Dielectrics and the Polarisation of Matter
An insulator in a field develops aligned dipoles whose own field opposes the applied one, cutting the field inside by the dielectric constant. That is why filling a capacitor with plastic raises its capacitance.
What a learner can do afterwards
- Explains polarisation for polar and non-polar molecules and the field it produces
- Relates the dielectric constant to the reduction of the field inside the material
- Predicts how capacitance, stored charge and voltage change when a dielectric is inserted
1 · Read
An insulator in a field still responds: its molecules become aligned dipoles whose own field opposes the applied one. The net field inside drops by the dielectric constant kappa. Polar molecules already own dipoles that swing into line, while nonpolar ones grow induced dipoles as their electron clouds shift. No free charges move, yet the slab reshapes the whole field.
Slide plastic into a capacitor and the capacitance multiplies by kappa. With charge fixed and lonely, the same Q needs less voltage: V drops by kappa and the stored energy drops too. With a battery holding voltage fixed, extra charge flows on instead, raising stored charge and energy. A slab of kappa 2 on an isolated capacitor halves its voltage. Either way the bound charge layer on the slab faces does the work.
The molecular picture explains the faces: aligned dipoles leave bound charge on the slab surfaces, and that layer makes the opposing field. Shape sets the base capacitance, since bigger plates or a smaller gap raise it, and the dielectric multiplies whatever shape gives. Capacitance is charge per volt, and stored energy grows with the square of the voltage.
Predict before computing: name what is fixed, Q or V. Fixed Q means V falls by kappa; fixed V means Q climbs by kappa. Mixing up the two cases is the classic slip. Kim's version, where capacitance falls by kappa, gets it backwards: capacitance multiplies while voltage falls.
Aligned dipoles oppose the field, capacitance grows by kappa, and the fixed quantity decides what changes.
More to read
8.1 Capacitors and Capacitance
A capacitor stores charge on two facing plates and energy in the field between them. Capacitance measures charge per volt, set purely by shape and size: bigger plates or a smaller gap raise it. Charging a capacitor parks positive charge on one plate and negative on the other. The voltage across grows with stored charge, and the stored energy grows with the square of that voltage.
8.4 Capacitor with a Dielectric
Sliding an insulator into a capacitor multiplies its capacitance by the dielectric constant kappa. The molecules inside align with the field, and their own opposing field cuts the net field for the same plate charge. With charge fixed, the voltage drops by kappa and the stored energy drops too. With voltage fixed by a battery, extra charge flows on instead, raising the stored charge and energy.
8.5 Molecular Model of a Dielectric
Inside a dielectric sit molecules that stretch or rotate in an applied field. Nonpolar molecules grow induced dipoles as their electron clouds shift, while polar molecules already own dipoles that swing into line. The aligned dipoles leave bound charge on the slab faces, and that layer makes the opposing field. No free charges move, yet the material still reshapes the field, which is why plastic between plates changes the whole circuit.
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.