Electricity

• A force field is an area in which an object experiences a non-contact force.
• Force fields can be represented as vectors, which describe the direction of the force that would be exerted on the object, from this knowledge you can deduce the direction of the field.
• Force fields can also be represented as diagrams containing field lines, the distance between field lines represents the strength of the force exerted by the field in that region.
• Fields are formed during the interaction of masses, static charge or moving charges.
• Gravitational fields are formed during the interaction of masses.
• Electric fields are formed during the interaction of charges.
• Both fields follow an inverse-square law.
• In gravitational fields, the force exerted is always attractive, while in electric fields the force can be either repulsive or attractive.
• Both fields have equipotential surfaces.
• The Earth’s gravitational field is radial, however very close to the surface it is almost completely uniform.
• Gravitational field strength (g) is the force per unit mass exerted by a gravitational field on an object.
• 35000 Vs = 2 × 50 × 750 = 0.75 MW of power is transmitted along 225 km of wire, which has a resistance of 0.2 Ω per kilometre, if the transmission voltage is V.
• Gravitational potential (V) at a point is the work done per unit mass when moving an object from infinity to that point.
• The power wasted is calculated using the formula Ip = V/R, where R is the total resistance along the whole length of wire.
• The energy wasted per second is found to be
• The energy wasted per second is calculated using the formula R × I².
• R =
• 8 × 10^7 W.
• The output voltage of the transformer is found using the formula Vs = Vp × Ns/Np.
• The current through the wire is calculated using the formula I = P/V, where P is the power transmitted.
• The total resistance along the wire is found using the formula
• Alternating electric current leaves a power station at 25000 V, and enters the primary coil of a step-up transformer with 50 turns on its primary coil and 750 turns on its secondary coil.
• Gravitational potential at infinity is zero, and as an object moves from infinity to a point, energy is released as the gravitational potential energy is reduced, therefore gravitational potential is always negative.
• The gravitational potential difference ( ) is the energy needed to move a unit mass V Δ between two points and therefore can be used to find the work done when moving an object in a gravitational field.
• Magnetic flux ( ϕ ) is a value which describes the magnetic field or magnetic field lines passing through a given area, calculated by finding the product of magnetic flux density ( B ) and the given area ( A ) when the field is perpendicular to the area: A Φ = B.
• When a conducting rod moves relative to a magnetic field, the electrons in the rod will experience a force, causing an emf to be induced in the rod, this is known as electromagnetic induction.
• Lenz’s law states that the direction of induced current is such as to oppose the motion causing it.
• To demonstrate Lenz’s law, you can measure the speed of a magnet falling through a coil of wire, and its speed when falling from the same height without falling through the coil.
• You can use the above equation to derive a general formula for the magnitude of emf induced by a straight conductor of length l, moving in a magnetic field of flux density B.
• When particles reach the edge of the electrode, they are accelerated by the electric field, increasing the radius of their circular path as they move through the second electrode.
• Faraday’s law can be expressed in the following equation: ε = N Δ t ΔΦ, where ε is the magnitude of induced emf, and N is the rate of change of flux linkage.
• When particles reach the gap again, the alternating electric field changes direction, allowing the particles to be accelerated again.
• The process of particles being accelerated and increasing their speed until they exit the cyclotron is repeated several times.
• Faraday’s law states that the magnitude of induced emf is equal to the rate of change of flux linkage.
• Magnetic flux linkage (N ϕ ) is the magnetic flux multiplied by the number of turns N, of a coil: Φ AN N = B.
• The time constant can also be found by plotting a graph of ln(Q) against t, where the gradient of this graph is -1.
• The time constant is the value of time taken to discharge a capacitor to of its initial value (of charge, current or voltage) or to charge a capacitor to of its initial value (of charge or voltage).
• When current passes through a wire, a magnetic field is induced, and the field lines of the induced magnetic field form concentric rings around the wire.
• When a capacitor is discharging, the current flows in the opposite direction, and the current, charge and potential difference across the capacitor all fall exponentially, meaning it takes the same amount of time for these values to halve.
• The approximate value of time constant can be found by drawing a line across at 63% of its maximum value as the time at which this occurs will be the time constant.