, the greater the cost. The

cost of the electricity is calculated using this equation:

Total Cost = Number Of Units x Cost Per Unit

E.g- if 5 units of electricity are used at a cost of 8p per unity, the total

cost will be 5 x 8 =40p.

Uses Of Electricity

Filament LampsIn a tungsten filament lamp the energy is transferred in the resistance wire which makes up the

filament. If you look at it closely, you will see the filament glowing but it also gives off a lot of heat. Of the electrical energy that goes into the lamp as little as 10% is transferred as light and up to 90% as

heat. An electrical heater operates in the same way as a filament lamp in that the energy is transferred (From electrical to heat in this case) in a resistance wire, often called the element.

Tube LampsIn a discharge tube the energy is transferred in the gas inside the tube. A discharge tube lamp is more

efficient than a filament lamp because:- More of the electrical energy is transferred as light.- Less of the electrical energy is transferred as heat.

Physics - P1.3

Electrical HeatingMany electrical appliances used at home are

lamps or heaters. In a lamp, electrical energy is transferred as light and heat. There

are different types of lamps, including:- Tungsten Filament Lamps

- Discharge Tube Lamps

Electricity is distributed from power stations to consumers through the National Grid, which allows distant power stations to be used. It also allows a mix of different energy resources to be used efficiently to supply the country’s electricity, whatever the local demand.

Transformers:Electricity is transferred from power stations to consumers through the wires and cables of the National Grid. When a current flows through a wire some energy is lost as heat. The higher the current, the more heat is lost. To reduce these losses, the National Grid transmits electricity at a low current. This needs a high voltage.Transformers are used in the National Grid. A transformer is an electrical device that changes the voltage of an alternating current (ac) supply, such as the mains electrical supply. A transformer that:• Increases Voltage – Step-Up Transformer• Decreases Voltage – Step-Down Transformer

Power stations produce electricity at 25,000 V. Step-up transformers change the voltage to the very values needed to transmit electricity through the National Grid power lines. Electricity is sent through these at 400,000 V, 275,000 V or 132,000 V. This reduces energy losses during transmission but the voltages would be dangerous in homes. Step-down transformers are used locally to reduce the voltage to safe levels. The voltage of household electricity is about 230 V.

Main Features:Electricity from a power station goes to:1.Step-Up Transformers2. High Voltage Transmissions Lines.3. Step-Down Transformers4. Consumers – Homes, Factories, Shops.

National GridPhysics - P1.3

Sankey Diagrams

Sankey Diagram Showing The Energy Transfer In A Light Bulb.

The efficiency of a device can be calculated using this equation:Efficiency = Using Energy Output ÷ Useful Energy Input The efficiency of the electric fire in the example is 90 ÷ 100 = 0.9Note that the efficiency of a device will always be less than 1.

Physics - P1.3

Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved.This Sankey diagram for an electric lamp shows that most of the electrical energy is transferred as heat rather than light.

Energy can be transferred usefully, stored or dissipated. It cannot be created or destroyed. Notice that 100 J of electrical energy is supplied to the lamp. Of this, 10 J is transferred to the surroundings as light energy. The remainder, 90 J (100 J – 10 J) is transferred to the surroundings as heat energy.The energy transfer to light energy is the useful transfer. The rest is ‘wasted’: it is eventually transferred to the surroundings, making them warmer. This ‘wasted’ energy eventually becomes so spread out that it becomes less useful.