Resistance and V=IR

Resistance and V=IR

IGCSE Coordinated Physics

Quick Review

• Electromotive Force (EMF) VS Potential Difference (PD)

• Current

• Q= It

• Connecting Ammeter and Voltmeter

Electromotive Force (EMF)

• Amount of energy supplied to each coulomb of charge passing through that power supply

• 1J/C = 1V

• Measured using Voltmeter

Potential Difference (PD)

• Amount of electrical energy supplied to per coulomb of charge across a component

• 1J/C = 1V

• Measured using voltmeter

Current

• A flow of electric charge

• Conventional current move from the positive terminal to the negative terminal

• Electron flow from negative terminal to positive terminal

Q= It

• Q is electric charge in Coulomb

• I is current in Ampere

• t is time in seconds

Connecting Voltmeter

• Voltmeter must be connected in parallel/ across the component of which the PD / EMF is measured

Connecting Ammeter

• Ammeter must be connected in series within the circuit

Learning Objective 1

• State and use  $R=\frac{V}{I}$  

Resistance

• Resistance is the opposition to the current

• The higher the resistance, the lower the current

• Resistance measured in Ohms,

   $\Omega$  

• If the current stays constant, voltage will increase with resistance

3 A flows through a 240 V lamp. What is the resistance of the lamp?

 $R=\frac{V}{I}$  
 $R=\frac{240}{3}$  
 $R=\ 80\Omega$  

Multiple Choice

The opposition to a flow of current is:

• Resistance

• Repulsion

• Refraction

• Reflection

Multiple Choice

As the resistance of a circuit increases, the current will:

• Decrease

• Increase

• Stay the same

Multiple Choice

A resistor has a potential difference of 10V across it and a current of 2A flowing through it. What is the resistance of the resistor?

• 20Ω

• 5Ω

• 200Ω

• 0.2Ω

Multiple Choice

A light bulb has a resistance of 7Ω with a potential difference of 21V across it. What is the current flowing through the light bulb?

• 147 A

• 28 A

• 3A

• 30A

Multiple Choice

A current of 2A flows through a resistor with a resistance of 10Ω. What is the potential difference across the resistor?

• 5V

• 5J

• 20V

• 20J

Learning Objective 2

• Sketch and explain the current-voltage characteristic of an ohmic resistor and a filament lamp

IV graph

• X-axis, Potential Difference (V)

• Y-axis, Current (I)

•  $\frac{1}{gradient}$  is Resistance ( $\Omega$  )

• As the potential difference (voltage) across a component is increased, the current in the component also increases

IV graph of resistor and filament lamp

• IV graph of resistor has constant resistance (Gradient is constant), the current is proportional to the potential difference

• For filament lamp, the current increases at a proportionally slower rate than the potential difference. This is because the current heat up the filament and further increase the resistance that opposes the current

Multiple Choice

difference between ohmic and non-ohmic conductors is

• ohmic conductor have constant resistance when the voltage is varied across them and non ohmic have changing resistance

• resistance differs but temparature remain the same

• no difference

• not enough information provided

Multiple Choice

Which of these devices are ohmic.

• Lamp

• Resistor

Multiple Choice

Which of these describe an Current-voltage graph for an Ohmic conductor

• Straight line passes through origin

• Curved line passes through origin

• A gradient that changes

Multiple Choice

Which of these describe a current-voltage graph for a lamp (non-ohmic)

• Straight line through the origin

• A curved line showing resistance changes

Learning Objective 3

• Describe and predict how resistance changes with the length or cross-sectional area of a wire.

Resistance of a wire

• As electrons pass through a wire, they collide with the metal ions in the wire

• The ions get in the way of the electrons, resisting their flow

The longer the wire, the longer the resistance

• If the wire is longer, each electron will collide with more ions and so there will be more resistance

• If the length is doubled, the resistance is doubled

The thicker a wire, the smaller the resistance

• If the wire is thicker (greater diameter) there is more space for the electrons and so more electrons can flow

• If the cross-sectional area of a wire is doubled, its resistance will halve

Multiple Choice

Which has the lowest resistance and highest current flowing through it?

• R1

• R2

• R3

• All the same

Multiple Choice

Which copper wire has the lowest resistance?

• 1 metre of 1 mm diameter wire

• 0.5 metre of 1 mm diameter wire

• 0.5 metre of 0.5 mm diameter wire

• 1 meter of 0.5 mm diameter wire