Electricity, Magnetism and Atomic Physics: notes

Static Electricity and Current

• Static electricity: electric charge that stays on an object and does not flow. Rubbing two insulators can move electrons from one to the other, leaving one positive and one negative.
• Current: the rate of flow of charge. Measured in amperes (A) with an ammeter connected in series.
• Potential difference (p.d.): the energy given to each coulomb of charge as it moves around a circuit. Measured in volts (V) with a voltmeter connected in parallel.
• Resistance: how hard it is for current to flow. Measured in ohms (Ω).
• Ohm's law: at constant temperature, V = I × R, where V is p.d., I is current and R is resistance.

Circuits and Symbols

• Series circuit: components joined one after another in a single loop. The same current flows through each component.
• Parallel circuit: components on separate branches. Each branch gets the full p.d. of the supply, and the currents from the branches add up.
• Circuit symbols: learn the symbols for cell, battery, switch, lamp, resistor, ammeter, voltmeter and fuse.

Power, Energy and Household Safety

• Electrical power: the rate at which electrical energy is used. P = V × I, measured in watts (W).
• Electrical energy: E = P × t, measured in kilowatt-hours (kWh) for bills.
• Fuse: a thin wire that melts when current gets too high, cutting the supply.
• Earthing: a safety wire that carries current away from the metal case of an appliance if a fault happens.
• Cost of electricity: cost = energy in kWh × price per kWh.

Worked calculation: An electric kettle is rated at 2 kW and used for 30 minutes. Energy = 2 kW × 0.5 h = 1 kWh. At 10 cents per kWh, cost = 1 × 10 = 10 cents.

Magnetism and Electromagnetism

• Magnetic field: the region around a magnet where a magnetic force acts. Field lines run from north to south.
• Electromagnet: a coil of wire with a current through it and often an iron core. It is strong when the current is high and can be switched off.
• Motor effect: a wire carrying current in a magnetic field feels a force. This force turns the coil in a motor.
• Electromagnetic induction: a voltage is produced when a conductor moves through a magnetic field or when the field changes near a coil.

Generators, Transformers and Transmission

• Generator: converts kinetic energy into electrical energy using induction.
• Transformer: changes voltage using two coils on an iron core. It only works with alternating current.
• Transmission: high voltage reduces current in power lines, which cuts energy lost as heat.

The Atom and Radioactivity

• Atom: a nucleus of protons and neutrons, with electrons moving around it.
• Alpha (α) radiation: stopped by paper, most dangerous if swallowed or inhaled.
• Beta (β) radiation: stopped by thin aluminium, passes through paper.
• Gamma (γ) radiation: passes through most materials, stopped only by thick lead or concrete.
• Half-life: the time for half the nuclei in a sample to decay.
• Uses and dangers: radiation is used in medicine and smoke detectors but can damage living cells.

Common slips

• Mixing up ammeter (series) and voltmeter (parallel).
• Forgetting that resistance stays constant only at constant temperature.
• Using watts instead of kilowatts when working out kWh.
• Thinking gamma is stopped by paper.
• Confusing a fuse with an earth wire.