Energy, Power & Resistance

How energy moves through circuits - and what opposes it.

Spec Points Covered
  • OCR.4.1.2 Define EMFElectromotive force. The energy transferred per unit charge by a source in driving charge around a complete circuit. Measured in volts (V). and PD, and distinguish between them.
  • OCR.4.1.2 Use $R = V/I$ and state Ohm's law.
  • OCR.4.1.2 Use the resistivityA material property that quantifies how strongly it resists currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohm-metres (Ω m). equation and explain how temperature affects resistanceThe opposition to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A). flow. The ratio of potential difference to currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).. Measured in ohms (Ω)..
  • OCR.4.1.2 Sketch and interpret I-V characteristics for ohmic conductors, filament lamps, diodes, thermistors, and LDRs.
  • OCR.4.1.3 Calculate powerThe rate of energy transfer. Measured in watts (W). using P = IV, $P = I^2R$, and $P = V^2/R$.
  • OCR.4.1.3 Use $W = IVt$ and convert between joules, kWh, and electron volts.
  • OCR.4.1.3 Apply $eV = 1/2 mv^2$ to accelerating charged particles.
Notes
01 electromotive force (EMF) electromotive force (EMF) OCR.4.1.2 → 02 resistance resistance OCR.4.1.2 → 03 resistivity resistivity OCR.4.1.2 → 04 thermistor (NTC) thermistor (NTC) OCR.4.1.2 → 05 An I-V characteristic shows how current varies with PD for a component OCR.4.1.2 → 06 Power is the rate of energy transfer $P = IV$ OCR.4.1.3 → 07 electron volt (eV) electron volt (eV) OCR.4.1.3 →
Σ Key Equations Full Reference →
On Data Sheet
Not on Data Sheet
Resistance
$$R = \frac{V}{I}$$
  • Where:
    • $R$ = ohm
    • $V$ = V
    • $I$ = A
Definition of resistance. Applies to ALL components, not just ohmic conductors.
Resistivity
$$\rho = \frac{RA}{L}$$
  • Where:
    • $rho$ = ohm m
    • $R$ = ohm
    • $A$ = \(m^{2}\)
    • $L$ = m
rho is a material property. Always convert diameter to area in \(m^{2}\) first.
Power (current x voltage)
$$P = IV$$
  • Where:
    • $P$ = W
    • $I$ = A
    • $V$ = V
Fundamental power equation. Other forms derived from this + Ohm's law.
Power (current squared x resistance)
$$P = I^{2}R$$
  • Where:
    • $P$ = W
    • $I$ = A
    • $R$ = ohm
Useful when V is unknown. Shows that doubling current quadruples power.
Power (voltage squared / resistance)
$$P = \frac{V^{2}}{R}$$
  • Where:
    • $P$ = W
    • $V$ = V
    • $R$ = ohm
Useful when I is unknown. For components in parallel (same V), lower R means more power.
Energy transferred
$$W = IVt$$
  • Where:
    • $W$ = J
    • $I$ = A
    • $V$ = V
    • $t$ = s
W is energy in joules, t is time in seconds. Also written as E = Pt.
Accelerating charged particles
$$eV = \frac{1}{2}mv^{2}$$
  • Where:
    • $e$ = C
    • $V$ = V
    • $m$ = kg
    • $v$ = m/s
Energy gained by charge e through PD V becomes kinetic energy. Assumes particle starts from rest. Combine E_k = ½mv² (on data sheet) with eV definition - this combined form is not a single data sheet equation.