4.2. Energy, Power and Resistance

Potential Difference (P.D.)

Electromotive Force (E.M.F)

• Defenitions

• Symbols

• Units → defenition

• Measured using (x2)

• P.D. (V)→ the energy transferred per unit charge that is lost from electrical to other forms as charge passes between two points (components)

• E.M.F (E)→ the energy transferred per unit charge that is gained from other forms to electrical as charge passes between two points (power supplies / cells)

• Measured in Volts (V) (1 volt = 1 joule per coulomb)

• Measured using a voltmeter

  • Must have a very high resistance → only a negligible current flows through it → does not disturb circuit

  • Must be connected in parallel

Kirchoff’s Second Law (defenition x3)

• In any closed loop within a circuit → sum of EMFs = sum of PDs

• This is a special case of the law of the conservation of energy → it is only true because energy is conserved

• In the case of closed loops without EMFs (no cells / power supplies) → some components experience the current in the opposite direction → PDs are negative → they all sum to 0

Resistance (defenition, unit → defenition)

• Potential difference per unit current

• Measured in Ohms (Ω) (1 ohm = 1 volt per amp)

Ohm’s Law (x5)

• At constant external conditions (temp)

• Current flowing through a component is directly proportional to the PD across it

• I.e. the resistance of the component is always constant

• The I-V graph of the component is a straight line through the origin

• If a component is ohmic → follows ohm’s law

Fixed Resistor: V-I Characteristics

• Ohmic → a straight line through the origin

Filament Bulb: V-I Characteristics

• Non-Ohmic → a curve with a decreasing gradient which passes through the origin#

• Because as current increases → filament heats up → resistance increases

Diode: V-I Characteristics

• Only allows current to flow in one direction → no current for negative voltages

• At threshold voltage → resistance suddenly decreases → current begins to increase rapidly

Thermistor: V-I Characteristics

• Non-Ohmic → a curve with an increasing gradient that passes through the origin

• As current increases → temperature increases → resistance decreases

LED: V-I Characteristics

(same as normal diode)

• Different colour LEDs have different threshold voltages

LDR: Resistance Characteristics

• Resistance is inversely proportional to light intensity

• As light intensity increases → resistance decreases

• (Semiconductor: more energy input → more available e- (n increases) → resistance decreases)

NTC Thermistor: Resistance Characteristics

• Resistance is inversely proportional to temperature

• As temperature increases → resistance decreases

• (Semiconductor: more energy input → more available e- (n increases) → resistance decreases)

Resistivity (defenition x4, units, equation)

• A measure of how electrically conductive a material is

• All objects of the same material will have the same resistivity (but not necesarily the same resistance)

• Measured in Ohm-metres (Ωm)

Resistivity = (Resistance x cross-sectional area) / Length

• 1 Ωm means a 1m³ block of that material has a resistance of 1Ω

• Resistivities for conductors tend to be very small → 1m³ of material has a very large surface area and a relatively low length → thus a very low resistance

How does Resistivity vary with Temperature for Metals & Semiconductors (x4)

• Metals → + temp → fixed metal ions vibrate with greater frequency & amplitude → increased frequency of collisions between e- and ions → increased resistance

• Semiconductors →+ temp → n increases → resistance decreases

• Resistivity is a constance for the material → links resistance to object’s shape

• So resistivity will change in the same way as resistance with temperature for that material

Power (defenition, units → defenition, equations x4)

• Rate of transfer of energy (energy transferred per unit time)

• Measured in Watts (W) → 1 watt = 1 joule per second

P = E/t

P = IV

P = I²R

P = V²/R

Kilowatt-hours (defenition x2, why it is used x3)

• 1kWh = 1000W * 3600s = 3.6MJ

• The energy transferred when a power of 1kW is maintained for 1h

• The S.I. Unit for energy is Joules (J)

• But this is a very small measure of energy

• So when measuring energy used for domestic or industrial purposes →kWh are used instead