4.2.1 - Current, Potential Difference and Resistance

Electricity

Draw the circuit symbol for a cell.

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Draw the circuit symbol for a battery.

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Draw the circuit symbol for a lamp.

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Draw the circuit symbol for a fuse.

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Draw the circuit symbol for a voltmeter.

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Draw the circuit symbol for an ammeter.

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Draw the circuit symbol for a diode.

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Draw the circuit symbol for a resistor.

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Draw the circuit symbol for a thermistor.

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Draw the circuit symbol for a variable resistor.

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Draw the circuit symbol for a LDR.

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Draw the circuit symbol for a LED.

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What is an electric current?

The flow of electrical charge

State the equation linking charge, current and time. Give the units for the quantities involved.

Q = I T

Charge (coulombs), Current (amperes), Time (seconds)

What can be said about the value of current at any point in a single closed loop?

Current is the same at all points in a closed loop.

What two factors does the current in a circuit depend on?

1. Potential Difference (V)
2. Resistance (R)

What equation should be used to calculate potential difference if current and resistance are known? State the units for all 3 quantities.

V = I R

Potential difference (V), Current (A), Resistance (Ohms)

What is an ‘Ohmic Conductor’? State the condition required.

A conductor for which current and potential difference are directly proportional

Resistance remains constant as current changes

Temperature must be constant

List FOUR components for which resistance is not constant as current changes.

1. Lamps
2. Diodes
3. Thermistors
4. Light Dependant Resistors (LDRs)

What happens to the resistance of a filament lamp as the temperature increases? Why?

Resistance increases

Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow

What is different about current flow through a diode?

The current only flows in one direction

Resistance is very high in the other direction, preventing current flow

State what happens to the resistance of a thermistor as temperature increases.

The thermistor’s resistance decreases

Give TWO examples of when a thermistor may be used.

1. In a thermostat to turn a heater on below a certain temperature
2. In a freezer to turn on a cooler when the temperature becomes too high

State what happens to the resistance of a LDR as light intensity decreases.

The LDR’s resistance increases

Give an application for a LDR.

Street light often use LDRs

When light levels become too low, the light gains sufficient current to turn on