# Physics: Unit Four

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- Created by: hannahh.hm
- Created on: 12-08-17 20:29

P4.1 - Electrical charges and fields

Protons and neutrons make up the nucleus of an atom. Electrons move about in the space around the nucleus.

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A proton has a positive charge. An electron has an equal negative charge. A neutron is uncharged.

Rubbing a polythene rod with a dry cloth transfers electrons to the surface atoms of the rod from the cloth. So the polythene rod becomes negatively charged.

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Rubbing a perspex rod with a dry cloth transfers electrons from the surface atoms of the rod on to the cloth. So the perspex rod becomes positively charged. Its positive charge is equal to the negative charge on the dry cloth.

Two charged objects exert a non-contact force on each other because of their charge. This is because a charged object creates an electric field around itself.

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Like charges repel. Unlike charges attract.

P4.2 - Current and charge

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A cell: A cell is necessary to push electrons around a complete circuit. A battery consists of two or more cells. The '+' symbol next to the long line of the cell indicates that this is the positive terminal of the cell.

A switch: A switch enables the current in a circuit to be switched on or off.

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An indicator: An indicator, such as a light bulb, is designed to emit light as a signal when a current passes through it.

A diode: A diode allows current through in one direction only.

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A light-emitting diode (LED): A light-emitting diode (LED) emits light when a current passes through it.

An ammeter: An ammeter is used to measure electric current.

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A fixed resistor: A fixed resistor limits the current in a circuit.

A variable resistor: A variable resistor allows the current to be varied.

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A fuse: A fuse is designed to melt and therefore 'break' the circuit if the current through it is greater than a certain amount.

A heater: A heater is designed to transfer the energy from an electric current to heat the surroundings.

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A voltmeter: A voltmeter is used to measure potential difference (i.e. voltage).

An electric current is a flow of charge. The bigger the number of electrons that pass through a component each second, the bigger is the current passing through it.

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Electric charge is measured in coulombs (C). Electric current is measured in amperes (A).

An electric current of 1 amp is a rate of flow if charge of 1 coulomb per second.

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charge flow, Q (coulombs, C) = current, I (amps, A) x time taken, t (seconds, s)

P4.3 - Potential difference and reistance

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An ammeter measures the current, and is connected in series with the bulb, so the current through them is the same.

A voltmeter measures the potential difference, and is connected parallel with the bulb.

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Potential difference is the energy tranferred energy transferred to the bulb or the work done on it by each coulomb of charge that passes through it, and it measured in volts, V.

potential difference across a component, V (volts, V) = energy transferred, E (joules, J) / charge, Q (coulombs, C)

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resistance, R (ohms, ) = potential difference, V (volts, V) / current, I (amperes, A).

The current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.

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A wire is called an ohmic conductor because its resistance stays constant as the current changes, provided its temperature is constant.

The gradient of the line depends on the resistance of the resistor. The greater the resistance of the resistor, the less steep the line.

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Reversing the potential difference across a resistor reverses the current through it.

P4.4 - Component characteristics

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Resistance increases as current increases.

Reversing the potential difference reverses the current and makes no difference to the shape of the curve. The resistance is the same for the same current, regardless of its direction.

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In the forward direction, the line curves towards the y-axis. So the current is not directly proportional to the potential difference. The resistance changes as the current changes. A diode is a non-ohmic conductor.

In the reverse direction, the current is virtually zero. So the diode's resistance in the reverse direction is a lot higher than its resistance is in the forward direction.

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A thermistor is a temperature-dependent resistor, and its resistance decreases if its temperature increases, and increases if the temperature decreases.

The resistance of a light-dependent resistor decreases if the lights intensity increases, and increases if the light intensity decreases.

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P4.5 - Series circuits

In a series circuit, the same current passes through each component.

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In a series circuit, the total potential difference of the power supply is shared between the components.

The total potential difference of cells in series is the sum of the potential difference of each cell.

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The total resistance of two (or more) components in series is equal to the sum of the resistance of each component.

Adding more resistors in series increases the total resistance because the current through the resistors is reduced and the total potential difference across them is unchanged.

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P4.6 - Parallel circuits

The total current through the whole circuit is the sum of the currents through the separate branches.

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For components in parallel, the potential difference across each component is the same.

In parallel, charge flows separately through each component. Therefore, the total current is the sum of the currents through each component.

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current, I (amperes, A) = potential difference, V (volts, V) / component resistance, R (ohms, )

The total resistance of two (or more) components in parallel is less than the resistance of the resistor with the least resistance.

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Adding more resistors in parallel decreases the total resistance because the total current through the resistors is increased and the total potential difference across them is unchanged.

The bigger the resistance of the component, the smaller the current that will pass through it.

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## Other cards in this set

### Card 2

#### Front

A proton has a positive charge. An electron has an equal negative charge. A neutron is uncharged.

#### Back

Rubbing a polythene rod with a dry cloth transfers electrons to the surface atoms of the rod from the cloth. So the polythene rod becomes negatively charged.

### Card 3

#### Front

Rubbing a perspex rod with a dry cloth transfers electrons from the surface atoms of the rod on to the cloth. So the perspex rod becomes positively charged. Its positive charge is equal to the negative charge on the dry cloth.

#### Back

### Card 4

#### Front

Like charges repel. Unlike charges attract.

#### Back

### Card 5

#### Front

A cell: A cell is necessary to push electrons around a complete circuit. A battery consists of two or more cells. The '+' symbol next to the long line of the cell indicates that this is the positive terminal of the cell.

#### Back

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