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4.2.13Electricity and Magnetism

Electrical Energy

5 min read 9 parts Lesson 17 of 31 1 quiz

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Section 1 of 9Energy transfer in circuits

Energy transfer in circuits
Supply
  • Supply energy is transferred from the power source to the charge as electrons pass through it.
Components
  • Components energy is transferred from the electrons to each component, which often dissipates some to the surroundings.
Energy depends on
  • Energy depends on how long the appliance runs and its power rating.

Section 2 of 9Electrical energy equations

E
I V t
What the symbols mean: E or W = energy transferred (J) · Q = charge (C) · V = potential difference (V) · I = current (A) · t = time (s)
  • E is energy (J), I is current (A), V is p.d.
  • (V) and t is time (s).
E = Q V
  • E = Q V it follows from V = W / Q and Q = I t.
Other forms
  • Other forms using V = I R the energy can be written E = I2R t = V2t / R.
  • Choose the form that matches the data given.

Section 3 of 9Energy transfers in devices

DeviceEnergy transfer
Kettle, heater, toasterElectrical to thermal
LampElectrical to light and thermal; a filament lamp wastes most as heat
Motor (washing machine, vacuum cleaner)Electrical to kinetic, with some thermal
LoudspeakerElectrical to sound
Conservation
  • Conservation no energy is lost overall, because the electrical energy supplied equals the useful plus wasted energy.
Efficiency
  • Efficiency useful output energy divided by total input energy.

Section 4 of 9The kilowatt-hour

Why
  • Why the joule is too small for household use, so electricity is charged in kilowatt-hours.
Kilowatt-hour (kW h)
  • Kilowatt-hour (kW h) the energy transferred by a 1 kW device operating for 1 hour.
Conversion
  • 1 kW h = 1000 W × 3600 s = 3.6 × 106 J.
  • kW h to J: multiply by 3.6 × 106.
  • J to kW hdivide by 3.6 × 106.
Formula
  • E = P t with power in kW and time in hours.

Section 5 of 9Cost of electricity

1
Convert the power to kilowatts.
2
Convert the time to hours.
3
Multiply power by time to get energy in kW h.
4
Multiply the energy by the price per kW h.
  • Bill
    • the cost depends only on the kW h, so a 100 W lamp on for 10 hours costs the same as a 1 kW heater on for 1 hour.

Section 6 of 9Measuring electrical energy in the lab

Calculation method
  • Calculation method connect an ammeter in series and a voltmeter across the heater, run it for a measured time and calculate E = I V t.
Direct method
  • Direct method a joulemeter reads the energy directly.
Specific heat capacity
  • Specific heat capacity an immersion heater in a metal block gives I V t = m c Δθ.
  • Energy escaping to the surroundings makes the measured heating less than the electrical energy supplied.
  • Insulating the block reduces this error.

Section 7 of 9More from the specification

1
Electrical Energy Transfer
  • The total energy delivered by an electric current through a resistive circuit element is calculated from E = VIt = I2 R t, measured in joules (J).
2
Electroscope Operation
  • Bringing a charged polythene rod near the top plate of a gold-leaf electroscope induces charge separation;
    • like charges repelling on the plate and leaf cause observable deflection.
3
Potential Difference and Electron Flow
  • Potential difference represents the electrical driving force pushing mobile electrons from the negative terminal toward the positive terminal against circuit resistance.
4
Electrostatic Fields from Insulators
  • Frictionally charging insulating materials traps excess positive or negative charges on their surface, creating static electric fields that exert forces on adjacent conductors.
5
Electromotive force
  • Energy transferred from other forms to electrical energy per unit charge.
6
Gold-leaf electroscope
  • A device in which a charged gold leaf falls when photoelectrons discharge the plate.
7
Mains supply
  • Domestic a.c. supply, rated 230 V rms at 50 Hz in the UK.
8
Resistance
  • Opposition to current, R = V / I, in ohms (Ω).
9
Electroscope
  • Instrument with a metal cap, stem and gold leaf used to detect charge.
10
Conductor
  • Material that allows current to flow.
11
Terminal
  • Connection point of a cell, labelled + or −.
12
Potential difference
  • Work done by a unit charge passing through a component.
13
Electric field
  • Region of space in which a charge experiences a force.
14
Alternating current
  • Current that continually reverses direction.
15
Field lines
  • Lines showing the direction and strength of a magnetic field.

Section 8 of 9Key terms

TermMeaning
Electrical energyE = I V t, in joules.
Kilowatt-hourEnergy unit equal to 3.6 × 106 J.
JoulemeterInstrument that reads electrical energy directly.
EfficiencyUseful output energy divided by total input energy.
Energy transferMovement of energy from one store to another.

Section 9 of 9Exam technique

1
Using minutes or hours with joules — E = I V t needs seconds, whereas kW h needs hours.
2
Mixing up watts and kilowatts — 500 W is 0.5 kW.
3
Saying energy is used up — it is transferred to other stores, mostly as heat.
4
Using the e.m.f. instead of the p.d. across the component when finding the energy in that component.
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