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Do you guys still remember electric current in AS/A-level Physics?

Electric Current

- Electric current is a flow of electric charges. A lamp lights because charged particles are moving through it.
- Ions can only vibrate about their fixed point, but electrons are free to move randomly from one ion to another through the lattice.
- When a battery is attached to a wire. The free electrons are repelled by the negative terminal but are attracted to the positive one.
- They still have random movement but now move in the same direction. This is a flow of charge- an electric current!

Charge

- Electric charge flows along wires.
- It flows along the complete conducting path.
- Charge is the source of electric force.
- Electric charges can be either positive or negative.

Coulomb

- The unit of electric charge is the coulomb.
- 1 coulomb is the amount of charge that passes a point when a current of 1 ampere flows for 1 second.
- The charge on 1 electron is 1.6 x 10 ^-19 Coulombs.

Potential difference

- The coulombs entering a lamp have electrical potential energy; those leaving have very little potential energy.
- The p.d between two points is the electrical potential energy transferred to other forms, per coulomb of charge that passes between two points.

The Volt

- P.d is measured in Volts and is often called Voltage.
- If the p.d is one volt, then 1 joule of electrical energy is transferred for each coulomb of charge.

Resistance

- The greater the resistance of a component the more difficult it is for charge to flow through it.
- The electrons make many collisions with a tungsten fillament which gives it high resistance.
- The resistance of a conductor is the ratio of the p.d applied across it, to the current passing through it.

The ohm

- Resistance is measured in Ohms.
- The current through a metal wire is directly proportional to the p.d across it ( providing that temperature remains constant)- this is Ohm’s law.

Resistivity

- The resistance of a wire at a
**constant temperature**depends on it’s**dimensions and the material**from which it’s made. - Every material has a property called it’s resistivity ‘rho’ it is measured in ohm metres.
- If we know the resistivity, the
**cross-sectional area and the length of a sample of material**we can calculate it’s resistance.

Thermistors

- A thermistor is a
**semiconductor device**made of materials whose r**esistance varies as a function of temperature**; can be used to compensate for temperature variation in other components of a circuit.

This is the end of the topic