What is Static Electricity?
- When you rub your hair on a balloon, your hair becomes charged.
- Electrons move from your hair, to the balloon.
- Each hair is Positively charged, like charges repel, so the hairs get as far away from each other as possible.
- There are attractive forces between opposite charges, positively charged hair is attracted to the negatively charged balloon.
What is an electric current ?
- Electic current- A flow of charge (electrons).
- Metal conductors have many charges (electrons) that are free to move.
- Insulators do not conduct electricity- they have few charges that are free to move.
What is electrical power?
- When electric charge flows a through a component in a circuit, Work is done by the power supply; Energy is transferred from the power supply to the component and its surroundings.
- Power- Measure of rate an electrical power suppply transfers energy to an appliance or its surroundings.
- Power (Watts, W) = Voltage (Volts, V) x Current (Amperes, A)
- Bigger Resistance = Smaller Current.
- Current through metal conductor is proportional to the voltage across it:
- Resistance (Ohms) = Voltage (Volts, V) / Current (Amperes, A)
- Resistors get hotter when electric current passes through them, Moving electrons bump into stationary ions in the wire.
- 2 Resistors in Series have more resistance than 1 on its own; The battery must push charges through both resistors.
- 2 Resistors in parallel have a smaller total resistance than 1 on its own; There are more paths for the electric charges to flow along.
- The resistance of Light-dependent resistors (LDR) change with light intensity; Its resistance in the dark is greater than in the light.
- Resistance of a Thermistor changes with temperature; hotter temp = lower resistance.
How do Series and Parallel circuits work?
- Voltage = Push on the charges in a circuit.
- Potential difference (p.d.) = Voltage.
- Greater p.d. between 2 points in a circuit, more work done to make charges move between the points.
- Voltmeter (connected in parallel) is used to measure Potential Difference.
- Adding an extra battery in parallel gives no greater p.d. but in series it does; more push is given to the charges in series.
- 3 components in series;
- Same current through all, p.d. across components = p.d. across the battery, p.d. is greater along the component with the greatest resistance.
- 3 components in parallel;
- Current near the battery = total current through components, Current is largest through component with the smallest resistance.
- The p.d. across each component = the p.d. across the battery.
What is electromagnetic induction?
- Electromagnetic induction- Move a magnet into coil of wire, voltage is induced across ends of coil.
- Join up ends of the coil to make a circuit, a current flows.
- You can induce a voltage in the opposite direction by:
- Moving the magnet out of the coil.
- Moving the other pole of the magnet into the coil.
How is mains electricity generated?
- Generators make electricity by electromagnetic induction.
- In generator, a magnet or electromagnet turns near a coil of wire and induces a voltage along the ends of the wire.
- The direction of the votage changes as the magnet rotates.
- Increase the voltage size by:
-Turning the magnet faster - Adding more loops to the coil -Putting an iron core inside the coil -Making the magnetic field stronger.
- Alternating current (a.c.)- the magnet in a generator turns all the time and its magnetic field constantly changes direction.
- Direct current (d.c.)- the current from a battery always is in the same direction.
- Mains electricity is a.c. (230v) as;
- It is easier to generate than d.c.
- It can be distributed more efficiently as less energy is wasted as heat.
What are transformers?
- If current in a wire coil changes, its magnetic field changes and induces a voltage in a nearby coil.
- Tranformer- 2 coils of wire wound onto an iron core, it changes the size of an alternating voltage.
- Voltage across primary coil (v1) / Voltage across secondary coil (v2) = Number of turns on primary coil (n1) / Number of turns on a secondary coil (n2).
How are electric motors made?
- If current is flowing through a wire or coil it can exert a force on:
-a nearby permanent magnet.
-another current-carrying coil nearby.
For current carrying wire in a magnetic field:
- If magnet's lines of force are at 90' to the wire, the wire experiences a force at 90' to the current direction and the lines of force of the field.
- If the magnet's line of force are parallel to the wire, the wire experiences no force.
The motor effect results in a turning force on a rectangular coil in a uniform magnetic field. Adding a Commutator allows the coil to rotate continuously; the Commutator swaps the current direction enery time the coil is vertical.